Elements of the Nature and Properties of Soils, 4th edition Ray R Weil Instructor Manual with Test B

Instructor’s Manual with Test Item File to accompany

Elements of the Nature and Properties of Soils 4th Edition Ray R. Weil, Ph.D. Professor of Soil Science University of Maryland College Park, Maryland

Nyle C. Brady, Ph.D. Late Emeritus Professor of Soil Science Cornell University Ithaca, New York

TABLE OF CONTENTS

Preface

iv

Chapter 1

The Soils Around Us

1

Chapter 2

Formation of Soils from Parent Materials

5

Chapter 3

Soil Classification

11

Chapter 4

Soil Architecture and Physical Properties

18

Chapter 5

Soil Water: Characteristics and Behavior

23

Chapter 6

Soil and the Hydrologic Cycle

28

Chapter 7

Soil Aeration and Soil Temperature

34

Chapter 8

The Colloidal Fraction: Seat of Soil Chemical and Physical Activity

39

Chapter 9

Soil Acidity, Alkalinity, Salinity, and Sodicity

46

Chapter 10

Organisms and Ecology of the Soil

57

Chapter 11

Soil Organic Matter

62

Chapter 12

Nutrient Cycles and Soil Fertility

67

Chapter 13

Practical Nutrient Management

85

Chapter 14

Soil Erosion and Its Control

93

Chapter 15

Soils and Chemical Pollution

99

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Chapter 1. The Soils Around Us Overview Roles of Soil Soils are critical to life on Earth. They play many roles in addressing the environmental challenges facing our world. Whether we realize it or not, we are dependent on the ecological functions of soil. Soil plays six key roles in the ecosystem: (1) supporting plant growth, (2) largely controlling the flow of water through the hydrologic cycle, (3) recycling waste products of society and nature, (4) modifying the composition and properties of the atmosphere, (5) providing habitat for an enormous diversity of organisms, and (6) functioning in built environments as construction material and support for building foundations.

Soils affect human health, both directly and indirectly. Soils influence the nutrient value of the food we eat, and also harbor certain microbial pathogens which can cause human disease. On the other hand, soils also harbor organisms, chemicals, and genes that benefit human health.

Medium for Plant Growth Most plants depend on the soil for a suitable medium for growth. Soils provide at least six factors for plant growth: (1) physical support, (2) aeration for roots, (3) moisture supply and storage, (4) moderation of root zone and near-ground air temperature, (5) an environment relatively free of phytotoxins, and (6) 13 of the 17 essential nutrient elements. Usually animals and people obtain their mineral nutrients indirectly from the soil via consumption of plant materials; however, for various reasons, geophagy-intentional eating of soils-is practiced by millions of people around the world.

The Pedosphere Concept The importance of soil as a natural body derives in large part from its roles as an interface between the worlds of rock (the lithosphere), air (the atmosphere), water (the hydrosphere), and living things (the biosphere). Environments where all four of these worlds interact are often the most complex and productive on Earth. The soil, or pedosphere, is an example of such an environment.

Soil as a Natural Body A soil is a three-dimensional natural body that exists in the landscape, much as a mountain or river does. The soil forms in the upper part of the regolith. Soil horizons develop as horizontal layers of the regolith become differentiated and a soil profile is formed, typically with O, A, E, B, and C master horizons. Organic matter is added at the surface, clay and salts are translocated, and parent material weathers into new soil material. In the process, topsoil becomes quite different from the subsoil. The nature of the soil at all depths in the profile—from as shallow as the upper 1 cm, to as deep as 5 or 10 meters—can be important for making wise land management decisions.

Soil: The Interface of Air, Minerals, Water, and Life An ideal soil in good condition for plant growth would have a volume composition of about 50% solid (about 45% mineral and 5% organic matter) and 50% pore space (about half filled with air and half with water). Pore space is particularly important for the organisms living in soil.

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Mineral (Inorganic) Constituents of Soils Mineral soil particles come in all sizes and are grouped by size into coarse fragments (>2 mm), sand (2–.05 mm), silt (.05–.002 mm), and clay (<0.002). Clay differs from the sand and silt, not only in size but also in mineral make-up. Clay consists of both primary and secondary minerals.

Soil structure describes the manner in which soil particles are grouped together.

Soil Organic Matter Soil organic matter generally accounts for only 1 to 6% of a soil’s dry weight, but greatly influences nearly all soil properties and uses. The soil organic matter includes living macro and micro organisms that make up the biomass portion. Humus, dark brown or black, is a collection of organic compounds accumulated in soil when partially broken down plant and animal residues are protected from complete decay. Much of the soil’s humus is colloidal in size, and has highly charged surfaces.

The presence of humus greatly increases nutrient and water holding capacity, and the potential to offer other components that may further stimulate plant growth.

Soil Water: A Dynamic Solution Water in the soil is different from the water encountered in everyday life, for two reasons: (1) the liquid properties of soil water are modified by the attraction between water molecules and soil particles and (2) the water in the soil is never pure, but always has a tremendous variety of substances dissolved in it. The soil solution contains, among other solutes, most of the elements essential for plant growth. Eighteen elements (Table 1.1 lists both the micronutrients and the macronutrients) are now considered to be essential to plant growth (nickel was recently recognized as such), and all but C, H, and O are supplied to plants from the soil solution. The pH of the soil solution may range from very strongly acid (<4) to very strongly alkaline (pH>10), but most soils have pH values between 5 and 8.

Soil Air: A Changing Mixture of Gases Soil air is different from atmospheric air because carbon dioxide is produced and oxygen is used up by soil organisms. Soil air is generally several times more concentrated in carbon dioxide than is atmospheric air. The exchange of gases with the atmosphere is of critical importance to plants.

Interaction of Four Components to Supply Plant Nutrients Mineral weathering and organic matter decomposition release nutrient elements from unavailable storage forms to more available adsorbed or dissolved forms. The great bulk of most soil nutrients is present in unavailable forms stored in the solid framework of the soil.

Nutrient Uptake by Plant Roots Soluble nutrients reach plant roots from the bulk soil by root interception, mass flow, and diffusion. Nutrients are taken up into the root by a biologically active carrier mechanism that transports nutrient ions across cell membranes.

Soil Quality, Degradation, and Resilience Continued growth of the human population forces us to produce more food from the same or smaller area of land. Intensified agriculture on the best soils may help spare land for wildlife habitat 2 .

and other uses, but only if soil quality can be maintained. Soil quality is a measure of the ability of a soil to carry out ecological functions, such as plant growth, water supply, and nutrient recycling. Soil quality reflects a combination of chemical, physical, and biological properties. Soil erosion, salt accumulation, nutrient depletion, and organic matter depletion are some of the processes by which poor management can lead to soil degradation. Some soils are more resilient than other as evidenced by their ability to recover their quality more rapidly after such disturbances. By improving the soil health, we strengthen the ecosystem that is the soil, and the ecosystem services that it provides us all.

Model Answers to Study Questions 1. See Section 1.1 and Figure 1.1.

- Need to transform solar energy into food energy over huge areas of the Earth’s surface.

- Society’s increased appreciation of and continued dependency on the services of natural soilbased ecosystems for water, wildlife, natural cycles, and recreation.

- Increasing replacement of petroleum by soil-grown plant products as feedstocks for chemical industry (e.g., soy bean based inks).

- Increasing replacement of finite fossil fuels with renewable plant-based biofuels (e.g., ethanol).

2. See Section 1.9.

A soil is an organized, three-dimensional body that is a component of a landscape. Mere soil is some material from such a body that can be moved around by a shovel or bulldozer.

3. See Sections 1.1–1.2 Six soil roles or functions: (1) medium for plant growth; (2) regulator of water supplies; (3) recycler of nutrients and carbon; (4) modifier of the atmosphere; (5) habitat for soil organisms; (6) engineering medium. Examples of interactions between these roles: Plant cover (medium for plant growth) enhances infiltration of rainwater (regulator of water supplies); an abundance of soil organisms (habitat for soil organisms) enhances the release of nutrients (recycler of nutrients) by the decay of plant residues.

4. Individual experiences. Examples: observing erosion at a construction site; washing “grit” from a batch of fresh spinach; watering house plants; planting a tree; observing earthworms on the sidewalk after a rain; eating almost any food that contains nutrients from the soil in which it was grown; getting a car stuck in the mud; feeling the texture of soil while cleaning it from the soles of your shoes.

5. See Section 1.12.

Approximate percentages after compaction: air (10%), Water (30%), mineral (54%), organic (6%).

Approximate percentages on a weight (mass) basis: air (0%), mineral (80%), water (18%), organic (2%) from the following approximate calculation:

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Component

Volume, cm3

Density, g/cm3

Mass, g

A

B

C=AxB

Fraction of total mass

air

25

0

0

0

water

25

1.0

25

25/135.5 = 0.18

mineral

45

2.65

108

108/135.5 = 0.80

organic

5

0.5

2.5

2.5/135.5 = 0.02

total

100

-

135.5

1.00

6. See Section 1.17 and Figure 1.18.

Most of the soil’s nutrient “assets” are in complex and very slowly available mineral and organic forms (analogous to long term investments), but these release smaller amounts of nutrients into somewhat more available colloid fraction, which in turn help keep up the supply of adsorbed or exchangeable nutrients on colloid surfaces, which in turn resupply the nutrient content of the soil solution (analogous to pocket cash).

7. See columns 2 and 3 in Table 1.1.

Plants derive these essential elements mainly from the soil: N, P, K, S, Ca, Mg, Fe, Mn, B, Zn, Cu, Cl, Mo, Ni. In addition, plants also take up these quasi-essential elements: Si, Na, Co. 8. See Section 1.2 and Appendix B. No, plants also take up elements which are not essential to the plant’s growth. Examples are silica and sodium (not essential for some plants), selenium, vanadium, lead. Some of these are nutrients for animals eating the plants, others (such as lead) are not, but are of concern because of their potential for animal toxicity.

9. See Glossary for definitions.

10. See Section 1.19.

Some soil degrading processes: erosion, salt accumulation (salinization), nutrient depletion, and chemical pollution.

11. See Section 1.9.

Pedologists study soils as natural bodies in the landscape without a particular land use in mind.

Pedology is therefore closely related to surficial geology. The edaphological approach to soils aims at understanding the soil system as it relates to the suitability of the soil as a medium for plant growth. Edaphology is therefore a kind of ecology.

12. Individual student responses.

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Chapter 2. Formation of Soils from Parent Materials Overview Soils vary greatly from one location to another around the world. This variation can largely be understood in terms of the way in which certain environmental factors of soil formation affect the four basic processes by which geologic parent materials are changed into soils.

Weathering of Rocks and Minerals The Earth’s mantle was originally covered by water and solidified magma, the latter being comprised of igneous rocks such as granite and gabbro. These rocks in turn were made up of mixtures of specific primary minerals such as quartz, feldspars, micas, hornblende, and biotite. Weathering broke down the rocks and minerals, destroying some in the process, but also synthesizing new ones (secondary minerals). Some of the weathering products were transported by water and wind to new locations where they become sediments in lake or ocean bottoms. These sediments were later recemented into sedimentary rocks such as sandstone and shale. Some igneous and sedimentary rocks were later subjected to high pressures and temperatures bringing about a metamorphosis (change) in form and structure, and creating metamorphic rocks. Gneiss, schists, and slate are examples of metamorphic rocks.

Weathering Processes (1) Physical weathering (disintegration) results in physical breakdown of the rock/minerals into smaller particles without significant change in chemical composition. Disintegration is enhanced by temperature changes and differential expansion of minerals, frost action, and peeling of layers from the parent mass (exfoliation). Erosion, transport, and deposition by wind, water, and ice are physical processes that grind and break down the rock and mineral particles.

(2) Biogeochemical weathering (decomposition) results in the destruction of the primary minerals and in some cases the simultaneous generation of secondary minerals such as silicate clays. The latter are formed either by alteration of primary minerals or by recrystallization of decay products into new minerals. Biogeochemical breakdown is enhanced by the (a) hydrolysis or the destruction of a mineral by reaction with H+ and OH− ions in water, (b) hydration or the chemical combining of the mineral with intact water molecules, and (c) complexation of metal cations by organic ligands. Oxidation of elements such as iron or manganese in minerals helps breakdown rocks and releases secondary minerals. Carbonation and other reactions utilizing acids formed by plants and microbes (e.g., H2CO3, HNO3, and H2SO4 and some organic acids) result in the formation of secondary minerals such as silicate clays. All these reactions require or are enhanced by the presence of water, and many of them may lead to the synthesis of secondary minerals.

Five Factors of Soil Formation 1. Parent material

2. Climate

4. Topography

5. Time

3. Living organisms

Parent materials are the starting point for soil development, and their nature profoundly influences soil properties, especially in areas where chemical weathering has not destroyed or greatly modified the original minerals. Soil texture is often determined by the nature of the parent materials as 5 .

are the rate and nature of some weathering processes. For example, limestones resist the acidifying processes while shales deter the rapid downward movement of water. Also, the type of silicate clays formed in a given area are commonly related to the nature of the parent materials.

Parent materials may have been formed in place from the native rock (residual or sedentary), or they may have been transported from one area to another by water, ice, wind, or gravity. Residual parent materials are common in upland areas that have not been covered by materials transported from elsewhere. They are the most extensive of all parent materials.

Water-transported materials may have been deposited in former lake bottoms (lacustrine), alongside streams (alluvial), or in ocean bottoms that have since been elevated (marine). If deposited near the seashore or riverbank they are generally coarse textured, but if deposited further from the shore and in still water, they are commonly fine in texture, giving rise to soils high in silt and clay.

Repeated depositions from multiple flooding events may create stratification of alluvial soils.

Parent materials transported by glaciers (ice) that once covered much of the world’s northern latitudes include a mixture of rocks and minerals. Deposited by the melting glaciers, these heterogeneous materials are known collectively as glacial till. Some till is found in irregular deposits known as moraines. Coarse textured glacial outwash materials are another locally important parent material.

Wind blown (aeolian) materials have resulted from the action of strong winds that carried fine-textured materials from one area to another. These include, in order of decreasing particle size, dune sands, loess, and aerosolic dusts. The latter can move across oceans such that soils in North America may contain dust from China and those in South America contain dust from Africa. Parent materials that move downhill slopes by gravity (colluvium) are not very extensive, but are locally important. Another type of locally important parent material is the accumulated organic debris of partially decomposed plant tissues in which organic soils form, mainly in bogs and other wet areas.

Climate largely determines the nature of the weathering that occurs and profoundly influences the living organisms found in an area. Temperature and precipitation affect the rates of chemical, physical, and biological processes responsible for weathering and for soil profile development. For every 10oC rise in temperature, the rates of biochemical reactions double. Also, weathering reactions involve water. Effective precipitation in soil formation is the water that moves through the regolith.

The greater the effective precipitation, the greater is the development of the soil profile. As a result, soils in desert areas are generally relatively shallow, being influenced mostly by mechanical weathering and by less intensive chemical reactions. Many soils of the humid tropics are deep, and in them most primary minerals have been destroyed by intensive weathering.

Climate also has a profound influence on the natural vegetation. By its effects on temperature, moisture, natural vegetation, and soil organisms, climate plays a major role in determining weathering patterns and in influencing the kinds of soils that develop.

Living organisms, especially natural vegetation, influence the development of soil characteristics in several ways. First, they are sources of organic matter essential for soil chemical and physical properties. The accumulation of organic matter in the upper layers of soil is one of the first steps in the development of soil profiles. Also, living organisms facilitate the cycling of essential plant nutrients. The nutrients are absorbed from the soil by plants, are returned to the soil surface in plant residues, and then back to the soil when soil organisms decompose the plant residues. Earthworms, termites, and rodents act as mixing agents, moving weathered materials and organic matter up and down the soil profile. Natural vegetation also helps stabilize the soil and protect it from the ravages of soil erosion.

Soils of grasslands tend to have higher organic matter contents than forested soils. This results in generally more stable soil aggregates in areas with natural grassland vegetation. Soils developed under pine forests are generally more acid than those formed under deciduous forests since pine tree needles are lower in non-acid cations such as Ca2+, Mg2+, and K+.

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Human intervention may have large impacts, from engineering new construction sites, to application of dredged sediments, to draining and cultivating tracts of land for agriculture. These actions may encourage or discourage the growth of vegetation, leading to further changes in the soil.

Topography influences the rate of runoff and erosion from the soil surface and consequently the infiltration of the water. Consequently, soils on hillsides are commonly not as deep as those on flat terrain, but the removal of excess water is more difficult in the flat lands. Excess water in depressed areas leads to the formation of peat bogs and in turn organic soils. Soils under shallow coastal waters are called subaqueous soils.

The amount of time that parent materials have been subjected to weathering and soil forming processes influences soil properties. Residual parent materials have generally been subjected to soil forming processes longer than transported parent materials. Recently deposited alluvium alongside a stream has had too little time for significant soil profile development to take place. Likewise, soils developed from glacial parent materials and coastal plain areas are generally less weathered than those developed from residual materials.

Four major soil forming processes 1. Transformations such as rock weathering and organic matter decomposition that destroy some soil constituents and synthesize others.

2. Translocations of organic and inorganic materials laterally or vertically. Materials may move out of the profile or from one horizon up or down to another. Often translocations result in accumulations of materials in a particular horizon, such as the accumulation of carbonates in the lower horizons of a semi-arid region soil.

3. Additions of materials, such as plant residues, dust, and salts, to the soil profile from outside sources.

4. Losses of materials, such as water, eroded particles, oxidized organic matter, and leached salts, from the soil profile.

The combined action of these processes brings about vertical differentiation in the regolith, and leads to the formation of distinct horizontal layers called soil horizons.

The Soil Profile The layering described above gradually gives rise to natural bodies called soils, each of which has a characteristic sequence of soil horizons referred to as a soil profile. The master horizons of these profiles include the following:

O horizons: Organic horizons that generally form above the mineral soil or within an organic soil.

A horizons: Topmost mineral horizons, darkened somewhat by organic matter accumulation. Some finer materials have moved downward.

E horizons: Zones of maximum leaching (eluviation) of clay and of oxides of Fe and Al, generally lighter in color than A horizon which overlays it.

B horizons: Zones of accumulation (illuviation) of materials such as silicate clays and oxides of Fe and Al, and sulfates and carbonates of Ca, and Mg. Materials may have formed in place or may have moved in from other horizons.

C horizons: Material underlying the A, E, and B horizons that is generally little affected by the processes that formed the horizons above it. It may or may not be the same as the material from which the upper horizon formed.

7 .

R layers: Underlying consolidated rock.

Transition horizons: Transitional horizons between master horizons, designated by using both capital letters, the dominant horizon being listed first (e.g., AE, EB, BE, and BC).

Subordinate horizon designations help clarify the specific nature of a soil horizon. For example, a Bt horizon has accumulated silicate clays, while a Bk horizon has accumulated carbonates.

Urban Soils Urban land now accounts for nearly 3% of the Earth’s land surface. Therefore, urban soils are taking on new significance. Urban soil materials are often reworked by human activities ranging from shoveling to bulldozing as the land surface is excavated and graded. Commonly, materials are trucked in from nearby excavations or from off-site locations and spread as a fill layer (typically an ^Au horizon) on top of the existing soil. This process also creates various buried horizons (typically designated 2Ab, 2Cb, etc.). Compaction, high pH, contamination, and altered biological communities are common urban soils.

Model Answers to Study Questions 1. See Section 2.1 and Figure 2.4.

During weathering the original rocks and minerals are destroyed or broken down into smaller particles or simpler compounds, but these products may also be combined so that new minerals are synthesized. Example: orthoclase breaks down to release potassium, aluminum, and silicon into the weathering solution and from these components new minerals, perhaps silicate clays, are formed.

2. See Section 2.1.

During hydration reactions intact water molecules bind to the hydrating mineral. During hydrolysis water molecules split into H+ and OH− ions, with the H+ often replacing a metal cation from a mineral structure. During dissolution, water molecules surround ions from the soluble mineral until they become dissociated from each other. During carbonation, water dissolves carbon dioxide to form carbonic acid. Drainage water removes the products of mineral breakdown, encouraging the reactions to proceed to the right.

3. See Figure 2.4.

As the weathering of alumina-silicate minerals proceeds, the ratio of silicon to aluminum tends to decrease because silicon dissolves in the weathering solution much more readily than does aluminum. Therefore, a low ratio of silicon to aluminum in a soil usually indicates a highly weathered stage.

4. See Figures 2.8, 2.11, 2.12, 2.13, 2.14, and 2.15.

Example: Glacial till deposits cover thousands of km2 in the northern United States, but in a given landscape in this till deposit may overlay glacial outwash (see Figure 2.15) resulting in two parent materials within a single soil profile. Figure 2.14 illustrates how coastal plain layering can cause sandy soils to lie adjacent to clayey ones.

8 .

5. See Sections 2.2–2.7.

One possible set of comparisons is given here in tabular form.

Factors of Soil Formation

Forested mountain

Grassland plain

parent material

residual sedimentary and igneous rocks

sedimentary rock, outwash, other

climate

cooler, more humid—more effective precipitation

more arid, warmer—less effective precipitation

biota

coniferous trees, under story ferns, shrubs, grasses

mainly perennial grasses, forbs, herds of grazing animals, prairie dogs

topography

steep

relatively level

time

relatively young due to rapid erosion losses

relatively old surfaces

6. See Section 2.3.

Colluvium is material transported downhill mainly by gravity. It includes an unsorted jumble of all sizes of particles, often with sharp edges. Glacial till is material transported and dropped by glaciers and also includes an unsorted mixture of all particle sizes, but generally with rounded shapes. Alluvium is material transported by flowing river water and is usually sorted by particle size in different horizontal layers with particles generally quite rounded in shape.

7. See Section 2.3 and Figure 2.16.

Loess is material transported and deposited by wind and consisting primarily of silt-sized particles.

As a parent material, loess has a high capacity to hold water, but is generally quite permeable.

Commonly, plant productivity is high and so is the rate of soil formation.

8. See Section 2.8.

Transformations might include weathering of mica to form clay and the decomposition of plant residues to form humus. Translocations might include the upward movement of salts from groundwater to the soil surface in arid regions, and the downward illuviation of clay from the surface horizons to the B horizons. Additions might include calcium added with the deposition of dust and organic matter added with the annual shedding of deciduous tree leaves. Losses might include cations leached down out of the profile and into the groundwater and clay particles from the soil surface eroded away by runoff water.

9. Information from all parts of the chapter may be applied to this answer. See also Figure 2.27.

If we assume that the age (time factor) is the same in both cases, then the soil differences will be mainly due to the biota and climate factors of soil formation. The soil under the cool humid pine forest will likely be acid, relatively deeply weathered, with a thick O horizon and probably a thin A horizon. The soil under the semiarid grassland will likely be more shallow, neutral to alkaline in

9 .

reaction, with little O horizon, but a thick A horizon. Both soils are likely to be relatively coarse in texture.

10. See Figures 2.20, 2.27, 2.30.

The soil on the forested mountainside might include these horizons: Oi-Oa-Oe-A-E-Bw-C. The soil on the semiarid grassland plain might include these horizons: Oi-A1-A2-Bk-C. Many other answers would be reasonable.

11. See Section 2.6.

The answer will vary with each location, but should include such changes as increasing thickness of A horizon, differences on development of the B horizon, color changes in the subsurface horizons (usually from bright reds and yellows in the upper members to grays and black in the lower ones), changes in parent material (e.g., colluvium near the bottom of the sequence). Changes in vegetation might be from trees to sedges, etc.

12. See Sections 2.9–2.10.

The four horizons could be labeled, from top to bottom, as: ^Au, ^2Cu, 3Apb, 3Btb

The carat (^) indicates human transported material (e.g., fill brought in from elsewhere, indicated by the contrasting textures compared to the underlying soil), the “uˮ indicates human artifacts (nails, bricks, etc.). The leading “2” and “3” indicate lithologic discontinuities (changes in parent material).

The “b” indicates buried horizons from the original soil prior to urban overburden fill. The top horizon could also be a ^Cu if the matter was in the material used as fill. The “A” designation implies that we believe the organic matter accumulated in place as an early soil formation process. The abrupt boundary suggests that this is an Ap.

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Chapter 3 Soil Classification Overview To make sense of our world, and to know how best to live in it we need to know what kinds of soils we are dealing with, and how these soils differ from those at another location. Classification systems make it possible to communicate with each other about these different soils and their management. Soils are separated into individuals, with the smallest unit displaying the characteristics called a pedon. Multiple pedons in a landscape make up a polypedon, with a large group in the landscape recognized as a soil individual.

Comprehensive Classification System: Soil Taxonomy There are many systems of classifying soils in use today, each meeting the needs of the country or countries in which it is used. In this text, we focus on Soil Taxonomy, a system used in the United States and, at least in part, in some 50 other countries. Soil Taxonomy utilizes the concept of soils as natural bodies and is based on observable and measurable soil chemical, physical, and biological properties including soil moisture and soil temperature status.

The Soil Taxonomy system also employs a unique nomenclature to connote the major characteristics of the soils in question. In addition, the presence or absence of certain diagnostic horizons in the soil profile helps ascertain the soil’s classification category. A diagnostic horizon is a layer or soil zone whose properties meet certain criteria specified for the purposes of classification.

A diagnostic horizon may be comprised of one or more of the genetic horizons discussed in Chapter 2.

Eight of these diagnostic horizons, called epipedons, are surface horizons. Twenty diagnostic subsurface horizons are used to characterize the different soils in Soil Taxonomy. In addition, five soil moisture regimes and ten soil temperature regimes help identify the category in which a soil belongs.

Categories of Soil Taxonomy In the Soil Taxonomy all the world’s soils are classed in the following categories (from the most general grouping to the most specific soil): orders (12), suborders (68), great groups (444), subgroups (about 2500), families (about 8,000), and series about 25,000). All soils belong to one of the 12 orders which are differentiated from each other primarily by the presence or absence of specific diagnostic horizons. In each order are found suborders that are distinguished from each other by different soil moisture and soil temperature regimes, and by dominant chemical and physical properties. In turn, each suborder is subdivided into great groups differing in the presence or absence of diagnostic horizons (including impervious pans), and in levels of certain chemicals such as clays and salts. Each great group is comprised of a number of subgroups that are characterized by a central (typic) member and by other members that are intergrades toward other orders, suborders, or great groups, or that have characteristics not shared with the typic member. Within each subgroup are soil families that vary in properties that are important for plant growth or for engineering uses of the soil.

Finally, within each family are soil series, the most specific category of the system. Soil series are identified in localized surveys and are given names of local significance.

Terminology for Soil Taxonomy The names applied to the different soil classes in Soil Taxonomy connote much about the soil’s properties. The nomenclature is logical and relatively simple. For example, the root of most of the names applied to the different soil orders is coined from words from Latin, Greek, or one of the 11 .

modern languages. To this root is added a vowel and the letters “sols”. Thus, the root of Mollisols stems from the Latin mollis, meaning soft, which implies a friable, easily worked soil.

The names of specific class members in categories within that of soil orders include the root elements of the soil order name plus specific formative elements or expressions that suggest distinctive soil properties. For example, consider the names of the lower categories of Alfisols and Mollisols that are formed under somewhat dry conditions (Ustic-Latin ustus, burnt implying dryness), have a natric (high sodium (Na)) horizon and are typical of the subgroup in which they are classed:

Order

Alfisols

Mollisols

Suborder (dry)

Ustalfs

Ustolls

Great group (natric)

Natrustalfs

Natrustolls

Subgroup (typical)

Typic Natrustalfs

Typic Natrustolls

Family names indicate the soil texture, mineral composition, and soil temperature class of upper 50 cm such as Typic Natrustolls, fine smectitic, mesic. The soil series names, however, relate to the location where they were first classified, examples being the Ontario, Norfolk, and Miami soil series.

Soil Orders

Entisols (little if any profile development) include Aquents (wet), Fluvents (alluvial deposits), Orthents (typical), Psamments (sandy), and Wassents (under water) suborders. They occur in varied environmental conditions, their properties being controlled largely by the parent materials from which they form since minimal soil development has occurred. Their productivity varies greatly, from very fertile alluvial soils (Fluvents) to recently deposited sandy (Psamments), and shallow rocky soils.

Sustainable management practices vary from intensive cropping of alluvial soils to maintenance of natural vegetation on some sandy areas and steep slopes.

Inceptisols have few diagnostic features, the process of soil formation being in its early stages.

Suborders for Inceptisols, as for most other orders, include those with very cold temperature regimes (Cryepts) and permafrost (Gelepts), and those with moisture regimes that are wet (Aquepts), humid (Udepts), alternately moist and dry (Ustepts), and dry in summer but moist in winter (Xerepts).

Inceptisols are about as variable as Entisols and have equally variable management requirements.

Andisols are developed on recent volcanic ejecta and have been subjected to only mild weathering.

Suborders are divided by temperature and moisture regime as for Inceptisols (Aquands and so on), plus Torrands (hot, dry) and Vitrands (high in volcanic glass). Though limited in their extent, Andisols are very productive. They are commonly located at high elevations near the volcanoes. Andisols are dark in color, low in density, and are quite easily managed. Their colloidal fraction is dominated by poorly crystallized minerals such as allophane and imogolite, giving these soils a high capacity to hold phosphorus in unavailable forms, a problem surmountable with judicious use of manures and fertilizers.

Gelisols are soils with permanently frozen layers (permafrost) in the upper 1 m, or in the upper 2 m if horizons display cryoturbation. They may have a histic, umbric, calcic, or mollic diagnostic horizon.

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The three suborders of Gelisols are Histels (organic material), Turbels (cryoturbation), and Orthels (no special features). These soils are found in very cold regions and are therefore of very limited agricultural significance. They typically support tundra vegetation.

Histosols are organic soils mainly formed in wetland accumulations of plant residues. The four suborders are Fibrist (fibers of plants obvious), Folists (leaf mat accumulations), Hemists (fibers partly decomposed), Saprists (fibers not recognizable), and Wassists (submerged most of the time). Even though they cover only 1% of the Earth’s land surface, Histosols are found all over the world. They are most extensive in cool and cold climates. They are light in weight and have high water holding capacities per unit mass. When drained and fertilized they are quite productive, being used extensively for flower and vegetable production. The peat is also used for containerized plant production and as landscaping mulch. The water level in drained Histosols should be maintained as high as crop production will permit to reduce oxidation of the peat and wind erosion of the soil. Also, some drained Histosols should best be returned to wetland conditions to provide habitats for wild species.

Aridisols are dry soils. They are found globally on about 12% of the land area, and are prominent in the western part of the United States. Seven suborders are recognized, Argids (clay), Calcids (calcium carbonate), Gypsids (gypsum), Cryids (cold), Durids (duripan), Salids (soluble salts), and Cambids (only a cambic horizon with no other diagnostic subsurface horizons). Most Aridisols are either Argids or Cambids (most Cambids were formerly classified as Orthids). Aridisols dominate the land in most of the western part of the United States and in vast areas such as the deserts in Africa, Asia, and the former Soviet Union. They support a diversity of desert flora and fauna, and are used to some extent for low-intensity grazing of sheep and goats, but their productivity is generally low. When irrigated, however, these soils can be very productive especially if supplementary nutrients are supplied through manure or chemical fertilizers. In any case, care must be taken not to over exploit these soils.

Vertisols are characterized by high levels of swelling-type clays. Suborders include Aquerts (wet), Cryerts (cold), Torrerts (hot/dry), Uderts (humid), Usterts (moist/dry), and Xererts (very dry).

Vertisols occupy large areas mostly in Australia, India, and Sudan, and smaller areas in the United States (Texas, Mississippi, Alabama, and the Dakotas). Vertisols are sticky and plastic when wet, and hard and cloddy when dry. This complicates their proper management especially by poor farmers with little or no access to power equipment to handle these heavy soils. Despite their limitations, Vertisols are widely tilled. However if conservation practices and mechanized equipment are not available to till the soil promptly at just the right moisture content, yields can be low and the soils are subject to severe erosion if the land is sloping.

Mollisols (dark soils of grasslands) are among the world’s most productive, having high organic matter contents and good physical condition. Suborders include Albolls (albic horizon), Aquolls (wet), Cryolls (cold), Gelolls (very cold), Rendolls (calcareous), Udolls (humid), Ustolls (moist/dry), and Xerolls (dry summers/moist winter). Large areas of Mollisols are found in the Great Plains of the North America, the steppes of Russia, and the grasslands of Central Asia and China. Many areas are intensively used for grain production.

Alfisols are found under conditions of mild acid weathering common under broad-leafed forests and savannas. Suborders include Aqualfs (wet), Cryalfs (cold), Udalfs (humid), Ustalfs (moist/dry), and Xeralfs (dry summers, moist winters). About 15% of the land area is classified as Alfisols in the United States and about 10% globally. Alfisols are prominent in east central and northeastern parts of the United States, in Northern Europe, and in tropical, and subtropical areas of Asia and Africa.

Alfisols have an Argillic (high clay) subsurface horizon that is at least 35% saturated with non-acid 13 .

cations. In general, Alfisols are quite productive. When properly managed, limed, and fertilized they rank with Mollisols in their crop productive capacity.

Ultisols are highly weathered soils developed under forests in the humid tropics and subtropics. They are more highly weathered and acidic than Alfisols, but less weathered than Oxisols. Suborders are Aquults (wet), Humults (high humus), Udults (humid), Ustults (moist/dry), and Xerults (dry summers, moist winters). They occur on 10% of the land areas in the United States (mainly in the southeast) and 9% worldwide. In tropical areas they are found in association with Oxisols. Ultisols are not naturally as productive as Mollisols or Alfisols, but when properly fertilized and managed they produce well.

The clays present in Ultisols are mostly of the non-sticky low activity type which helps assure easy workability of these soils in the field.

Spodosols are extremely acid soils of forested areas in moist (usually cold) regions where coarsetextured parent materials are present. Spodosols are found in about 3% of the land area in the United States and globally. The suborders include: Aquods (wet), Cryods (cold), Gelods (very cold), Humods (humus), and Orthods (typical). Spodosols are common in northeastern North America and northern Europe. Because of their high acidity and coarse texture, these soils are not naturally very fertile. They are poorly buffered, especially in the lower horizons, and so associated watersheds are quite susceptible to acid rain damage. When heavily fertilized, however, they can become quite productive as evidenced by the potato growing areas of Maine. Most are used for coniferous forest production.

Where continued crop production is practiced, excessive applications of chemical fertilizers must be avoided to prevent the buildup of chemicals in drainage waters to undesirable levels.

Oxisols are the most highly weathered of the soil orders, being most prominent in the humid tropics where rain forest is the dominant natural vegetation. Six suborders are found: Aquox (wet), Perox (very humid), Torrox (hot/dry), Udox (humid), and Ustox (moist/dry). They have a deep oxic subsurface horizon dominated by oxides of iron and aluminum. Primary minerals have been essentially all destroyed. Non-sticky low activity clays prevail so the soils are physically well structured but chemically poor. Their nutrient-supplying power is low and much of the soil nutrient supply is associated with the organic matter that occurs in the surface horizons.

Soil Suborders to Soil Series The lower level categories in Soil Taxonomy each provide an increased degree of specificity in the classification scheme. Properties such as soil moisture and soil temperature are used to differentiate suborders. Specific diagnostic horizons help differentiate Great Groups, and the concept of a core or typical soil is the basis for subgroup differentiation. Mineralogical and textural differences are used to differentiate families, while the most specific category, the soil series, is the basis for local considerations including the conceptual framework for local soil surveys. In utilizing such local surveys, the users should be mindful of the relationship of the soil series to the family, the subgroup, great group, suborder, the order, and even the soil that provides a mantle for all global land areas.

Mapping Soils Soil Description. A soil pit is a rectangular hole large enough and deep enough to allow one or more soil scientist to enter and study a typical pedon as exposed on the pit face. After cleaning away lose debris from the pit face, the soil scientist will examine the colors, texture, consistence, structure, plant rooting patterns, and other soil features, to determine what horizons are present and at what depths their boundaries occur. A soil description is then written in a standard format. The soil scientists will 14 .

learn certain unique characteristics that they can look for to identify a soil quickly and distinguish it from other soils in the area.

Delineating Soil Boundaries. Instead of digging many soil pits, the soil scientist will bring up soil material from numerous small bore holes made with a hand auger. The texture, color, and other properties of the soil material from various depths can be compared to characteristics of the known soils in the region. The soil auger is primarily used to confirm that the type of soil predicted to occur in a particular landscape position is the type actually there. An efficient soil mapper will use clues from changes in topography, vegetation, and soil surface colors as a guide in locating sites to make borings.

Soil Surveys A soil survey is a systematic examination, description, classification, and mapping of the soils in a given area. Once the natural bodies are delineated and their properties described, the soil survey will be useful in making interpretations for all kinds of soil uses, not just those uses which were a high priority at the time the soil survey was conducted.

The basic steps in making a soil survey are: 1) Mapping of the soils; 2) Characterization of the mapping; 3) Classification of the mapping units; 4) Correlation with other soil surveys to make sure that the soil boundaries match up with those mapped for adjacent areas, and that the characterization and classification of mapping units are consistent with other soil surveys; 5) Interpretation of soil suitability to accompany the soils map in order to describe the suitability of each mapping unit for various land uses; and 6) Organization of the information to make it available via maps, electronic data files, and tabular information.

In the United States, this soil survey is available online through the Web Soil Survey (WSS).

Using the WSS, you first delineate your Area of Interest (AOI) and then find detailed information about that specific site. There are also smartphone applications that are currently in use. Use of WSS for land use or site planning requires that the geographic information on the map be integrated with the descriptive and quantitative information found under the Soil Explorer tab. Tables and maps can be made that provide soil characterization data and interpretive rankings for the mapping units on such uses as wildlife habitat, forestry, landscaping, waste disposal, building construction, and source of roadbed materials.

Model Answers to Study Questions 1. See Section 3.2 and Figures 3.3 and 3.42.

Diagnostic horizons are layers in the soil profile that have certain specified properties used to classify soils. In contrast, genetic horizons are layers in the soil that differ from adjacent layers in some observable property as a result of pedogenic processes. A diagnostic horizon may contain several genetic horizons. For example, a Mollic (diagnostic) horizon found in a Mollisol might be 50 cm thick and include an Ap, A, AB, B/A, and a Bw horizon, provided that all five of these genetic horizons meet the color, base saturation (non-acid saturation), and other criteria for a Mollic horizon.

2. See Section 3.1 and Figure 3.1.

A pedon is the smallest area of a soil, typically 1 to 10 m2, that can embody all the significant properties of that soil. A group of very similar pedons is called a polypedon. A polypedon that includes all of the contiguous pedons of one type is a soil individual. An area of land called a landscape usually includes a variety of landforms, each made up of several soil individuals.

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3. See Section 3.4 and Figure 3.7.

Entisols (least); Inceptisols; Mollisols; Alfisols; Oxisols (most weathered)

4. See Section 3.4.

Ultisols have a lower percentage base saturation (<35%) deep in the profile than do Alfisols.

Inceptisols have some kind of easily developed B horizon while Entisols have no B horizon.

5. See Section 3.4, Figure 3.9.

Always begin at the top of the key. The presence of permafrost causes the soil to key out as a Gelisol. Only a “no” answer for the first two queries would allow the spodic horizon to come into play in keying out a Spodosol.

6. See Sections 3.11, 3.12, 3.15, and 3.16.

Vertisols: parent material, climate (dry season) Mollisols: organisms (grass vegetation), parent material (high calcium) Spodosols: climate (humid, high effective precipitation); parent material (sand or sand-forming) Oxisols: climate (hot, humid), time (very old)

7. See Section 3.4 (Table 3.3).

Psamments (Entisols); Udolls (Mollisols); Argids (Aridisols); Udepts (Inceptisols); Fragiudalfs (Alfisols); Haplustox (Oxisols); Calciusterts (Vertisols)

8. See Tables 3.4, 3.5, and 3.6.

Aquic Argixerolls: Mollic epipedon (oll), dry summers and moist winters (xer), silicate clay accumulation in an argillic horizon (argi), gleyed B horizon (aquic). Possible genetic horizon sequence: Ap-A-A/B-Btg-Cg. Some possible interpretations: Wetness limitations for basements, for some crops, and for construction. Organic-rich, fertile, deep A horizons. High water holding capacity for crops and grasses. May have both percolation and wetness limitations for on site sewage treatment.

9. See Section 3.3.

Soil Taxonomy is hierarchical because at each level, several taxa are group together within a higher level category, e.g. several great groups are included in each suborder and several suborders, in turn, are included in each order.

10. See Section 3.3, Section 3.17.

Aquic Paleudults: subgroup, wetness problems for road beds, basements.

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Fragiudults: great group, Fragipan is dense, brittle, and difficult to hand-excavate, very slow permeability, water likely to pond after rainfall, slow percolation limits use for septic drain field.

Haplusterts, great group, Vertisols order, high content of expanding clays, very unstable cuts and very poor subgrade for road or for foundations, pipeline damage likely due to swelling and shrinking, very difficult to till or excavate when either very dry or very wet.

Turbels: suborder, Gelisols with permafrost within 2 m depth, cryoturbation present indicating that soil may suffer serious frost-heaving, foundations or pipelines that might melt the permafrost could cause damaging settlement.

11. See Section 3.18.

Soil pits are dug to enable soil scientists to make detailed observations and collect precise samples from profiles thought to be typical of the major soil series they expect to encounter in a soil survey. The data from the soil pit allow the scientists to write a detailed soil description and to learn enough about each soil series to be able to recognize it when augering in the field.

12. See Section 3.18.

Using Web Soil Survey: 1.

2.

3.

4.

Point your browser to www.websoilsurvey.nrcs.usda.gov/app/ Open Web Soil Survey.

Define your Area of Interest (AOI) by outlining an area on the map that includes the site.

Use Soil Data Explorer tab to show soils by suitability rating for orchards (if available).

13. To answer one must be online. Since the street address or GPS coordinates are not given, once you have found the nearby towns, you will have to use landmarks, such as the stream meanders, to locate the exact place. One can print out or save a PDF the resulting map.

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Chapter 4. Soil Architecture and Physical Properties Overview Soil Color Soil color affects the absorption of solar radiation, but the main reason for studying soil color is that color provides clues to the nature of other soil properties and conditions. The principal causes of soil colors are: (1) organic matter (darkens soils); (2) iron oxides (provides brown and reddish coatings); (3) carbonates (impart whitish colors); and (4) oxidation state (reduced iron oxides impart gray and greenish colors). Dry soil is generally lighter (higher in value) than moist soil. Soil colors are described using the Munsell system that includes the hue, value, and chroma.

Soil Texture Coarse fragments (bigger than 2 mm) are not considered to be part of the fine Earth fraction to which the term texture applies. Organic matter is disregarded in determining soil texture. Soil texture describes the distribution of particles into different size classes, the main divisions being sand (2 mm to 0.05 mm), silt (0.05 mm to 0.002 mm), and clay (smaller than 0.002 mm). The size of soil particles (i.e., the texture) greatly influences the surface area present in a given mass of soil. The surface of soil particles is the site of water adsorption, gas adsorption, mineral weathering, cation exchange, and much microbial colonization. Therefore, many processes and properties are very much affected by soil texture and soil surface area. Mineralogically, sand and silt fractions usually consist mainly of quartz and other primary minerals. The clay fraction, by contrast, contains mostly secondary minerals, often silicate clays. Sandy soils are generally loose and freely draining. Silty soils hold high amounts of plant-available water but are subject to rapid erosion by piping. Clayey soils are stick and plastic when wet and hard when dry. Texture affects soil properties largely because of the relationship between particle surface area and particle size. The texture of a soil can be described by designating a textural class, such as sandy loam, silty clay, or loam. These can be determined from a textural triangle chart if the percentage of two of the three size fractions is known. The percentages of sand, silt, and clay can be determined by the “feel” methods, laser diffraction, or by various sedimentation methods.

Soil Structure The term structure relates to the grouping of primary soil particles into aggregates or peds.

Soil structure may greatly modify the influence of soil texture on water and gas movement, heat transfer, aeration, and tilth. Soil structure is categorized by its type (shape of the peds), size, and strength of expression. The main types of structure are: (1) granular and crumb (roughly spheroidal, crumb being more porous than granular); (2) platy; (3) prism-like (columnar and prismatic); and block-or-nut-like (angular or subangular blocky). Structureless conditions include massive clays and single grain sands.

The formation of soil structure is promoted by chemical (flocculation) processes, physical processes (e.g., freezing, drying), and, especially, biological processes (e.g., binding of particles by microbial glues and fungal hyphae, as well as by fine plant roots). Soil aggregates have a hierarchical organization in that macro aggregates (about 1–5 mm) are comprised of clusters of micro aggregates (about 0.1 to 1mm), which in turn consist of sub-micro aggregates (about 0.01 to 0.1 mm) which are finally comprised of primary mineral and organic particles held together by iron oxides and clayhumus domains. Biological factors mostly affect the larger aggregates. Mycorrhizal fungi can dramatically increase aggregation, enhancing the soil environment for the growth of both plant roots and their own hyphae.

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Tillage and Structural Management of Soils Tilth refers to the physical condition of soil in relation to plant growth. A major component of tilth is soil friability, when the tensile strength (i.e. force required to pull apart) is much greater for individual aggregates than for the soil clods they make up. Soil tilth can change rapidly in response to management practices and conditions. Conventional tillage usually involves both primary tillage (plowing) and secondary tillage (harrowing) to create a seedbed, bury residues, and kill weeds.

Negative long term effects of conventional tillage include the formation of dense plow pans and the accelerated loss of soil organic matter and hence structural stability. Conservation tillage generally keeps the soil surface covered, manipulates the soil less thoroughly and promotes stable macropores.

Compaction, from foot or vehicular traffic, acts by breaking down soil aggregates. Beating rain and excess tillage also destroy soil structure. Deep tap-rooted plants are used to reduce compaction in fields when grown in rotation or as a cover crop.

Aggregate stability is encouraged by fungal hyphae, especially those that are mycorrhizal.

Microbial polysaccharides act as glue to hold particles together in aggregates. Humus also plays a role in stabilizing aggregates. Multivalent cations encourage both flocculation and clay-organic matter complexes. Hard crusts that form when surface structure is broken down cause serious problems by reducing both water infiltration and seedling emergence. Certain synthetic soil conditioners hold promise in stabilizing surface structure.

Soil management practices that promote good soil structure include (1) minimizing tillage; (2) timing tillage for optimum moisture conditions (approximately field capacity or a little drier); (3) maintaining plant litter/residues on the soil surface; (4) incorporating a constant supply of decomposable organic material; (5) using sod crops whenever possible; (6) using green manure and cover crops whenever possible; and (7) applying gypsum and soil conditioners.

Soil Density When the mass (weight) per unit volume of soil, or density of soil, is expressed with regard to the volume of only the solids, the property is called the particle density, and ranges from less than 1 (for organic soils) to over 3 (for some mineral soils) Mg/m3. For most mineral soils, 2.65 Mg/m3, the density of quartz, is assumed to be the particle density.

When the volume of the entire (usually undisturbed) soil sample (solids plus pores) is considered, the resulting property is called the bulk density. Soil bulk density is influenced by both texture (sands are generally more dense) and structure (granular structure lowers bulk density). Bulk density is indicative of the pore space in a soil and the ease with which roots may penetrate a soil.

Percent Pore Space = 100 − (100 × (Bulk Density/Particle Density)). Mineral surface soils typically have bulk densities between 1.1 and 1.5 Mg/m3, while that of compact subsoils may be 2.0 or higher.

Bulk density greater than 1.6 greatly impairs the growth of most plant roots.

Pore space in mineral soils is typically about 50% of the soil volume. The total pore space can be divided into macropores, mostly between peds, and micro pores, mostly within peds. Under moisture conditions favorable for plant growth, most of the macro pores are air-filled and most of the micropores are water-filled. Minimum tillage and good organic matter management practices in arable soils promote macropore space approaching that of virgin soils. The influence of bulk density on root penetration in compacted soils is related to soil strength (penetration resistance) as modified by soil texture and water content. By a process dubbed biodrilling, roots of perennial plants and certain cover crops can produce channels that allow easier penetration by roots of plants subsequently growing in the same soil.

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Engineering Properties Soil consistence describes the resistance of a soil to deformation at various moisture contents.

Consistence may be hard, loose, or soft when dry and sticky and plastic when moist. Friable soil crumbles easily. Some soil horizons are cemented or indurated. Consistency is an engineering term used to describe the resistance of soil to penetration by an object such as a thumbnail. Bearing strength is the capacity of a soil to withstand stresses without rupturing or becoming deformed.

Soil strength helps predict the susceptibility of a soil to sudden failure or rupture under a stress; it is a measure of the coherence among soil particles and is closely related to the content and type of clay. It can be assessed by the unconfined compression test. The stability of non-cohesive soil material is related to its angle of repose. In order to avoid uneven settlement, engineers may strive to completely compact a soil before loading it with a building or highway. The optimum water content for such compaction can be found by a Proctor Test. The plastic limit and liquid limit are moisture contents at which a given soil just begins to act to be malleable or flowable. The difference between these moisture contents, the plasticity index, is used by engineers to classify soil material with regard to its suitability for such construction purposes as roadbeds. Certain clay minerals engender a high degree of stickiness and plasticity to soils, as well as swell/shrink properties that can threaten the stability of construction foundations.

Model Answers to Study Questions 1. See Section 4.1.

Colors of chroma <2 may indicate water-saturated conditions during at least part of the year. The depth to the seasonal water table can therefore be estimated by these gray or mottled red and gray colors. A shallow water table could be a problem for houses with basements, septic drainfields, and for road subgrades.

2. See Section 4.2, especially Table 4.1.

Sandy soils. Advantages: warm up faster in spring, better drainage, easier to till, easy to change pH, better trafficability, and less compaction and rutting. Disadvantages: high leaching potential for chemicals, low nutrient and water-holding capacities.

Clayey soils. Advantages: better nutrient and water-holding capacities, low leaching potential for chemicals. Disadvantages: difficult to till, low infiltration rate, slow drainage, susceptible to compaction and rutting.

3. See Sections 4.4–4.6.

Clayey soil: Improved aggregation and surface soil structure could increase the rate of infiltration and make tillage much easier. Strong structure in deeper horizons could enhance drainage and possibly increase leaching potential.

Sandy soil: Aggregation with organic matter might increase infiltration rate to even higher levels, and could increase water and nutrient-holding capacity, the latter effects being due more to the organic matter level than to the structure per se.

4. See Section 4.7.

A higher bulk density indicates greater compaction and less pore space, particularly macropore space. The bulk density in plot A is very high for such a forested soil, and is indicative of serious 20 .

compaction and will likely have significant negative impacts on forest productivity and hydraulic function. The increase in plot B is much less serious, so the timber harvest system used in plot B should be preferred. While probably not restrictive of root penetration directly, the increase in bulk density in plot A suggests reduced infiltration capacity and aeration of the surface soil which could seriously hinder root growth and biological activity.

5. See Section 4.3.

15% clay, 45% silt: Loam 10% clay and 80% sand: borderline between loamy sand and sandy loam.

6. See Section 4.8.

Using equation developed in Box 4.6, for Db = 1.29, % pore space = 100 − (100 × 1.29/2.65) = 51.3. For Db = 1.1 (before timber harvest), % pore space = 100 − (100 × 1.1/2.65) = 58.5. This is a loss of 7.2% pore space (58.5 − 51.3). Since the change came about by compaction, this small loss of total pore space likely masks a much greater loss of macro-pore spaces as macropores were crushed into micropores. Bulk density Figures by themselves give no quantitative information on the relative sizes of pores.

7. See Sections 4.6–4.7.

Tillage done when the soil is at the proper moisture content can temporarily loosen the soil and increase porosity. However, this improvement in porosity often lasts only until the first few heavy rains. Over the longer term, tillage tends to hasten the loss of organic matter, thus weakening aggregation. It also bares the soil surface to the impact of rain drops, increasing soil erosion and causing the formation of surface crusts. Many tillage implements also cause subsoil compaction.

On balance, minimizing tillage has a beneficial influence on soil structure by allowing natural structure-producing processes (fungal networks, earthworms, etc.) to proceed unimpeded. Another physical property that must be considered is soil temperature. Minimizing tillage (conservation tillage) generally results in surface residues covering the soil, a factor which can cause soil temperatures to be lower in spring. Therefore, some tillage to bare at least a small portion of the soil over the crop row may be justified on the grounds of soil temperature considerations, especially in poorly drained soils in cool regions.

8. See Sections 4.6–4.8.

Answers will vary. Some of the best things: add organic matter to soil, grow cover crops, encourage earthworms (by lime, or organic additions), minimize tillage, establish paths for all traffic, keep soil covered with mulch, stand on wooden boards if planting when soil is wet. Worst things: use excessive tillage, tillage when soil is too wet, trample soil when wet.

9. See Section 4.9 and Figure 4.40.

The Proctor test indicates the water content at which the soil can be most effectively compacted in preparation for a roadbed or foundation. The engineer may add water to bring the soil to this optimum water content so that maximum compaction can be achieved before the soil is loaded with a structure, thus preventing settlement of the foundation or road bed.

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10. See Section 4.9.

The building was most likely constructed when the soil was moist and in the expanded state. The water content of the soil would not change under the foundation, until the tree roots penetrated that zone and extracted water for transpiration. Thus as the tree grew larger, the soil under the foundation was dried causing it to shrink.

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Chapter 5. Soil Water: Characteristics and Behavior Overview Water participates directly in dozens of soil and plant reactions and indirectly affects many others. Water’s polar structure with its hydrogen (positive) end and hydroxyl (negative) end is responsible for the attraction of water molecules, first for each other (cohesion) and then for solid surfaces (adhesion or adsorption), especially if these surfaces have either positive or negative charges.

Cations and anions likewise attract water molecules, being surrounded by a swarm of these as the ions move in the soil. The attraction of water for soil solids accounts for capillary movement in soils and for the soils’ ability to hold water against the pull of gravity and plant roots.

Energy Relations The retention and movement of water in soils, its uptake and translocation in plants, and its loss to the atmosphere are all energy phenomena. In fact, the difference in energy levels of water at one site or condition (e.g., wet soil) from that at another site or condition (e.g., dry) determines the direction and rate of water movement in soils and plants, the movement always being from a zone of high energy level to one of low energy level. Scientists identify the energy status of a soil in relation to that of pure water outside the soil at standard conditions (temperature, pressure, elevation). Kinetic energy is typically negligible; potential energy is most important in determining the status and movement of soil water. Scientists use the term soil water potential to describe the soils’ energy status.

The components of the overall soil water potential are that due to gravitational forces, called gravitational potential, that due to osmotic forces associated with the presence of salts, osmotic potential, and that related to the attraction forces of the soil solids (the soil matrix) matrix potential.

Soil water potential is commonly expressed in terms of standard atmospheric pressure at sea level (bar) or in terms of Newtons/m2 expressed as the Pascal (Pa) or kilopascal (kPa) in the international system of units (SI) used in this text.

Soil Moisture Levels Soil moisture levels are generally correlated with soil water potential. In wet soils, the water molecules are loosely held, their energy status being relatively high compared to that of pure water. In dry soils, the water is held tightly by the soils, solids and its energy status (water potential) is much lower than that of pure water.

Soil texture influences the amount of water held at a given soil water potential level. Clay soils with numerous small capillary pores have high water holding capacities; while sandy soils with fewer micropores have low ones. Likewise, a well-granulated soil with high total pore space generally holds more water than a poorly structured soil, especially if the latter has been compacted.

Soil water content is measured directly by weighing a sample of soil before and after drying in an oven (105oC). Other indirect means of estimating soil moisture status take advantage of the correlation between soil water levels and characteristics such as (a) the electrical resistance of a gypsum or nylon block in equilibrium with the soil, (b) the tension of water due to attraction of soil solids, (c) the scattering of neutrons by the hydrogen molecule in water (neutron probe), (d) the time it takes for a reflected electromagnetic pulse to move a few centimeters in soil (TDR), and (e) the electrical capacitance of soil moist which varies mainly with water content.

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Movement in Soils Water flows from sites with high soil water potential to those with lower potentials. Three types of water movement are recognized: (a) liquid flow in soils saturated with water (saturated flow), (b) liquid flow in unsaturated soils (unsaturated flow), and (c) vapor flow. Saturated flow takes place after a heavy rain or irrigation water application where all soil pores are filled with water.

It moves quite rapidly, its rate of movement being determined by the size and configuration of the soil pores and by the driving force (hydraulic gradient). It is largely responsible for most of the ready drainage from soils including the downward movement of pesticides, organic wastes, and plant nutrients. Preferential flow through interconnected pores and crevices may greatly increase the chance for groundwater pollution.

Unsaturated flow is the dominant means of movement in upland soils. It moves slowly, traveling only in the finer soil pores and being driven by the matric potential gradient. Unsaturated flow is higher in the clay soils.

The quantity of water moving in the form of vapor is relatively small, being of significance in meeting plant water needs only in arid and semi-arid areas. However, water movement from the soil to the atmosphere is in the vapor form.

A stratified layer in the soil that differs sharply in texture from the layers below and above it acts as a moisture barrier, resisting water movement up or down the profile. The same principle holds for a layer of coarse sand or gravel in the bottom of a flower pot. In fact, the behavior of water when it encounters an abrupt change in pore sizes may be used to advantage in the design of such facilities as golf course sand greens and nuclear waste storage sites.

Qualitative Description of Soil Wetness There are no distinct forms of soil water. However, certain qualitative terms are commonly used to describe practical aspects of the degree of soil wetness. A soil is said to be at maximum retentive capacity when it is essentially water saturated; when gravitational water has drained away it is said to be at field capacity (water potential approximately −10 to −30 kPa). Capillary water occupies the finer soil pores and moves according to the laws governing capillarity (water potential between −10 to −30 and −1500 kPa). Hygroscopic water is held very tightly by soil colloids (water potential lower than −3100). Available soil water is that held between the field capacity and the wilting coefficient (plants permanently wilted from lack of water, moisture potential is −1500 kPa); of this, the water held in the larger pores is the more easily available. The Least Limiting Water Range (LLWR) is the water contents ranging between when the soil is so hard that roots cannot penetrate to when the soil is so wet that root cannot get enough oxygen, encompassing the conditions where soil conditions do not severely restrict root growth. The LLWR is made more narrow by soil compaction.

Water Supplied to Plants Water is supplied to plant roots by capillary movement of the water to root surfaces and by the growth of roots into the soil. Since roots are in contact with less than 1% of the total soil surface area most of the water moves from the soil to the plant roots, although the actual distance of movement needed is very small.

24 .

Model Answers to Study Questions 1. See Section 5.3.

Although determination of the absolute energy level of soil water is difficult, water behavior is governed by the relative energy level at various locations in the soil. By using a standard reference state of water, energy levels can be determined relative to this reference state. Customarily for soil investigations, the reference state is pure water (no osmotic potential), not influenced by soil solids (zero matric potential), and at the bottom of the soil profile or at the water table (gravitational potential defined as 0).

2. See Section 5.3.

The root must overcome matric forces attracting the water to the soil solids, osmotic forces attracting water to solutes (salts) dissolved in the soil solution, and gravitational forces pulling downward on the water. Compaction would likely increase the matric forces by reducing the size of pores, thus bringing more of the water close to a solid surface.

3. See Section 5.3 plus Box 5.1 and Figures 5.5, 5.6.

Free water at the water table is attracted by adhesion to the hydrophilic surfaces of soil mineral particles, causing the water to spread out in a thin film with a contact angle of essentially zero.

Simultaneously, cohesion among water molecules tends to hold them together. The fact that the cohesion is much stronger than adhesion of water to the air in the soil pore causes a membranelike surface tension at the air-water interface. However, because the water cohesion is much weaker than the adhesion of the water to the solid surfaces of the pore, a curved meniscus forms at the water-air interface. While atmospheric pressure acts on the water at the water Table, the countervailing pressure of the water under the meniscus is less than atmospheric pressure (because pressure is always lower on the convex side of a curved interface), therefore the water rises in the pore until the difference in pressure is just balanced by the weight of the water pushed above the water Table.

4. See Section 5.4 and Table 5.2.

Tensiometers are sensitive enough in moist soils for use in the flower beds, but cannot measure beyond about −85 kPa and so could not be used for the rough turf. The resistance blocks are fine for the rough turf, but are not sensitive enough at very high potentials to be useful in the flower beds.

5. See Section 5.4.

The capacitance method measures the soil water content, while the tensiometer and electrical resistance block methods measure the water potential. Therefore, TDR requires that a calibration be made to allow one to know at which soil water contents the desired potentials stated would pertain for a particular soil.

25 .

6. See Section 5.2, especially Figure 5.5.

The depth of soil in the pot will determine the height of capillary columns possible. Smaller diameter pores can create taller capillary columns. Therefore, near the top of the 30 cm tall pot, larger pores will be air-filled because they cannot create a 30 cm tall capillary column. Fewer pores will be air-filled near the top of a 15 cm tall pot because even relatively large pores can create a 15 cm capillary column. If the same soil is used in both pots, there will be a larger volume of well-aerated soil in the taller pot.

7. See Section 5.9 and Box 5.3, where the density of water (1Mg/m3) is included for completeness and to show how units cancel. Note that the Bx horizon (a fragipan) does not count in the rooting depth).

A horizon: (1.2 Mg/m3/1 Mg/m3) × (0.28 kg/kg – 0.08 kg/kg) × 30 cm = 7.2 cm Bt horizon: (1.4 Mg/m3/1 Mg/m3) × (0.30 kg/kg – 0.15 kg/kg) × (70 cm – 30 cm) = 8.4 cm Total (no significant rooting in a Fragipan) = 7.2 cm + 8.4 cm = 15.6 cm

8. See Section 5.4, Box 5.2. θm = g water/g soil = [(972 − 300) − (870 − 300)]/ (870 − 300) = (672 − 570)/(570) θv = cm3 water/cm3 soil

= 0.179 (or 17.9%)

= θm × Db = θm × (dry mass soil/volume soil) = 17.9% × (dry mass soil/volume soil) = 17.9% × (570 g/π r2 h) = 17.9% × (570 g/3.14 × 3.25 cm × 3.25 cm × 15 cm) = 20.5%

9. See Section 5.9 and Figures 5.39–5.40. a. Compaction increases soil strength and thus limits root penetration to reach more water. b. Compaction reduces macropores that would hold water at field capacity. c. Compaction will reduce aeration at field capacity, limiting plant ability to take up water. d. Compaction creates more micropores which hold more water at potentials below wilting point.

10. See Section 5.10.

Roots continually extend themselves into new volumes of moist soil, and soil water films conduct water to the root by capillary flow as the root depletes the water adjacent to it.

11. Step one: What is the volume of the compacted soil with bulk density 1.10g/cm3?

0.75 g/1 cm3 = 1.10 g/× cm3 » 0.75/1.1 = x = 0.68 cm3 Step two: What is the water to soil volume ratio in the compacted soil, assuming all the water remained during the compaction process?

Water in uncompacted soil = 0.25 cm3/cm3 as indicated by ◄ on the bar graph.

26 .

After compaction, this same 0.25 cm3 water is contained in the 0.68 cm3 of soil calculated in the previous step. Therefore, in the compacted soil θv = 0.25cm3 water/0.68cm3 soil = 0.36cm3 /cm3.

This is the value indicated by ◄ on the bar graph for the sandy loam with severe compaction.

12. Unsaturated flow: movement of a wetting front; capillary rise from a water table; movement of water toward a root as the root removes water.

Vapor movement: moistening of a fertilizer granule in wilting point dry soil; movement and condensation of moisture on the underside of surface rock during a cold night following a warm dat; maintenance of near 100% humidity in soil air.

13. Detailed calculation for first cell: a. Convert % to ratio: 27.1% = 27.1 g H2O/100 g dry soil = 0.271 g/g b. From Eq.5.7, but with density of water = 1 added (Box 5.2): θv = θm *Db * DH20 : 1.49 Mg soil/m3 * 1 Mg H2O/m3 * 0.271g H2O /g soil = 0.403 m3/m3 Water in layer = θv * L, where L = cm layer thickness Water in 0–30 cm layer = 0.403 m3/m3 * 30 cm = 12.1 cm and from Eq.5.11 (Box 5.4)

A W C = (θ m F C −θ m W P ) ⋅ D b ⋅ L for 0–30 cm layer: AWC = (12.1 − 8.0)= 4.1 cm for 0–90, AWC0-120 = AWC0-30 + AWC30-60 + AWC60-90 =4.1+4.3+3.3 = 10.69 cm Filled in table. Calculated values in bold type.

Field capacity

Wilting point

Available water

Soil depth

Bulk density, ρb

θm

θd

θm

θd

θm

θd

cm

(Mg m−3)

%

cm

%

cm

%

cm

0–30

1.49

27.1

12.1

17.9

8.0

9.2

4.1

30–60

1.51

27.5

12.5

18.1

8.2

9.4

4.3

60–90

1.55

27.1

12.6

20.0

9.3

7.1

3.3

0–90

37.2

27 .

25.5

10.7

Chapter 6. Soil and the Hydrologic Cycle Overview More than 99% of the total global water is found in the oceans, glaciers, and deep ground water and is largely inaccessible to the hydrologic cycle with its rapid exchange between the atmosphere and the Earth’s surface. Of the water that does participate in the human-scale, hydrologic cycle, one-third of the cycle water resides in the soil. The soil plays an important role in several of the processes of the cycle. It provides direct and indirect support for evaporation from the land. It supplies water for plants which is subsequently returned to the atmosphere through plant leaf surfaces. Soil properties as well as the management of soils can help determine the fate of water that reaches the soil in rain or snow. Some water moves downward as gravitational water that can carry with it chemicals that move into streams, rivers, and ground water. Some is held temporarily in the soil pores. The remainder runs off the soil surface removing part of the soil itself that is later deposited as sediment in ponds, lakes, and river harbors, much to the detriment of humans and aquatic life. The disposition of water in a watershed can be expressed by the water balance relationship: precipitation (P) = evapotranspiration (ET) + soil storage (SS) + discharge (D).

Low Impact Development Soil compaction during construction results in drastically increased surface runoff during storms. The runoff burden carried by streams in urbanized watersheds is increased even more as a result a large proportion of the land being covered with completely impermeable surfaces (roof tops, paved streets and parking lots). Erosion of stream banks, toppling of trees, scouring of stream beds and exposure of once-buried pipelines are typical signs of the environmental degradation that ensues as streams become overwhelmed. Features such as permeable pavers that allow some infiltration even in parking lots and rain gardens that catch runoff and release it slowly by infiltration are part of what is termed low impact urban design.

The soil-plant atmosphere continuum Water moves from soil to plants to the atmosphere and back to the soil in a continuum that is subject to the same energy relations as discussed in Chapter 5. Water that is commonly held in the soil at water potentials of −30 to −50 kPa moves into plant roots at a potential of about −70 kPa, up the plant stems (−75 kPa), through the leaf surfaces to the atmosphere (−500 kPa), and resides finally in the atmosphere at a water potential of −20,000 kPa. Upon being condensed to rainwater and falling back to the Earth, the water once again enters the soil and the process is repeated. Hydraulic redistribution by plant roots brings water from moist soil zones to dry soil zones, particularly hydraulic lift, as carried out by deep tree roots.

Vapor losses of water Vapor losses of water from soils occur by evaporation (E) at the soil surface and by transpiration (T) from the plant leaf surfaces after entering the plants from the soil. The combined loss, termed evapotranspiration (ET) is responsible for the water vapor movement from soils during the period of plant growth. ET is affected by factors such as the level of moisture in the soil, solar radiation, and the angle of the sun’s rays as they reach the soil, soil temperature and wind movement over the surface, the shading of the soil by plants (leaf area index), and the vapor pressure of the atmosphere. Annual ET losses range from 35 to 75 cm (14–30 in.) in non-irrigated humid to semi-arid regions, to 50–125 cm (20–50 in.) in hot, dry, arid regions where irrigation is used. The potential 28 .

evapotranspiration (PET) is the vapor loss that would occur if the soil were kept at optimum water content. PET can be most simply estimated as 0.65 times the rate of evaporation from a class A evaporation pan. The difference PET − ET is the water deficit, a measure of plant stress.

The evaporative water required to produce 1 kg of plant dry matter (termed the transpiration ratio) varies greatly from region to region and from one plant to another. In humid regions this requirement is 200–500 kg water and is almost twice as much in arid regions. Some cereal crops such as corn and sorghum have relatively low water requirements compared to some forage crops such as alfalfa.

Transpiration (T) s a plant process subject to only minor control if other plant processes are proceeding normally. In contrast, evaporation (E) from the soil surface is subject to some degree of control by soil and crop management. For example, dense vegetation cover (high leaf area index) reduces E by shading the soil. Similarly the maintenance of crop residues on the soil surface through the use of conservation tillage practices can reduce E. Likewise, in semi-arid areas farming systems that alternate summer fallow (no plants growing) with traditional cropping can leave more moisture in the soil for the cropped year. The use of mulches of either organic materials such as sawdust, manure, wheat, straw, or plastic sheets placed over the soil surface can help control weeds and reduce evaporation from the soil surface. In irrigated areas the use of trickle or drip irrigation applied directly to the plants can reduce evaporation losses from the soil between plants.

Liquid Losses Liquid water losses from the soil occur in two ways: (a) through percolation or drainage that carries with it plant nutrients, pesticides, and other dissolved chemicals that can move into groundwater and nearby streams and (b) through runoff water that carries with it plant nutrients, as well as the finer fractions of the soil itself, through soil erosion. In urban watersheds with vast expanses of impermeable surfaces, runoff due to rainfall events can be extreme, and greatly exceeds the discharge and base flow from adjacent forested watersheds.

Water percolating down through the aerated or vadose zone soil may eventually reach the water table, the upper surface of the zone of saturation which holds groundwater that has built up from water that percolated earlier. Groundwater flows slowly through the porous aquifer layer until it emerges as surface water in a spring or stream. Under certain circumstances most of the percolating water may move rapidly by preferential flow through very large macropores such as those formed by earthworms and old root channels, bypassing the bulk of the soil matrix and any chemicals that may be held in there in the much smaller pores. The preferential flow would, however, carry downward any surface applied nutrients or pesticides that were either soluble in water or attached to dispersed colloids.

Drainage The draining of excess water from soils is essential for the growth of land plants other than aquatic species such as rice, cattails, and wetland forest species. Unfortunately, however, the drainage water removes essential plant nutrients from the soil, nutrients that can later accumulate in lakes and reservoirs and give rise to the undesirable process of eutrophication. Drainage water also carries some pesticides and other organic waste products into ground water and ultimately into water sources used for human and animal consumption. Such chemical pollution can be enhanced by the presence on soils of macropores, through which the chemical-bearing water can move rapidly downward into the groundwater.

Artificial drainage systems are sometimes required to remove excess soil moisture and increase the oxygen supply in the upper soil layers. Surface field drain systems with shallow ditches and gentle side slopes are used to help remove the water even before it moves into the soil. Some land 29 .

forming may be needed to permit water movements to the shallow ditches. Subsurface (underground) drains that involve the placing of perforated tubing or tiles lines horizontally underground enhance the draining of land whether it is in an agricultural field or around the foundation of a house.

Draining the land promotes many conditions favorable to higher plants and soil organisms, and provides greater stability of building foundations and roadbeds. Heaving, the bad effects of alternate expansion and contractions due to the freezing and thawing of soil water, is alleviated. Wet soils are 3-8o C cooler in the spring than dry soils. Soil aeration is largely dependent on soil drainage.

Soil drainage may also need to be installed to prevent water seepage into basements and under foundations.

Septic Tank Drain Fields A special case of soil drainage is the percolation of waste water through the soil profile for treatment and disposal. Wastewater from homes and businesses is piped to a concrete box called a septic tank where the solids settle out and slowly decompose. The liquid effluent overflows into a series of perforated pipes buried in a septic drain field which operates in reverse from how s drainage system works. For effective operation, soil must be permeable, but not too permeable, deep over the water table or rock, and not too steeply sloped. Suitable soil permeability is determined by a perc test.

Irrigation For centuries, farmers and landowners have supplemented nature’s water supply with irrigation water. Such water use has increased ten-fold globally in the past century. It includes not only the diversion of river water into reservoirs and through canals to farm lands, but also the pumping of underground water for agricultural purposes. Such irrigation is utilizing a major proportion of the world’s water resources. Approximately 80% of the consumptive (not returned) use of water globally is for irrigation.

Applying water by flooding the soil or through furrows of row crops is most economical but also is least efficient and is often wasteful of the water. Sprinkle irrigation gives more uniform coverage and is more efficient. Micro-irrigation systems (trickle or drip) that apply very small quantities of water directly to or very near the plants are most efficient since they minimize the evaporation from the soil surface (E).

Irrigation must be accompanied by good internal drainage that prevents the buildup of salts on irrigated soils. Throughout history, failure to prevent such salinization has resulted in land degradation and the ultimate abandonment of once productive irrigated areas.

Model Answers to Study Questions 1. See Section 6.1, Eq. 6.1.

The water balance equation states that P = ET + SS + D. We are being asked to estimate discharge (D), so we rearrange the equation: D = P − ET − SS. It is given that P for the period is 450 mm and that ET averages 4 mm of water per day. For the 90-day period, ET therefore equals 4 × 90 or 360 mm. We need now only find soil storage (SS) to be able to estimate D. The soil mantle averages 150 cm deep and holds 0.2 cm per cm depth at field capacity. The initial water content is 0.1 cm/cm, so the remaining capacity to store water against drainage is approximately 0.2 − 0.1 = 0.1 cm per cm or 0.1 × 150 cm = 15 cm (or 150 mm). Therefore, D = 450 mm − 360 mm − 150 mm = −60 mm. The calculated discharge is negative, therefore no discharge is expected from the P falling during the study period. Some discharge may occur from earlier precipitation already on its way to the groundwater and streams.

30 .

interception precipitation

2. See Section 6.1 and Figure 6.1 and Figure 6.2.

Example sketch.

transpiration

infiltration

evaporation surface storage

percolation plant uptake

soil storage

3. See Section 6.2.

One example is the increased infiltration and reduced runoff that occurs where the soil is well covered by living plants or plant residues.

Another example is the dramatic reduction in evaporation from the soil surface when a heavy cover crop is terminated and left as a surface much.

4. See Section 6.3 and Figure 6.11.

Water always moves to where its potential will be lowest. Where there is a large resistance to flow, there must be a large potential gradient to overcome that resistance. The root membrane provides one such resistance, so the root must maintain a water potential inside that is considerably lower than the water potential of the soil solution around the root. Similarly, the stomata present a resistance to the flow of water from the leaf tissues to the atmosphere, and a very high potential gradient exists during most days to drive this movement.

5. See Section 6.3.

The potential evapotranspiration (PET) is the rate of water loss to the atmosphere if the soil were maintained at an optimum water content. It is a measure of the evaporative demand resulting from a set of climatic conditions. If soil water availability restricts actual ET to a level less than PET, the difference PET-ET is termed the water deficit, and is a measure of plant water stress.

6. See Sections 6.3, 6.4, and 6.9.

Since ET consists of the sum of T and E, any water lost as E increases ET and lowers water use efficiency because E does not contribute to increased plant production. Losses by E can be controlled by such practices as (a) mulching the soil surface, (b) increased density of vegetation (substitutes ET for E), and (c) drip irrigation that does moistens only the rooting zone of each plant leaving most of the soil surface dry.

7. See Section 6.4.

Evapotranspiration (ET).

31 .

8. See Section 6.3.

Plant roots in moist soil zones act as conduits that move water to dry soil zones, especially at night. This process is termed hydraulic redistribution. A common example is hydraulic pumping of deep moisture to the surface soil by tree roots. In semi-arid conditions, deep rooted trees and shrubs may bring enough water to the surface soil to allow the growth of more shallow rooted grasses and other understory plants, including annual crops.

9. See Section 6.4.

Plastic mulches result in elevated soil temperature, which is advantageous in cool spring conditions, but disadvantageous during hot summer growing seasons. Black plastic can also effectively inhibit weed growth. Organic mulches moderate soil temperature, keeping the soil cooler during hot, sunny weather-an advantage in summer, but a disadvantage in early spring.

Organic mulches can contribute to soil organic matter and nutrient supply, roles that plastic cannot play. Plastic mulch material also has the disadvantages of increasing the carbon footprint because of fossil fuel derived material, accumulating in the soil, causing aesthetic problems and litter, and interfering with water movement in the soil.

10. See Section 6.4.

Conservation tillage conserves water (by reducing evaporation and increasing infiltration), soil (by protecting the soil from raindrop impact) and sometimes energy (by requiring less fuel for tillage and irrigation) relative to clean tillage.

11.

Cubic m of water

Water remaining

total water collected

2000000.00

lost in storage

400000.00

1600000

lost in distribution

400000.00

1200000

delivered to field

1200000.00

lost by evaporation

360000.00

lost by runoff

240000.00

lost by percolation

240000.00

used in transpiration

360000.00

total dry matter produced

396000.00

total available for ET

720000.00

total ET per ha

12600.00

32 .

Answers

(a) overall water efficiency (kg/m3)

0.20

0.20

(b) water application efficiency (%)

18.00

18.00

(c) field water efficiency (%)

30.00

30.00

(d) total ha that can be irrigated (ha)

57.14

57.14

12. See Section 6.6 (especially Figure 6.23).

When a chemical is mixed into the soil matrix and a heavy rain or irrigation causes free water to pond and flow on the soil surface. The gravitational water flows freely down earthworm channels, by-passing most of the chemical in the smaller pores.

13. See Section 6.7 and Figure 6.26.

No effect. Water will not flow from the soil where its potential is slightly negative to the drain pipe where it would be zero. In order to collect any water, the pipe must be below the water table where the soil water has a positive potential.

14. See Table 6.2.

Steep slope; too low a percolation rate; too high percolation rate (>150 mm/h); too shallow to bedrock or water Table; susceptibility to flooding; too stony.

15. See Section 6.9, especially Table 6.4.

(1) micro-irrigations systems such as drip.

(2) surface irrigation systems such as furrow or basin flooding.

33 .

Chapter 7. Soil Aeration and Soil Temperature Overview Soil air and soil temperature influence most chemical and physical processes in soils. The quality and composition of soil air influences soil formation processes, the rates of breakdown of organic residues, soil color, the availability and uptake of plant nutrients, and the growth of all soil organisms as well as terrestrial plants. The quantity of soil air is related primarily to the total pore space in soils and to the proportion of that space that is filled with water. But the composition of soil air has far-reaching effects on soil organisms, higher plants and the chemistry of the soil. Most plant roots use oxygen in soil air for respiration, however some hydrophytic plants have special structures, such as aerenchyma tissues and pneumatophores, that allow them to thrive in water-saturated soils by obtaining oxygen for their roots from the atmosphere.

Gases in Soil Air Soil air contains the three main gases that characterize the atmosphere, nitrogen (N2), oxygen (O2), and carbon dioxide (CO2). Since gaseous N2 is mostly inert in the soil, its content in soil air is about the same as that of the atmosphere, some 78%. The remaining nearly 22% is comprised mostly of O2 and CO2 with some water vapor. But the soil air O2 level is generally less than the 21% found in the atmosphere, and CO2 is definitely higher than the .035% found in the atmosphere. When O2 in soil is exhausted, the soil is said to be anaerobic. Oxygen is depleted more frequently in saturated soils under warm conditions.

Characterizing Aeration Status The ratio between these two gases (O2 and CO2) in soil air determines the air’s suitability for plant and animal life. This ratio, in turn, is determined primarily by two processes: (a) the consumption of O2 and release of CO2 through the respiration of plant roots and microbial metabolism, and (b) the exchange of these gases between soil pores and the atmosphere. The latter process is accomplished primarily through diffusion of the two gases between soil pores and the atmosphere, with the net flow bringing O2 into the soil and releasing CO2 to the atmosphere.

The aeration status of a soil can be characterized by determining: (a) the O2 content of the soil, (b) the proportion of the pore space filled with air, and ( c) the oxidation-reduction (redox) potential of the soil. The latter also gives an indication of the tendency of the soil to accept or donate electrons which influences the ionic species of several elements present in soils. Molecular oxygen readily accepts electrons and is a strong oxidizing agent. Under conditions of oxygen deficiency, however, some soil microorganisms can use other chemicals such as Fe3+ and Mn4+ as electron acceptors, the latter thereby being reduced to lower valent forms (Fe2+ and Mn2+). Such oxidation-reduction reactions influence the valence state and consequent availability of several essential elements including iron, manganese, nitrogen, sulfur, and carbon. The redox-potentials (Eh) are measured in volts (V) or millivolts (mV). Redox potentials of +0.4 to +0.7 V indicate oxidizing conditions. Redox potentials between +0.3 and −0.3 V indicate reducing conditions.

Effects of Soil Redox Status Soil aeration influences the rate and nature of microbial breakdown of organic residues, CO2 being the prime oxidation product while nitrogen oxides (NOx), methane gas and organic acids are common under reduced conditions. Methane, a very active greenhouse gas, is produced only under 34 .

very low redox potentials, below −1.5 V. N2O, another active greenhouse gas, is produced mainly under more mild redox conditions (+1.8 to +0.4 V).

Through its effect on redox potential the level of soil oxygen determines the oxidation status of iron and manganese. In poorly drained acid soils, the reduced forms of these elements may be present in toxic quantities while in well-drained alkaline soils the oxidized forms may be tied up in insoluble compounds, resulting in nutrient deficiencies. The redox status of a soil also has color implications, the red and brown colors being characteristic of oxidized Fe and Mn compounds, the blues and grays predominating under reduced conditions.

Management Practices to Improve Soil Aeration Terrestrial plants vary in their tolerance of poor aeration, although most will show some growth reduction if the soil air is less than about 10% of the soil volume. The uptake of both water and plant nutrients is adversely affected by poor aeration.

Both surface and subsurface drainage are essential if an aerobic environment is to be maintained. The maintenance of a stable soil structure that encourages pores of macro size will enhance good aeration. Organic matter additions through farm manure and crop residues will help assure stable soil structure. In poorly drained heavy-textured soils, some mechanical stirring of the soil (cultivation) may be necessary to assure good aeration. Heavy trafficked lawns may require special "core" cultivation to permit gas exchange to take place more easily. Trees and other vegetation in urban landscapes may benefit from special management practices to avoid compaction, covering breather roots, and planting into containers or compacted soils that will hold water in such a way as to create saturated conditions.

Wetlands and Their Poorly Aerated Soils Wetlands are ecosystems characterized by anaerobic soil conditions. Among the many functions of wetlands are the provision of habitat for wildlife, the moderation of flooding, and the removal of certain nutrients from runoff water. The three basic indicators of a wetland are the presence of hydrophytic vegetation, periods of water-saturated conditions, and hydric soils. Hydric soils are saturated long enough during periods of enough biological activity to cause anaerobic conditions to occur and redoximorphic features and other hydric soil indicators to form. Wetland delineation is critical for land use decisions. Because of the important ecosystem services provided, wetlands are protected legally in the USA and other countries.

Soil Temperature Effects Soil temperature affects physical, biological, and chemical processes occurring in soils. In cold soils biological decomposition is very slow, thereby limiting the release of plant nutrients such as nitrogen, phosphorus, and sulfur. Microbial activity does continue in cold temperatures, with some adapted organisms remaining active in permafrost, but rates of activity double for every 10oC rise in temperature and are optimized at between 35-40 oC. Ammonia-containing fertilizers applied early in the spring when the soil is cold are not readily oxidized to the nitrate form that could easily be leached from the soil. Some soil borne plant pathogens are influenced by the temperature of the soil, and are subject to control by artificially warming the soil by covering it with plastic sheets. Forest fires can heat the surface layers of soils to levels that volatilize some organic compounds which condense at lower depths in the profile and later constrain the percolation of water through the soil. Alternate freezing and thawing subjects the soil aggregates to pressures as ice crystals form and expand.

These pressures are relieved by pushing upward the top soil horizons along with associated plant roots,

35 .

shallow building foundations, sidewalks, and highway beds. Severe damage to the plants, the buildings, walkways, and highways results. Seed germination is often controlled by specific soil temperature requirements and plant growth is much more responsive to soil temperatures than to air temperatures.

Factors Influencing Soil Temperature The temperature of a soil is determined by three primary factors: (a) solar radiation, (b) the specific heat of the soil, and (c) energy needed to evaporate soil water. Only about 10% of the solar energy is used to warm the soil, but in the summer when the sun’s rays are more nearly perpendicular to the soil surface, soil temperatures are increased significantly. The specific heat of a soil (energy required to increase the temperature 1oC) helps control the rate at which soils warm up in the spring.

Because of water’s high specific heat compared to that of dry soils, wet soils warm up very slowly, usually causing a delay in early spring plant growth. Likewise, the very large amount of energy needed to evaporate water from the soil (540 calories per gram of water) also delays the rapid warming up of soils in the spring. Soil water also affects the thermal conductivity of soils by forming bridges between most of the soil particles, to help conduct the heat through the soil. The major role soil water plays in controlling soil temperatures is thus demonstrated.

Variations in Soil Temperature Considerable seasonal and monthly variations of soil temperatures occur, especially in the upper soil horizons. Subsoils are generally warmer in the late fall and winter and cooler in the spring and summer than the surface soil layers or the above ground atmosphere. Advantage is taken of this fact in the operation of heat pumps that cool houses during the summer months and warm them in the winter. A network of circulating water pipes laid underground near the building to be heated/cooled maximizes contact with the subsoil and takes advantage of the subsoil’s cooler summer temperatures and warmer winter temperatures to moderate temperature extremes in the building.

Control of Soil Temperature The temperature of field soils is not subject to radical human regulation. However, soil cover and especially organic residues or other types of mulch placed on the soil surface significantly affect soil temperatures, cooling the soil in warm weather and warming it during cold spells. Conservation tillage practices that leave much, if not all, the crop residues on the soil surface definitely buffer these temperature extremes.

Another means of some control over soil temperature is soil moisture. Poorly drained soils in temperate regions that are wet in the spring are 3-6oC cooler than comparable well-drained soils. Only by removing this water can temperature depression be alleviated. The controlling influence of soil water on soil aeration and soil temperature is apparent everywhere. Practices that prevent excess buildup of water in soils enhance the usefulness of soils for both engineering and agriculture purposes.

Model Answers to Study Questions 1. See Sections 7.1−7.2.

Oxygen and carbon dioxide, the concentration of the former decreasing and the latter increasing with depth in the soil.

36 .

2. See Section 7.1.

Hollow, tube-like structures that allow the movement of air down to the roots. This means of getting oxygen to roots allows certain plants to grow in water-saturated soils because it relieves these plants of the dependency on oxygen in the soil for root respiration.

3. See Section 7.3 (especially Figure 7.4).

FeOOH → Fe2+; MnO2 → Mn2+; If high levels of nitrate are present, the reduction of Fe3+ and Mn3+ will be delayed until most of the nitrate-nitrogen has been reduced to N2.

4. See Section 7.3.

Organisms use the sulfur or nitrogen in sulfate and nitrate, not the oxygen, as electron acceptors under anaerobic conditions. The sulfur or nitrogen accepts electrons, lowering the valence of these elements. For example, N (V) in nitrate goes to N (III) in nitrite.

5. See Section 7.3 and Figure 7.5.

The gases might include nitrogen oxides, hydrogen sulfide and methane, in that order of appearance.

6. See Sections 7.4 and 7.7.

If the temperature is too cold for microbial activity, the oxygen dissolved in the soil water would not become depleted and so anaerobic conditions would not prevail, despite complete water saturation.

7. See Section 7.7.

Among the properties typical of wetland soils are redox depletions, redox concentrations, hard, black (Mn) nodules, and oxidized root traces. Other indicators of a wetland could include saturated soils during long period in the growing season, plants with brace roots, plants with pneumatophores (knees).

8. See Section 7.2, 7.3, 7.5, and 7.7.

Ethylene, a partially oxidized product of anaerobic metabolism, is toxic to many organisms, including many decomposer microorganisms, helping to account for the accumulation of organic matter in wetlands.

Methane is produced under very low redox potential conditions in wetlands.

Nitrous oxide, one of the first reduction products to appear as oxygen gas is depleted during period of saturation in wetlands, represents one of the pathways by which these soils remove nitrogen of incoming water.

Oxygen gas is absent for extended periods in wetland soils; it becomes depleted during the early stages of saturation in a seasonal wetland.

Hydrogen sulfide, smelling like rotten eggs, is produced by the reduction of sulfates when wetland soils become highly anaerobic and low redox potential prevails.

37 .

9. See Section 7.7.

Wetland hydrology; hydric soils; hydrophytic vegetation

10. See Section 7.8.

Seed vernalization; seed germination; growth rate; root functions of water and nutrient uptake.

11. See Section 7.8.

The heat of the fire may vaporize certain waxy components of soil organic matter, forcing them to diffuse in vapor form down into the soil where they condense on the cooler soil particles, forming a water-repellent hydrophobic layer. Next winter when heavy rains come, the soil above the hydrophobic layer becomes water-saturated and most of the rain subsequently runs downhill rather than infiltrating into the soil. The saturated soil layer may then slide downhill, in one of many types of mud slides common on burnt-over forest lands.

12. See Section 7.10.

Low soil thermal conductivity is desirable for underground houses which would allow the thick covering of soil to insulate the house against extremes of outside air temperature. Because more pores are filled with water and fewer with air, and because more mineral to mineral contact area exists in a compacted soil, compaction would increase the thermal conductivity, making the soil less effective as an insulator. Therefore, it would be desirable to use loose, un-compacted, well-structured, organic matter-rich soil as a covering for maximum insulating properties.

13. See Section 7.10.

Assuming the soil is undergoing a long term warming trend in spring or early summer, the soil at 15 cm deep would not get as hot as the air above the soil. The maximum temperature in the soil might be about 22 oC and this maximum would be reached a few hours later, say about 3 pm.

14. See Section 7.11.

Lower soil temperature resulting from the surface residue mulch under conservation tillage will delay the germination of warm-season crops, such as corn and soybeans. In cool northern regions with a short growing season, this delay can seriously reduce yields, and may therefore outweigh the conservation benefits in the minds of farmers whose livelihoods depend on yields. In warmer climates with a longer growing season, the delay is shorter and of less consequence.

38 .

Chapter 8. The Colloidal Fraction: Seat of Soil Chemical and Physical Activity Overview Colloidal particles (clays and humus) are the seat of most chemical, physical, and biological properties of soils. They are extremely small in size, being mostly less than 2 micrometers in diameter, a size below the range of the ordinary light microscope. Their small size, coupled with plate-like shapes, contribute to very high external surface areas that are complemented in some cases with even more extensive internal surface areas. These surfaces characteristically carry negative and/or positive charges that influence the attraction and repulsion of the particles toward each other and that attract (adsorb) swarms of ions of the opposite charge along with numerous water molecules. The adsorbed ions are subject to exchange with plant roots thereby providing plants with most of their essential mineral elements.

There are four major types of colloids in soils: (a) crystalline silicate clays, (b) allophane and associated non-crystalline silicate clays, (c) hydrous iron and aluminum oxide clays, and (d) humus.

The silicate clays are found around the world but are more widespread in temperate areas. Oxides of iron and aluminum are most prominent in soils of the tropics but are widespread elsewhere as the red colors associated with iron oxides would suggest. Allophane is most prominent in recent volcanic deposits. Humus is found wherever organic residues are returned to the soil and undergo decomposition and resynthesis by soil organisms.

Structure of Layer Silicate Clays Silicate clay particles are crystalline, each particle layer being comprised of two basic individual sheets. One of these sheets is dominated by a plane of silicon atoms surrounded by oxygen atoms. The oxygen atoms, in turn, act as connective links to a companion sheet dominated by an aluminum and/or magnesium plane surrounded by linkage oxygens and a few hydroxyl groups. The silica sheet is called a tetrahedral sheet because of the four-sided configuration of a given silicon atom and its associated oxygen atoms. Similarly, the aluminum/magnesium sheets are known as octahedral sheets because each aluminum/magnesium atom and associated oxygens and hydroxyls comprise an eight sided building block or octahedron.

In nature, ions having nearly the same radius as a silicon atom (e.g., aluminum) can fit in the tetrahedral sheet through a process called isomorphous substitution. If the substituting ion has a lower valence than silicon, an unsatisfied negative charge within the crystal results. This is the primary source of the negative charge on the crystal. Similar isomorphous substitution can take place in the octahedral sheet with aluminum being replaced by a similar-sized lower-valent cation (e.g., magnesium) likewise giving rise to a negative charge. These negative charges can then attract cations from the soil solution. Later, these cations may be subject to exchange with plant roots thereby becoming the primary source of plant nutrients in unfertilized soils.

Types of Clay Minerals Based on the number and arrangement of tetrahedral (silica) and octahedral (aluminamagnesia) sheets contained in the crystal units or layers, silicate clays are classified into two different groups: (a) the 1:1 type minerals (one tetrahedral to one octahedral sheet) and (b) 2:1 type minerals.

The 1:1 type crystals (e.g., kaolinite) are larger in size than the other types and have a fixed structure with no internal surfaces and little isomorphous substitution. Consequently, these minerals have relatively low surface area and low capacity to attract (absorb) cations. They do not swell when wetted 39 .

or shrink when dried. Kaolinite has many properties that make it useful for applications ranging from porcelain production to electrical insulation to medication, and is increasingly being applied as a physical barrier for pests in organic agriculture.

There are four general groups of minerals with 2:1 type crystal structures. One, the fine-grained mica group, resembles the 1:1 type in that they are somewhat larger in size than the other 2:1 types and are non-expanding, having little internal surface area. About 20% of the silicon atoms in the tetrahedral sheets have been replaced by aluminum and the very strong negative charge resulting is satisfied by potassium ions held rigidly between adjoining 2:1 layers and preventing expansion of the crystal. Cation adsorption capacity is higher than that of the 1:1 types but definitely lower than that of the other groups having 2:1 type structures.

Two clay groups with 2:1 type structures have expansive type crystals, the smectites, and vermiculites. Montmorillonite is the most prominent smectite in soils. The individual 2:1 layers are held together only loosely and exchangeable cations and water molecules are attracted between the layers resulting in enormous internal adsorptive surfaces. Consequently, these clays expand when wet and shrink when dry and have very high cation adsorption capacities. In the smectite group magnesium has substituted for some of the aluminum in the octahedral sheet. Some such substitution has also occurred of aluminum for silicon in the tetrahedral sheet giving rise to the high cation adsorption capacity of this mineral.

The vermiculite groups are less expansive than the smectites, since water and magnesium ions act as bridges holding the 2:1 type layers together. Vermiculites have very high cation adsorption capacities due to significant substitution of aluminum for silicon in the tetrahedral sheets as well as some substitution of magnesium for aluminum in the octahedral sheet.

Another 2:1 type mineral, chlorite, is non expansive since its interlayer between two 2:1 layers is occupied by a magnesium-dominated octahedral sheet that holds the adjacent layers together.

Chlorite has particle size, cation adsorption capacity, and physical properties similar to those of fine-grained micas. Layer silicate clays in which three out of three octahedral positions are occupied by metal cations are termed trioctahedral. Those with only two out of three positions occupied are dioctahedral. Much of what is known about the structures of crystalline clays has been discovered using a technique called X-ray diffraction which measures the manner in which x-rays are reflected off parallel planes of atoms.

The iron and aluminum oxides, common in highly weathered soils, have cation adsorption capacities even lower than that of kaolinite, are nonexpansive, and encourage good soil physical properties. Allophane, a noncrystalline silicate mineral, is nonexpansive but has a high capacity to adsorb both cations and anions. Humus is composed basically of carbon, hydrogen, and oxygen and its negative charges are associated with organic acid groups. The cation adsorption capacity of humus, like that of the hydrous oxide of iron and aluminum, is dependent on the pH, being much higher in alkaline than in acid soils.

The 2:1 type clays are most prominent in areas that have been subjected to only mild chemical weathering, while kaolinite and especially the iron and aluminum oxides dominate soils where chemical weathering has been intense. In the United States 2:1 type clays are most prominent in the northern and arid western states while the humid southeast is dominated by kaolinite and Fe, Al oxides.

Humus Although the nature of humus is discussed in more detail in chapter 11, here we note that humus colloids are not minerals, nor are they crystalline. They consist of a wide variety of partially decomposed or charred cell walls and biomolecules derived mainly from microorganisms and bits of tissues derived mainly from plants. Much of this material has complex chains and rings of carbon 40 .

atoms bonded to hydrogen, oxygen, and nitrogen. Humus particles are often among the smallest of soil colloids and exhibit very high capacities to adsorb water, but almost no plasticity or stickiness.

Humus has high amounts of both negative and positive charge per unit mass, but the net charge is always negative. Humus also has many hydrophobic sites that allow this colloidal constituent to absorb many organic molecules in soils.

Charge Characteristics The negative charges on 2:1 type clays are considered permanent since they are associated with the basic structure of the minerals and are not dependent on soil pH. With 1:1 type and Fe, Al oxide clays, and humus, however, the negative charges are pH dependent, being low in very acid soils and high in alkaline soils. As the pH rises, H+ ions are removed from surface OH groups on the colloids, leaving behind negatively charged O2- ions. In very acid soils kaolinite and Fe, Al oxides may have a net positive charge that results from the attachment of H+ ions to surface OH groups on the colloid. Such positive charges attract anions and are thus the source of anion adsorption capacities.

Adsorption of Cations and Anions Colloids attract and hold a complex swarm of cations and anions. Hydrated ions from the soil solution are surrounded by a shell of water molecules. These hydrated ions are attracted to sites on the colloid surface that bear a charge of the opposite sign to the charge of the hydrated ion. As they approach the surface of the colloid, they are loosely held by mutual attraction of the charge colloid site and the ion for opposite ends of the intervening water molecules. This type of sorption is termed outer sphere complexation, and is readily reversible. Exchangeable ions are examples. Other ions may bond directly with the atoms of the clay surface, forming much stronger, more difficult to reverse relationships known as inner sphere complexes. Phosphate ions strongly sorbed or fixed to the hydroxyl plane of the octahedral sheet and metal ions strongly sorbed to the oxygen plane of tetrahedral sheets are examples.

The ions attracted to and adsorbed on the surface of the soil colloids are subject to exchange with ions in the soil solution or on plant roots. Such exchangeable ions serve as valuable storage assets for plants, the ions being held from loss by leaching or runoff but remaining available for later uptake by plants. H+ ions created by root respiration will exchange for K+, Mg2+, and Ca2+ for example, that can then be taken up by plants.

The extent to which ions from the soil solution will exchange places with ions adsorbed on colloids in outer sphere complexes will depend on several a number of characteristics of cation exchange reactions, including: reversibility, charge equivalency (one mol of charge exchanges with one mol of charge), the ratio law (after equilibrium is approached, the ratio of species in the solution will equal the ratio of ion species adsorbed on the colloid), the anion effect, the selectivity for particular ion species, and the complementary ion effect.

While both cation and anion exchanges take place, because of the predominance of the negative charges on soil colloids cation exchange is by far the most important. Next to photosynthesis and respiration probably no other process in nature is as vital to plant and animal life as the exchange of ions between soil colloids and growing plant roots.

Cation Exchange Capacities The cation exchange capacity (CEC) of a soil is determined primarily by the amount and nature of the soil colloids and by the pH. Soils high in smectite and vermiculite clays have high CEC values at all pH levels, those high in fine-grained micas and chlorites being intermediate. The CEC of 41 .

kaolinite and the Fe, Al oxides is quite low. In very acid soils the CEC of humus is also quite low, but under neutral to alkaline conditions humus may have a CEC that, on a weight basis, several fold exceeds that of smectite.

The CEC is commonly determined as either the moles of charge of exchangeable cation that can be held at a high pH (usually 7.0 or 8.2) under which conditions the CEC is maximal, or it is determined as the moles of exchangeable cations that can be held at the existing pH of the soil. The latter is often termed the effective CEC or ECEC. If the soil pH is very low, the difference between the two can be great.

Physical Properties Soil colloids differ widely in their physical properties, including plasticity, swelling, shrinking, and contributions to aggregate stability. Smectites cause many engineering problems because their high potentials for shrinkage and swelling threatens to crack pavements and foundations and makes soil manipulation very difficult. The wetting and drying (or freezing) of such soils result in underground pressures that can destroy building foundations and highway roadbeds. However, the swelling tendency of smectites makes them ideally suited to seal monitoring wells and line landfills.

Adsorption of organic molecules The tendency for an organic compound, such as a pesticide or petroleum product, to become sorbed onto or into soil colloids is expressed by the distribution coefficient for the compound, Kd , which equals (mg chemical sorbed per kg soil)/(mg chemical dissolved per L solution). Since the humus fraction usually accounts for most organic molecules sorbed, another means of expression, the Koc is typically more characteristic of a particular compound, regardless of which soil it is exposed in.

Compounds with a low Kd or Koc usually are more subject to leaching and thus pose a greater potential threat to groundwater.

Biomolecules, such as DNA or microbial proteins, have been observed to sorb strongly to soil clays in such a way that they become difficult to detect, yet retain their biological activity. This behavior may have serious implications for understanding the risks involved with organic and biological contamination of soils.

Model Answers to Study Questions 1. See Section 8.1.

Soil complex components: (a) charged colloidal particles (clays and humus) surrounded by (b) swarm of cations and anions.

Nutrients are held and protected by colloids (serving as a bank) but are slowly released for plant uptake and leaching.

2. See Section 8.2 and Table 8.1.

Compared to kaolinite, smectites are smaller in size, resulting in 4–8 times larger external surface area.

Smectites also have very large internal surface area between layers; kaolinite has none.

42 .

3. See Figures 8.3, 8.4, 8.6, 8.7, and 8.10.

Clay Mineral

Tetrahedral to octahedral ratio in layers

Sheets held together by

Interlayer primary cation substitution

Interlayer exchangeable cations

Nonexpanding Kaolinite

1:1

H-bonding

None

None

Fine-grained micas

2:1

K+ ions

Al3+ for Si4+ in Tetrahedral sheet

few if any

Chlorites

2:1

Al(OH)3 or Mg(OH)2

Al3+ for Si4+ and Mg2+ for Al3+

few if any

Expanding Smectites

2:1

O to O and cation to O linkages

Mg2+ for Al3+ in Octahedral sheet

many cations

Vermiculites

2:1

Mg2+ and other cations

Al3+ for Si4+ and Mg2+ for Al3+

many cations

4. See Section 8.5.

Fine grained mica: Alteration since fine-grained micas have the same basic 2:1 type structure as muscovite, having merely lost K+ ions.

Kaolinite: Re-crystallization since muscovite (2:1 type) must decompose and decomposition products recombine to form kaolinite (1:1 type).

5. See Section 8.5 and Figure 8.13.

Kaolinite: hot humid area where intensive weathering would likely have taken place.

Smectite: dry or semi-arid area with only moderate weathering and with ample non-acid cations in parent material.

Vermiculite: area with mild weathering of micas, chlorite, or soda lime feldspars.

6. Foundation (see Box 8.1).

-most desired: kaolinite since it does not shrink and swell.

-least desired: smectites since they swell and shrink upon being wetted and dried and thereby break up foundations.

High CEC (see Figure 8.19). most desired: vermiculite-highest CEC of silicate clays. least desired: kaolinite-lowest charge and CEC of silicate clays.

Source of K (see Figures 8.7 and 8.10). most desired: fine-grained mica since K is present in between 2:1 layers. least desired: kaolinite since no K in crystal structure and low nutrient-holding capacity.

43 .

Hard clods (see Section 8.3). most desired: smectites-clods are very hard when dry. least desired: kaolinite-clods are not hard and are easily broken up by tillage.

7. See Figure 8.13. most sticky and plastic: soil in semi-arid area formed on materials high in non-acid cations which would most likely have smectite clay that is sticky and plastic when wet. least sticky and plastic: soil in well weathered acid soil that would likely be dominated by kaolinite and Fe, Al oxides neither of which are highly sticky and plastic when wet.

8. See Box 8.4 and Table 8.1.

Assume CECs of colloids expressed in cmolc/kg to be: Humus = 200, Montmorillonite (mt) = 100, Vermiculite (vm) = 140, and Fe Al oxides (Fe, Al) = 2 The cmolc associated with each of these colloids in 1 kg of the soil in question is:

Humus (4%):

.04 kg O.M. × 200 cmolc kg soil kg O.M.

Mt (10%):

0.1 kg M kg soil

Vm (10%):

0.1 kg Vm × 140 cmolc kg soil kg Vm

= 14 cmolc/kg soil

Fe, Al (10%):

0.1 kg Fe, Al × 2 cmolc kg soil kg Fe, Al

= 0.2 cmolc/kg soil

= 8 cmolc/kg soil

× 100 cmolc = 10 cmolc/kg soil kg Mt

Total

= 32.2 cmolc/kg soil

9. See Box 8.3. Since 1cmolc of any cation will replace 1cmolc of any other cation, it will take 10 cmolc of Al3+ to replace the 10 cmolc of Ca2+. The gram molecular weight of Aluminum (27g), divided by the charge of the Al3+ ion (3) gives the mass of 1 mol charge (molc) = 27g/3 = 9 g. One centimol charge (cmolc) is 1/100 of this, or 9g/100 = 0.09g. Since we need 10 cmolc of Al to replace the 10 cmolc of Ca, the total mass of Al needed is 0.09g × 10 = 0.9g.

10. See Box 8.3. 9 cmolc Ca2+ + 3 cmolc Mg2+ + 1 cmolc K+ + 3 cmolc Al3+ = CEC of 16 cmolc (assuming other cations such as Na+ and NH4+are negligible in comparison to those listed).

11. See Section 8.9, Box 8.4. Convert the mass of each element into the centimoles of charge it represents by (1) dividing by the atomic weight then (2) multiplying by the valence, and finally (3) converting from mmol to cmol by dividing by 10:90 mg Ca2+ = 90 mg/(40 mg/mmol) * 44 .

(2 charges/mol)/10 mmol/cmol = 0.45 cmolc; 35 mg Mg2+ = 35 mg/(24 mg/mmol) * (2 molc/mol)/10 mmol/cmol = 0.29 cmolc , K+ = 28 mg/(39 mg/mmol) * (1 charges/mol)/ 10 mmol/cmol = 0.07 cmolc, Al3+ = 60 mg/(27 mg/mmol) * (3 molc/mol)/10 mmol/cmol = 0.67 cmolc. The CEC = 0.45 + 0.29 + 0.07 + 0.67 = 1.48 cmolc/100 g = 14.8 cmolc /kg. The Al saturation = (0.67/1.48) × 100 = 68%.

12. See Sections 8.8–8.9.

The Ba2+ in the final leachate is the Ba2+ that was adsorbed by the CEC. The large amount of Mg in the leachate represents the Mg from the MgCl2 used to replace the Ba2+ (minus the relatively small amount that was sorbed by the CEC in the process. Therefore the cmolc of CEC = the cmolc of Ba2+ in the final leachate. Thus the CEC of the 100 g soil sample = (258 mg /137 mg/mmol Ba) * (2 mmolc/mmol Ba2+)/(10 mmol/cmol) = 0.377 cmolc/100g = 3.77 cmolc /kg.

13. See Section 8.12.

Kd and Koc indicated the degree of attraction of the soil (Kd) and organic carbon (Koc) for organic compounds, commonly organic pollutants. High Kd and Koc values suggest high sorption rates and low soil solution levels which, in turn, suggest low movement into drainage waters. Koc is more consistently characteristic of organic compounds than Kd since the organic compounds are commonly complexed with soil organic matter that in turn is correlated with the organic carbon (oc) content of the soil.

14. See Sections 8.8–8.10.

The memo could take many forms, but it should explain that strontium behaves chemically very much like calcium, and is likely to participate in cation exchange reactions with calcium and other cations already in the soil. The degree to which the strontium is available for uptake by roots or downward movement by leaching will be inversely related to how much of it is held in exchangeable form on soil colloids. The memo should include consideration of how degree of strontium adsorption by the cation exchange complex would be affected by such soil properties as: (1) the CEC of the soil, (2) the amount of strontium added to the soil relative to the amounts of other cations already on the exchange complex (ratio law), (3) the nature of the other adsorbed cations (complementary ion effect), and any specific selectivity for strontium or for the complementary ions by the particular colloids present (for example mica clays would tend to hold K in preference to Sr, while the reverse might be true for smectite, which preferentially sorbs Ca).

15. See Section 8.12.

If properly targeted and applied, antibiotic drugs and natural insecticides (such as Bt toxins) can perform valuable functions by fighting pathogens and killing insect pests. However, when adsorbed to soil colloids, these compounds may be protected from microbial degradation while maintaining their antibiotic or insecticidal properties and may therefore select for resistant bacteria, upset the microbial and faunal ecology of the soil system.

45 .

Chapter 9. Soil Acidity, Alkalinity, Salinity, and Sodicity Overview The degree of acidity or alkalinity (i.e., the soil reaction) is considered a master variable that affects nearly all soil properties, chemical, physical, and biological. Expressed as soil pH, it determines which plants dominate the natural landscapes and which crops can be grown successfully in farmers’ fields. It helps determine whether elements are present at optimum, deficient, or toxic levels for plant growth and affects aggregate stability as well as air and water movement. It determines the fate of many soil pollutants, stimulating their destruction or movement from the soil into the groundwater. Nature stimulates acidity in humid environments and alkalinity in arid environments.

Human intervention can accelerate or slow down these natural changes or even reverse them when it is desirable to do so.

The Process of Soil Acidification Processes that add H+ ions to the soil system cause acidification and lower soil pH. Processes that consume H+ reduce acidity and raise soil pH to neutral, or even higher. Hydrogen ion producing processes important in soils include: the formation of carbonic acid from carbon dioxide; dissociation of H+ ions from organic acids; oxidation of N, S, and Fe compounds; deposition of strong acids (sulfuric and nitric) from the atmosphere; cation uptake by plants; accumulation of acidic organic matter; precipitation of certain cations (as when Al3+ hydrolyzes water to form Al(OH)3); and deprotonation of pH-dependent charge on clays and humus. Alkalinizing processes that consume H+ ions are generally the opposite in nature to the acidifying reactions. For example, inputs of carbonate or bicarbonate, anion protonation, reduction of N, S and Fe compounds, and anion uptake by plants all consume H+ ions.

Acidification is a natural process in soil formation that is accentuated in humid regions where processes that produce H+ ions outpace those that consume them. Natural acidification is largely driven by the production of organic acids (including carbonic acid) and the leaching away of the non-acid cations (Ca2+, Mg2+, K+ and Na+) that the H+ ions from the acids displace from the exchange complex.

Emissions from power plants and vehicles, as well as inputs of nitrogen into agricultural systems are the principal means by which human activities have accelerated acidification during recent decades.

Role of Aluminum Aluminum is the other principal acid cation besides hydrogen. Its hydrolysis reactions produce H+ ions and its toxicity comprise one of the main detrimental effects of soil acidity. In strongly acid soils the colloidal complex is dominated by Al3+ ions and associated hydroxy-aluminum ions (e.g., Al(OH)2+) complemented by a low level of adsorbed H+ ions. Much of the Al3+ and H+ is bound tightly in the organic matter and on clay crystal edges, and is not readily exchangeable.

In moderately acid soils, aluminum and hydrogen ions continue to dominate, although the aluminum is present mostly as hydroxy-aluminum ions such as Al (OH)2+ and Al (OH)2+. Most of these complex ions are adsorbed on the soil colloids but in some 2:1 type clays (e.g., vermiculite) some of them may move into interlayer spaces in the clay structure, prevent interlayer expansion, and block some of the exchange sites.

In neutral to alkaline soils, the permanently charged sites on the clay minerals are occupied by exchangeable Ca2+ and other non-acid cations rather than by H+ and Al3+. The pH-dependent charges on humus, kaolinite and Fe, Al oxides become available at very high pH levels. If sodium ions become prominent, plant growth is generally curtailed.

46 .

The proportion of the CEC (as determined at the pH of the soil, or in a system buffered at a high pH such as 7.0 or 8.2) that is satisfied by Al3+ and H+ ions is known as the acid-saturation percentage. The proportion of non-acid cations, such as Ca2+ and Mg2+, is known as the percentage non-acid saturation (sometimes referred to as base saturation, although these cations are not true bases). In very acid soils the percentage non-acid saturation is low, since Al3+ and H+ dominate the exchange complex. As the pH increases, the non-acid saturation is increased approaching 100% in most soils at pH 5.5 to 6.5.

Pools of Acidity Three pools of acidity are recognized: (a) active acidity, (b) salt-replaceable acidity, and (c) residual acidity. The active acidity is a measure of the H ion activity in the soil solution. While it identifies the environment in the immediate vicinity of the plant roots, the quantity of H+ ions in this form is very small, being equivalent to no more than about 2 kg CaCO3 per hectare furrow slice.

Salt-replaceable acidity gives an indication of the exchangeable Al3+ and H+ present on the exchange complex and is equivalent to 200–2,000 kg per hectare furrow slice. The remaining acidity (residual) is mostly associated with aluminum and hydrogen ions that are tightly bound by the organic matter and silicate clays. The amount of residual acidity is commonly much higher than even the saltreplaceable form and may be as high as 5 to 20 Mg per hectare furrow slice.

Cation Saturation A typical mineral soil with mixed mineralogy and a moderate organic matter level exhibits a moderate decrease in effective cation exchange capacity as pH is lowered, suggesting that pH dependent charges and permanent charges each account for about half of the maximum CEC. As pH drops from 7.0 to about 5.5, the effective CEC is reduced because H+ ions and Al(OH)xy- ions (which may include AlOH2+, Al(OH)2+, etc.) are tightly bound to some of the pH-dependent charge sites. At pH values above 5.5, the concentrations of exchangeable aluminum or H+ ions are very low, and the effective CEC is essentially 100% saturated with exchangeable non-acid cations (Ca2+, Mg2+, K+ and Na+, the so-called base cations). As pH is further reduced from 5.5 to 4.0, aluminum ions (especially Al3+), along with some H+ ions, occupy an increasing portion of the remaining exchange sites.

Exchangeable H+ ions occupy a major portion of the exchange complex only at pH levels below 4.0.

The CEC of an organic soil is dominated by pH-dependent (variable) charges, with only a small amount of permanent charge. Therefore, as pH is lowered in an organic soil, the effective CEC declines more dramatically than in most mineral soils. At low pH levels, exchangeable H+ ions are more prominent and Al3+ less prominent in organic soils than in mineral soils.

Soil Buffering The total acidity in soils is the sum of the active, exchangeable (salt replaceable) and residual pools of soil acidity. These three forms of soil acidity suggest that a mechanism exists to resist rapid and uncontrolled drastic changes in the active pool of soil acidity. This resistance is called buffering.

Changes in the soil solution pH (the active acidity) are buffered by the presence of the other two pools.

Soil colloids and their reaction with H and Al compounds in the acid range and carbonates in alkaline ranges help account for soil buffering. In certain anaerobic soils and sediments, the presence of reduced sulfur provides the potential for the production of an enormous fourth pool of soil acidity if the material is exposed to air by drainage or excavation. The higher the amount of pH-dependant charge from humus and variable charge clays and the higher the amount tightly bound aluminum that can be released, the higher will be the buffering capacity of the soil, other factors being equal.

Consequently, soils high in organic matter and 2:1 type clays have higher buffering capacities than those dominated by sand or kaolinite and Fe, Al oxides and require more limestone (or sulfur) to significantly change the soil pH. 47 .

Factors Controlling Soil Reaction Climatic variables and the decomposition of organic residue provide natural controls on soil pH. In low rainfall areas the non-acid cations released as weathering proceeds are subject to little leaching but rather remain associated with the exchange complex in soils with high non-acid saturations and pH values greater than 7. Under humid conditions ample precipitation is available to remove most of the non-acid cations and they are replaced by Al3+ and H+ ions. This results in distinctly acid soils characterized by high percentages of acid saturation.

Human intervention has accentuated both the acid and alkaline tendencies in soils. Acidity is increased by the use of nitrogen-containing fertilizers that are oxidized in the soil leaving HNO3 as an end product. Application of organic materials such as animal manures, composts, sewage sludge, and some forms of plant litter can also decrease soil pH with oxidation of ammonium nitrogen. Some composts and plant litters contain enough non-acid cations to have no net effect on pH. Acid rain, stemming from the emission of nitrogen and sulfur-containing gases into the atmosphere, carries significant quantities of HNO3 and H2SO4 into the soil. The magnitude of the effects of acid rain on forest soils and aquatic environments are of great concern. The acidity of some coastal wetland soils increases dramatically if they are drained, since the sulfides and elemental sulfur they contain can be oxidized to release large quantities of sulfuric acid. Alkalinity is increased by the application of salt-bearing irrigation waters to arid region soils. Sodium salts are especially troublesome since sodium-saturated soils have extremely high pH values and a very poor physical condition.

pH, Nutrient Availability, and Plant Response The availability of most essential plant nutrients is affected by soil pH. In acid soils all nutrient elements except iron, manganese, and zinc are deficient, the latter three elements sometimes being present in toxic quantities. In contrast, these three elements along with copper and boron have low availability in alkaline soils. In general, intermediate pH levels of 5.5 to 7 provide the most overall satisfactory range for plant nutrient availability. Fungi seem to be relatively unaffected by soil pH, but bacteria and actinomycetes are generally less active in quite acid soils.

There is wide variation in the tolerance of plants to acidity and alkalinity. Some legumes such as alfalfa and sweet clover and some trees such as Beech, Maple, and Poplar grow best at near neutrality. At the other extreme, azaleas, rhododendrons, and cranberries must have strongly acid conditions for best growth. Black locusts, loblolly pine, and aspen trees also tolerate strongly acid conditions.

Aluminum toxicity is one of the most common problems associated with acid soils. Restricted to the roots, aluminum causes stunted root growth and symptoms of drought stress and phosphorus deficiency. Some species have greater Al-tolerance, and plant breeders are working on incorporating these adaptive traits in new varieties.

Soil pH is determined in the laboratory using an electrometric pH meter. In the field solutions of pH-sensitive dyes are mixed with the soil and the resulting colors are matched with calibrated color charts to give reasonably accurate measurements of soil pH.

Acidifying Agents To satisfy the acidity requirements of some plants such as azaleas and rhododendrons, and to reduce the extreme alkalinity in some arid region soils, it is necessary to reduce the pH of the soil. This can be done by applying generous quantities of organic matter such as pine needles, acid peat moss, and some sawdusts that are low in non-acid cations. Elemental sulfur, which upon oxidation in the soil 48 .

produces sulfuric acid, can be used to reduce the soil pH. Ferrous sulfate also reduces soil pH and simultaneously provides available iron that is often required to meet plant needs.

Liming Considerations Soil pH is largely controlled by the humus and clay fractions and their associated exchangeable cations. The maintenance of satisfactory soil fertility levels in humid regions depends considerably on the judicious use of lime to balance the losses of calcium and magnesium from the soil. Liming not only maintains the levels of exchangeable calcium and magnesium but in so doing also provides a chemical and physical environment that encourages the growth of most common plants. Lime is truly a foundation for much of modern humid-region agriculture. Gypsum and organic matter (either applied or grown) represent other tools that can be used to ameliorate soil acidity instead of, or in addition to, liming. On the other hand, it is sometimes most judicious to use acid-tolerant plants, rather than attempt to change the chemistry of the soil.

Three types of calcium and magnesium-containing liming materials are used to decrease soil acidity in humid regions: (a) carbonates, (b) oxides, and (c) hydroxides. Ground calcite (CaCO3) and dolomite (CaCO3 - MgCO3) are the primary carbonate sources that are widely used around the world.

They are inexpensive and if finely ground react well with the soil. The oxides and hydroxides are specialty materials used to provide prompt reaction with the soil and to raise the pH to high levels.

They are more expensive, however, and are somewhat caustic to handle.

The amount of limestone required to increase the pH to the desired level depends on the buffer capacity of the soil. Because of their high cation exchange capacities, clay soils, especially if they are high in organic matter, require more limestone than comparable coarse textured soils. Rates of application of up to 7–9 Mg/ha may be needed. Repeated applications of limestone are required since leaching losses and crop removal deplete the soils of calcium and magnesium and lower the pH. Limestone applications to the surface soil have little effect on the subsoil horizons since the carbonates do not move downward. To counteract aluminum toxicity and calcium deficiency in the subsoil, applications of gypsum (CaSO4•2H2O) may be needed. The gypsum slowly dissolves and is leached into the subsoil where it stimulates the penetration of plant roots and the uptake of calcium.

Causes of Alkalinity Alkalinity and alkaline soils simply refers to pH levels above neutral (>7.0) and is not to be confused with alkali conditions which involve high levels of exchangeable sodium. The two conditions do often occur together in the same soil. Alkalinity is basically caused by hydroxyl ions generated when carbonates and bicarbonates react with water. Since sodium carbonates are much more soluble than calcium carbonates, sodium is associated with greater hydroxyl production and higher pH levels (often above 8.5). Biological activity, which increases the carbon dioxide concentration in soil solutions, tends to drive the carbonate reaction to the left (towards precipitation of calcium carbonate) and therefore limits the rise in pH. High levels of soluble sodium or calcium salts also limits the dissolution of the carbonates by the common ion effect and thus also limits the rise on pH.

Salt Accumulation About 1/3 of the soils in arid and semi-arid regions of the United States suffer from some degree of salt accumulation. Salts tend to accumulate naturally in some soils of arid and semiarid regions because there is insufficient rainfall to flush them from the upper soil layers, and from upward capillary water movement that incidentally brings salts back up to or near the soil surface. Some saline seeps stem from the horizontal transport to low lying areas of fossil deposits of salts laid down in lake bottoms during geological time. Other saline areas are man-made and result from poorly designed irrigation 49 .

systems that bring large quantities of salts from upland areas into irrigation schemes and provide inadequate means of washing the salts out of the soil profile. Tragic experiences going back through the centuries illustrate how productive lowland soils can be rendered effectively useless by irrigation systems that do not allow for the effective removal of salts from the cultivated soils.

Characterizing Salt-affected Soils Salt-affected soils can be characterized by three measurements: (a) soil salinity level, (b) exchangeable sodium percentage, and (c) sodium adsorption ratio. Soil salinity is estimated by measuring the electrical conductivity (EC) of a saturated paste of soil (laboratory) or the bulk soil conductivity using electron probes in the field. Electromagnetic induction of electrical current is also used to estimate salinity in moist soils. This data can be collected with on-the-go instrumentation, and used in precision agriculture applications. The sodium status of highly alkaline soils is ascertained by determining the exchangeable sodium level as well as the cation exchange capacity. The percent of the CEC satisfied by Na+ ions gives the exchangeable sodium percentage (ESP). A closely related characteristic of saline/sodic soils, the sodium absorption ratio (SAR), is the ratio in the soil solution of the sodium concentration to the square root of one half the sum of the concentrations of Ca2+ and Mg2+, all concentrations being expressed in millimoles of charge per liter.

Dispersion of Soils Dispersion of soil colloids leads to formation impermeable surface crusts and soils highly impermeable to water and air. Sodic soils are an example. The dispersed condition is brought about by two basic mechanisms, the dominance of monovalent, large hydrated ions (mainly sodium) and low overall dissolved salt concentrations. High concentration of salt – even of sodium salts—will cause flocculation, while low salt concentration causes dispersion.

Classes of Salt-affected Soils Salt accumulation gives rise to three classes of salt-affected soils: (a) saline, (b) saline sodic, and (c) sodic. Saline soils have electrical conductivities of more than 4 deciSiemens per meter (ds/m), pH values of 8.5 or less and ESP values of about 15 or less. These soils are characterized by a white accumulation of neutral salts giving rise to the traditional name white alkali. Saline-sodic soils also have high levels of neutral salt accumulation, the EC being more than 4 ds/m and the pH 8.5 or less. But the ESP is greater than 15 and the SAR is at least 13. Crop growth is constrained by both salinity and sodium levels, although the physical condition of these soils is about the same as that of saline soils. The chemical conditions cause dispersal, which is the opposite of the desired flocculation in normal soils.

Sodic soils have the sodium levels of saline-sodic soils without the moderating influences of the neutral salts. As a result the EC is less than 4 ds/m and the pH greater than 8.5. A high percentage of sodium saturation causes the individual colloidal particles to repel each other and to disperse, leaving a very poor physical condition and a discolored appearance due to the dispersion of the humus.

Hence, the name black alkali is used traditionally to describe these soils. Plant growth on these soils is highly constrained or even restricted, but this may be localized with nearby soils supporting much more production.

Plant Tolerance of Salinity Salt damage is greatest in the early stages of growth, and salinity may hinder or prevent seed germination. There are considerable differences in the tolerance of plant species to salinity. Apple, peach, and almond trees along with beans, carrots, and potatoes are very sensitive to salinity. At the 50 .

other extreme are crops such as cotton, Bermuda grass, sugar beets, and Bougainvillea that are reasonably tolerant at high salt levels. Urban dwellers see the effects on trees and grass of excess salts used to deice their roads and walkways during winter months. Likewise, the buildup of salts in pots used to culture perennial plants can be a problem.

Irrigation projects that use waters high in salts, especially those dominated by sodium, and that have less than adequate drainage systems encourage the development of first saline-sodic soils and finally sodic soils. Through the ages highly productive valley soils have been degraded and finally abandoned because of these unsound water management schemes. Monitoring the quality of incoming water to test for SAR and for bicarbonates and being certain that effective drainage schemes are available to wash offending salts from the soil are two practices that must be followed. Genetic breeding to incorporate salt tolerance is underway.

Reclamation of Salt-affected Soils Successful attempts have been made to reclaim salt-affected soils whose productivity has declined. The reclamation of saline soils requires effective drainage and high-quality irrigation water to leach away excess soluble salts without bringing excess sodium into the system. Care must be used in disposing of the salt-containing drainage waters without overburdening the downstream users.

The reclamation of saline-sodic and sodic soils is more complicated. While soluble salt levels must be reduced in the saline-sodic soils, care must be taken to simultaneously reduce the levels of exchangeable sodium. To do so, sources of Ca2+ or H+ ion are used to replace the Na+ ions. Gypsum (CaSO4 2H2O) is the most popular choice to provide calcium since it is widely available, is inexpensive and acts reasonably rapidly. Several tons per hectare are mixed well with the soil, and after a few months the soil is thoroughly leached to remove the offending Na+ ions.

Sulfur is a second chemical that has been used to remove the sodium. Upon oxidation in the soil, sulfuric acid is formed, the Na+ ions are replaced with H+ ions, and the pH is lowered.

Unfortunately, the sulfur is more expensive than gypsum, is more toxic to handle, and acts somewhat more slowly than gypsum.

The reclamation effects of gypsum or sulfur are greatly accelerated by plants growing on the soil especially if they are somewhat tolerant of the salinity and if they have deep root systems to help provide channels to the lower horizons. Sugar beets and cotton are examples of such crops. Once saltaffected soils have been reclaimed, steps must be taken to prevent their reverting to the high salt status.

Surveillance of the soils’ electrical conductivity and of the SAR of both the soil and incoming irrigation water is essential as is the maintenance of both organic matter and chemical nutrient levels.

Model Answers to Study Questions 1. See Box 9.1.

Since the ion product of H+ and OH− ion concentrations in water solutions is a constant (10−14), the pH gives an indication of both the H+ ion and OH- ion concentrations.

2. See Section 9.2 and equation 9.10.

When H+ ions are adsorbed on a clay surface, they usually attack the structure of the minerals, releasing Al3+ ions, which react with water (hydrolyze) to form H+ ions. A single Al3+ ion can thus release up to three H+ ions as the following reversible reaction series proceeds to the right in stepwise fashion:

51 .

i. Al3+

+ H2O

Al(OH)2+ + H+

↔

ii. Al(OH)2+ + H2O

↔

Al(OH)2+ + H+

iii. Al(OH)2+ + H2O

↔

Al(OH)3 + H+

Most of the hydroxy aluminum ions (Al(OH)xy+) formed are tightly adsorbed on acid soil surfaces (internal and external) and block cation exchange sites, thereby reducing the effective CEC of the soil. As the pH is raised and more of the hydroxyl aluminum ions precipitate as uncharged Al(OH)3o, the negative sites on the colloids become available for cation exchange. This is one reason for the increase in soil CEC as the pH is raised from pH 4.5 to pH 7.0 (above which virtually all the aluminum cations have precipitated as Al(OH)3o).

3. See Sections 9.4 and 9.8.

The volume of soil is 0.16m × 10,000m2/ha = 1,600 m3. If we assume that the soil water content is about 25% by volume, then the upper 16 cm of soil contain about 400 m3 of solution. This is 400 m3 × 1,000 L/m3 = 4 × 105 L of solution. At pH = 5, the H+ ion concentration of this solution is 10−5 mol/L, giving a total of 4 × 105 L × 10−5 mol/L = 4 mol of H+. It would take 4 molc = 2 mol CaCO3 or 0.2 kg to react with and neutralize this amount of H+. In contrast, it is likely to require something like 2 Mg (10,000 fold more) of CaCO3 to raise the soil pH to 7.0. This is because most of the H+ ions in acid soils are not in the soil solution (active acidity), but are in exchangeable (salt replaceable) forms or are strongly bound by hydroxy aluminum ions, organic matter, and silicate clays (residual acidity). Measurements of pH indicate soil solution H+ ion levels, but to change the pH limestone additions need to neutralize the much more plentiful exchangeable and residual or bound forms of H+ ions.

4. See Section 9.4, especially Figure 9.7.

Buffering is the resistance to changes in soil pH. Buffering prevents drastic fluctuations in soil pH that could harm plants and microbes. Buffering affects the amount of amendments such as limestone or sulfur needed to change the soil pH. Buffering of the soil solution pH takes place because H+ ions in the salt replaceable (exchangeable) and bound (residual) forms tend to replenish most of the H+ ions that are removed from the soil solution. At low pH values, aluminum compounds react with added H+ ions to strongly resist pH changes. At intermediate pH levels cation exchange and the dissociation of H+ from pH-dependent charge sites on clay and humus provide a moderate degree of buffering. At higher pH values, the dissolution and precipitation of carbonates strongly resist pH changes.

5. See Section 9.6.

Acid rain is precipitation that is acidified in the atmosphere by HNO3 and H2SO4 formed by the reaction with water vapor of nitrogen and sulfur oxides released mainly by the burning of fossil fuels. Acid rain is more harmful to forests than to agricultural areas since ameliorating liming practices used routinely to neutralize the acidity in agriculture cannot be used economically in forested areas.

6. See Box 9.2.

The volume of rainwater is 0.5 m (= 500 mm) × 10,000 m2/ha = 5 × 103 m3. At 0.0001 mol H+/L (= pH 4.0) there would be 5 × 103 m3 × 103 L/m3 × 1 × 10−4 = 500 mol H+. It would take 250 mol or 25 kg CaCO3 to react and neutralize this amount of H+.

52 .

7. See Section 9.7, especially Figures 9.15, 9.17, and 9.18.

At low pH levels, Fe, Al, and Mn are often present at toxic levels while deficiencies of Mo, Cu, and B may occur. At high pH levels, Mo and Se may exist at toxic levels while Fe, Mn, Cu, B, and Zn are often deficient. Some native species such as azaleas, rhododendrons, and cranberries require acid soil conditions while other nature species such as eucalyptus, arborvitae, currants, and lilacs require neutral to alkaline conditions.

8. See Section 9.8.

First determine g of CaCO3 needed to provide one mole of charge (1 molc) i. gmol wt CaCO3

=

100 g

ii No. of molc in CaCO3

2 molc

=

50 g CaCO3/molc

To express this in terms of centimole (cmolc) we merely divide by 100: iii 50/100 = 0.5g/cmolc Since our soil has a CEC of 8 and is 60% Al-saturated, the amount of Al3+ to be eliminated by exchange reactions is: 8 × 0.6 = 4.8 cmolc of Al/kg of soil.

For 4.8 cmolc/kg soil we need 0.5g/cmolc × 4.8 cmolc/kg = 2.4 g CaCO3/kg or 0.0024 kg CaCO3/kg soil.

To convert this to amount of pure CaCO3 needed per hectare we simply multiply by 2 × 106 kg, the mass of one hectare of soil about 15 cm deep.

0.0024 kg CaCO3/kg × 2 × 106 kg/ha = 4.8 × 103 kg/ha or 4.8 Mg/ha Since the limestone in question has a CaCO3 equivalent of 90, one Mg of the limestone will provide 0.9 Mg of pure CaCO3. Consequently, the application rate must be adjusted: 4.8 Mg CaCO3 ha

×

Mg limestone = 0.9 Mg CaCO3

5.33 Mg limestone ha

9. Adjust for more shallow depth (you are attempting to alter less soil that assumed by the lab): 12 cm actual depth/18 cm assumed × 2 Mg/ha recommended = 1.33 Mg/ha CaCO3 equivalent needed.

Adjust for purity of limestone: 100% CaCO3 equiv. recommended/85% CaCO3 equiv. actual × 1.33 Mg/ha CaCO3 equiv. = 1.56 Mg/ha limestone needed.

Adjust for land area of job: 1.56 Mg/ha limestone × 2.5ha = 3.9 Mg limestone needed for job.

10. See Section 9.6, especially Figure 9.14. Also Figure 7.3.

The black “rich-appearing” soil was from a coastal wetland and was probably a potential acid sulfate soil material, being black as much from the color of iron sulfide as from humus. Freshly sampled from the marsh, the iron sulfides were still in a reduced state when the lab first measured a pH of around 6. After exposure to air for several months, oxidation of the iron sulfide took place, forming ferric iron compounds and sulfuric acid, greatly acidifying the soil to such an extremely 53 .

low pH that nearly all plant species would be killed. The best solution would have been to avoid this problem by testing the soil for reduced sulfur. Once installed, the soil will be very difficult to neutralize as acid will continue to form for years. Many tons of limestone would have to be worked in to the entire depth of the soil material. Alternatively, the problem soil could either be buried under a thick layer (about 1 m) of other soil to isolate the sulfides from air, or the material could be removed entirely and new soil installed. Each “after the fact” solution would be disruptive and costly.

11. See Section 9.10.

Azaleas are acid-loving, requiring strongly acid conditions (pH 5 or less) and the added limestone adversely affects the roots of azaleas. To counteract the effects of the limestone, FeSO4 or elemental S could be added, the FeSO4 to supply needed iron and the S (upon oxidation by microbes) to supply H2SO4 which will re-acidify the soil around the azaleas.

12. See Section 9.9.

Gypsum is sufficiently soluble to move downward in an acid aluminum-toxic soil thereby providing a Ca/Al ratio that permits the growth of plant roots in the lower soil layers. The sulfate may also exchange with hydroxyls that can then precipitate the Al ions from solution. Limestone is not soluble enough to permit such downward movement.

13. See Section 9.1, especially equations 9.6–9.9.

Three major sources of alkalinity: i.

Hydrolysis of carbonates and bicarbonates by reactions such as:

ii. Non-acid cations weathered from minerals replace Al3+, Al hydroxy ions, and H+ ions from the exchange complex. iii. Hydrolysis of adsorbed sodium to form NaOH. 14. See Section 9.7.

Availability in Nutrient

Acid soils

Alkaline soils

Iron

high

low

Nitrogen

medium

high

Molybdenum

low

high

Phosphorus

low

low

15. See Section 9.7.

Although the total Fe levels may be high in the soil, the Fe is mostly in insoluble forms and consequently is unavailable for absorption by the peach trees.

16. See Sections 9.1 and 9.12.

The difference is due primarily to the higher solubility of Na2CO3 and to its greater tendency to hydrolyze 54 .

2Na+ + 2OH− + H2CO3

Na2CO3 + 2H2O ↔

CaCO3, being much less soluble hydrolyzes only slightly giving only slightly alkaline solutions.

17. See Section 9.11, 9.13, and 9.16.

The soil properties developed are those characteristic of sodicity. The irrigation water used for this field likely has a high sodium adsorption ratio (SAR) indicating high levels of sodium compared to calcium and magnesium. This stimulates a similarly high SAR in the soil solution, a higher pH, a higher exchangeable Na level, and a consequent breakdown of aggregate stability once the colloids become highly sodium-saturated. The high pH and Na levels, along with poor soil structure, result in reduced plant growth.

18. See Section 9.17 and 9.18.

Note that the question should refer to the soil in question 17 (not 5). Additions of gypsum, elemental sulfur or H2SO4 can replace the exchangeable Na+ with Ca2+ or H+ ions, and irrigation water can be used to leach the Na+ ions from the soil.

Growth of Na- tolerant plants such as barley, rye and berseem clover can be grown to open up root channels. Some synthetic polymers can be used to help hold aggregates together and increase hydraulic conductivity.

19. See Box 8.3 and Section 9.12, especially Equation 9.27.

CEC is the sum of the cation concentrations. Since it is stated that soil is alkaline, there would be no exchangeable Al or H. ESP is calculated as the Exchangeable sodium/CEC × 100.

Exch. cation concentration, cmolc kg−1

%

Na

K

Ca

Mg

CEC

ESP

A

1

1.2

3

3

8.2

12.2

B

1

1.2

17.4

6

25.6

3.9

C

4.3

1.2

3

3

10.5

37.4

20. See Section 9.12, especially Equation 9.28. The calculations of SAR in question 8 first involve changing the units from the mg/L listed in the data table to the cmolc kg-1 used in the formulae.

Only then can the equations be applied. For example, the Na value is first divided by 23 mg/cmolc , the Ca value is divided by 20 mg/cmolc , the K value is divided by 39 mg/cmolc and the Mg value is divided by 12 mg/cmolc: SAR = Na/(0.5Ca + 0.5Mg)1/2 = Na/23/SQRT(SUM(Ca/20+Mg/12)/2) where Na, Ca, and Mg units are mg/L.

55 .

Concentrations of cations, (mg L–1) Soil

Na

K

Ca

Mg

SAR

A

230

46

40

24

7.1

B

230

46

230

48

3.6

C

990

46

40

24

30.4

21. See Section 9.11 and 9.15.

Irrigation waste water is apt to have high levels of soluble salts as well as potentially toxic levels of trace elements such as selenium, molybdenum and boron. Birds, fish and other wildlife consuming wetland plants that absorb excessive levels of these elements are poisoned or otherwise adversely affected.

22. See Section 9.17, especially Box 9.5.

Leaching Requirement = Eciw incoming irrigation water Ecdw acceptable drainage water

=

1.2 dS/m = 0.2 6ds/m

Assuming the volume percentage of water in the unirrigated and in the fully wet (irrigated) soil to be 15 and 45% respectively, the difference (30%) must be added to fully wet the soil to a depth of 45 cm. Thus 45 cm × .30 = 13.5 cm irrigation water is needed to wet the soil.

Calculate extra water needed to leach the soil by multiplying the centimeters of water needed to wet the soil by the leaching requirement ratio: 13.5 cm × 0.2 = 2.7 cm.

Thus, the total irrigation water needed is:

13.5 cm + 2.7 cm = 16.2 cm.

23. See Sections 9.9 and 9.18.

Gypsum (CaSO4•7H2O) is a very versatile soil amendment. In aluminum toxic acid soils, gypsum applied to the surface is soluble enough to allow the calcium and sulfate to move down to the toxic subsoil and replace the Al3+ from the CEC and OH- from the clay surfaces and precipitate the Al3+ with the hydroxyls, thus detoxifying the aluminum. In sodic soils, usually having a very high pH, the calcium from the gypsum replace the sodium on the CEC lowering the ESP. The sulfate accompanies the replaced sodium allowing to be easily leached out of the root zone.

24. Section 9.18.

Gypsum provides Ca2+ ions to replace Na+ ions on soil colloids, is easily and safely applied, and is generally less expensive than other ameliorating substances. In the following reactions the Na2SO4 is soluble and can easily be leached from the soil.

2NaHCO3 + CaSO4 ↔

Na2SO4 + CO2 + H2O

Na2CO3 + CaSO4 ↔

Na2SO4 + CaCO3

56 .

Chapter 10. Organisms and Ecology of the Soil Overview The Soil Ecosystem Soils are complex ecosystems. Even a handful of fertile soil contains thousands of species of organisms from most of the major groups of living things. Soil contains macro- (>2 mm), meso- (0.1 to 2 mm), and micro- (<0.1 mm) fauna, as well as microflora (algae, bacteria, fungi, and actinomycetes). The latter are the most diverse and numerous and generally provide the most biomass and metabolic activity, but their activity is enhanced in many ways by the fauna.

Like other ecosystems, soil ecosystems are characterized by food webs that include many species at each of the main trophic levels. Primary producers are mainly higher plants (partially via their roots) and algae. Primary consumers include herbivores and detritivores. Secondary and tertiary consumers may be predatory or mycophagous animals or microorganisms. The ultimate decomposers belong to various groups of microflora. Heterotrophs rely on organic compounds for carbon and energy needs, while autotrophs receive their carbon mainly from carbon dioxide, and energy from photosynthesis and from oxidation of various elements. The diversity of soil organisms is an important factor of soil quality and ecosystem function. As in any ecosystem, the smallest soil organisms are the greatest in number.

Organism abundance in soils is usually controlled by the supply of decomposable organic matter. The ratio of soil organic matter:detritus:microbial biomass: faunal biomass is often approximately 1,000:100:10:1. Most soil organisms are heterotrophs that use tissue from other organisms as a source of energy and carbon, however soils also are home to many unique and important autotrophic organisms (mostly bacteria).

Earthworms Earthworms are probably the most important macroanimal in most soils. These organisms ingest soil and detritus (dead tissues), feeding mainly on the bacteria and fungi in these materials.

They do not harm higher plants. Their activities affect soils in many ways that benefit soil productivity. Benefits include greater macroporosity and hence improved drainage, infiltration, aeration, and root penetration of soils. Earthworms also increase the availability of plant nutrients in the casts they produce from partially digested soil. These casts also become water-stable soil aggregates. Some effects may be detrimental. Earthworm channels in certain untilled soils have been shown to enhance the leaching of chemicals through the soil. The invasion of northern forest soils by exotic earthworms of European origin is believed to be having major impacts on ecosystems that have developed without earthworms for nearly 10,000 years. On balance, however, earthworms are thought to be very beneficial to both soil and environmental quality.

Earthworms are encouraged by soils high in organic matter, lime, and with good drainage.

Very acid and very sandy conditions, as well as extensive tillage, are detrimental to most earthworms.

Termites and Ants Termites and ants are social insects whose biomass and activity in soils can be comparable to that of earthworms. Termites and the soil nests they build (often including large above ground mounds) are most common in the semi-arid tropics. These insects do not produce such generally beneficial effects on soil productivity as do earthworms, but their activity is an important factor in the formation of certain tropical soils. Bacteria in their guts produce significant quantities of methane, adding to the global greenhouse effect.

57 .

Microanimals Nematodes and protozoa are the two most important groups of microscopic animals living in soils. Nematodes are a highly diverse group of tiny roundworms living in almost all soils, with almost 100,000 species thought to exist. Some are detritivores; some are mycophagous; others are predatory.

The few species that attack plant roots receive most of the attention of agriculturalists. Long-term crop rotations, fumigants and new biological methods are used to control nematode infestations. Protozoa are mobile, single-celled creatures, many of which prey upon fungal and bacterial colonies. Some are human and animal pathogens. Predation by nematodes and protozoa on fungi and bacteria helps to accelerate nutrient cycling by releasing the nutrients (especially N) tied up in the microbial biomass.

Plant Roots and Soil Algae Fine feeder roots and root hairs are often considered to be soil organisms because they greatly influence soil conditions in their rhizosphere, the soil in their immediate vicinity. They remove water and nutrients, and add organic carbon sources and growth factors. Dead roots and root exudates are major sources of soil organic matter that support microbial activity. The rhizosphere is usually populated by a greater abundance and diversity of microorganisms than is the bulk soil. Algae are single-celled plants which generally live by photosynthesis and so are found principally in the upper few millimeters of soil. In some soils they may provide a substantial amount of organic matter and their polysaccharides may help stabilize soil structure.

Soil Fungi While soil fungi include single-celled yeasts that live principally in anaerobic soils, most soil fungi are filamentous, producing masses of hyphae called mycelia. Some produce macroscopic fruiting bodies we call mushrooms. Fungi are efficient decomposers of a wide range of organic compounds. Fungi are even more important than bacteria in the production of humus and stabilization of soil aggregates. Some also produce antibiotics such as penicillin. A few, such as Aspergillus flavus, produce compounds, mycotoxins, that are highly toxic to humans. Some play important roles in the control of plant pathogens. Some fungi are, themselves, pathogenic to higher plants.

Closely related to certain pathogens are the fungi that form symbiotic mycorrhizae with plant roots. Both ectomycorrhizae (forming mainly on certain ericaceous and coniferous trees) and endomycorrhizae (forming on a wide range of plants) assist plants in obtaining nutrients (particularly phosphorus) from soil. Ectomycorrhizae are used routinely in forestry to assure vigorous growth of tree seedlings. Endomycorrhizae, which actually invade and form vesicles and arbuscules in plant root cells, are often called AM for short. Research continues to uncover the widespread influence of mycorrhizae on plant growth and ecosystems, including beneficial interaction with nitrogen-fixing bacteria in legume roots.

Bacteria and Archaea Bacteria and Archaea are single celled, prokaryotic organisms that often exist as colonies on the surfaces of soil particles and roots. As a group they have an extremely wide range of environmental adaptations and metabolic capabilities. Most soil bacteria are aerobic heterotrophs, but many others are facultative or obligate anaerobes. Many anaerobic processes in soils are carried out

by Archaea rather than bacteria. A wide range of autotrophic bacteria live in soils, including those responsible for nitrification and sulfur oxidation. Certain soil bacteria and cyanobacteria (formerly called blue-green algae) are responsible for much of the world’s nitrogen fixation.

58 .

Actinomycetes Actinomycetes are a class of bacteria, cells of which often form short filaments reminiscent of fungal hyphae. Actinomycetes have cell with size and structure similar to other bacteria. They are generally aerobic heterotrophs, and have much more limited environmental adaptations than fungi or bacteria. They are important in the later stages of plant residue decomposition, in composting, and for the production of numerous antibiotics.

Ecological Interactions Competition among the various organisms in the soil is very keen, and may involve the production of specialized compounds such as iron-binding siderophores and antibiotics. Soil management practices commonly used in agriculture may inhibit some soil organisms and stimulate others. Certain pesticides, such as fumigants, may sharply decrease microbial and faunal population, at least temporarily. However, most herbicides and insecticides stimulate at least some organisms (as a food source) while having little effect on most soil organisms. Tillage generally has a negative effect on soil life, while organic matter additions and diverse plant communities generally stimulate diversity and abundance in the soil community. A healthy soil community is likely to minimize problems with plant pathogens through a complex set of ecological interactions, including microbial antagonisms, development of disease suppressive soils, and induced systemic resistance.

Model Answers to Study Questions 1. See Section 10.1.

Functional redundancy describes the presence of more than a single species capable of carrying out a given ecosystem function. If a disturbance inhibits or kills off a number of species, there should be other species able to survive to carry out the functions of the lost species.

2. See Table 10.7 and Section 10.2.

Primary producers: sorghum and rye plants (not listed in the body of the table).

Primary consumers: some fungi, bacteria and actinomycetes that decompose the plant residues; also enchytraeids and earthworms, in so far as they eat plant residues.

Secondary consumers: nematodes, earthworms, some microarthropods that eat fungi, etc.

Tertiary consumers: some microarthropods that prey on nematodes and other microarthropods.

3. See Section 10.2.

Mesofauna stimulate microbial metabolism by grazing fungal colonies, macerating organic matter mixing and mixing it with microbes in their guts, and transporting both organic residues and microbial propagules.

4. See Table 10.2.

Chemo heterotrophs: combined carbon for energy and carbon Chemo autotrophs: combined carbon for carbon, inorganic elements (N, S, Fe, etc.) for energy

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Photoautotrophs: carbon dioxide for carbon, sunlight for energy Photoheterotrophs: combined carbon for carbon, sunlight for energy (rare) 5. See Section 10.11 (also Section 7.3).

Terminal electron acceptor, role played by N, Fe, S and the like for anaerobes.

6. See Section 10.9.

Plant (root) and fungal symbionts. Plant receives nutrients (especially P), water, and protection from some diseases to a much greater extent than if dependent solely on its own root system.

Fungus gets virtually unlimited supply of sugars without having to out compete all other soil heterotrophs for organic residues.

7. See Section 10.4 and Box 10.1.

Earthworms mix in organic residues, create macropore channels that can enhance infiltration, percolation and aeration. Their casts are enhanced in nutrient availability. Their grazing stimulates microbial colonies. Under certain circumstances earthworm channels may enhance the leaching loss of chemicals. The mixing action of certain earthworms may also alter habitat and ecosystem dynamics in certain forested soils.

8. See Section 10.7.

The rhizosphere is the zone of soil (about 2 mm) around living roots and significantly influenced by them. It contains many times more bacteria than the bulk soil, is enriched in growth factors and carbon sources excreted by the root, but is generally depleted of certain soluble mineral nutrients used by the roots. The mix of microbial species in the rhizosphere are usually different from those in the bulk soil.

9. See Section 10.14.

Manure application provides a food (carbon and energy) source for soil microbes and fauna, encouraging greater activity and a high diversity of species. Tillage, on the other hand, disrupts the soil environment to the detriment of earthworms, other fauna, and fungi. Bacteria may be encouraged as tillage breaks apart organic residues and aerate the soil. In the long term, tillage causes soil organic matter to be depleted and thus lowers biological activity and diversity in the soil.

10. See Section 10.13.

Induced systemic resistance describes a phenomenon whereby a plant root system is colonized by beneficial rhizobacteria that cause a signaling chemical to accumulate in the plant. This signal is translocated up to the shoot where it induces leaf cells to produce chemical changes that defend against attack by detrimental insect or fungi.

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11. See Section 10.13.

A disease suppressive soil is one in which both a virulent pathogen and a susceptible host are present by the plant disease fails to develop because of protective action by beneficial microbes.

General suppression is caused by high levels of heterotrophic, usually saprophytic, microbial activity that inhibit the activity of many pathogens. Specific suppression is caused by a particular beneficial microbe or group of microbes that is antagonistic against a particular pathogen.

12. See Sections 10.9 and 10.13.

Since most soils contain effective naïve mycorrhizal fungi, inoculation with such fungi rarely improves plant performance in normal soils. However, in denuded, sterilized or highly disturbed soils in which few mycorrhizal fungi exist, inoculation may be extremely beneficial. For example, in planting pine trees on strip mined land, the use of an ectomycorrhizal inoculant such as the fungus Pisolithus tinctorus can spell the difference between success and failure. Disease suppressive organisms are, like most microorganisms, nearly ubiquitous in all normal, biologically active soils, and it is more likely the soil conditions that limit their effectiveness rather than their presence. However, again, inoculating with specific disease suppressive microbes can be successful if the soil conditions are appropriate but populations are low or if a more rapid population build up will be important, such as when protecting seeds from rot organisms by coating them with antagonistic bacteria.

13. See Section 10.6 and Figures 10.14 and 10.16.

Soil nematodes feed on fungi, bacteria and algae or are predatory on other nematodes, protozoa, or insect larvae or parasitize plant roots. The different trophic groups of nematodes can often be distinguished by the type of mouth parts present. Figure 10.14 illustrates a predator and plant parasite. The head of a bacteria-eating nematode is detailed in Figure 10.15.

14. See Section 10.10 and Figure 10.32.

Actinomycetes are filamentous and often profusely branched (see), appearing somewhat like tiny fungi. However, their genetic makeup and cellular properties clearly place them in the Bacteria domain—they have no nuclear membrane, are about the same diameter as other bacteria, and often break up into spores that closely resemble cocci bacterial cells.

15. Given: There are 58 nematodes in a 1 g soil sample representing the upper 10 cm of a soil with bulk density = 1.3 Mg/m3. The number of nematodes in 1.0 m2 land area of this soil would be: 1 m2 to 0.1 m depth = 0.1 m2 volume of soil. The soil density is 1.3 Mg/m3 = 0.13 Mg/0.1 m3 = 1.3 × 105 g/0.1 m3. Therefore the 1 m2 contains 58 nematodes/g × 130,000 g = 7.54 × 106. One ha = 10,000 m2 of land or 1,000 m3 of soil to 10 cm depth. Therefore 1 ha of this land contains 1 × 103 × 7.54 × 106 = 7.5 billion/ha.

16. See Sections 10.2, 10.4 and 10.5.

Ecosystem engineer describes an organism (usually an animal) whose activities markedly alter the soil environment in which that organism and other organisms live. Earthworms (that alter soil hydrology, chemistry and structure with their burrowing activities) and dung beetles (that alter soil nutrient cycles and N supply by burying manure) are examples.

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Chapter 11. Soil Organic Matter Overview The Global Carbon Cycle Soils play a central role in the global carbon cycle as they are repositories, sources, and sinks for a major portion of the carbon in circulation around the world. About 3,000 Pg of carbon is contained in soil organic matter (the value depends on what depth of soil is considered and whether the O horizon is included), compared to 620 Pg in the standing vegetation, and 790 Pg in the atmosphere.

Carbon is the foundation of life. Carbon cycles from the atmosphere to plants to soil organic matter and back to the atmosphere. The oceans act as a buffer for atmospheric carbon dioxide by dissolving or releasing large quantities, but fossil fuel burning and soil organic matter depletion add more carbon to the atmosphere each year than the oceans take out, resulting in rising levels of carbon dioxide that are believed to be the main cause of global warming.

Decomposition of Plant Residues Green plant tissues consist mostly of water (60 to 90%). The dry matter is more than 90% C, H, and O. Only about 5 to 10% of the dry matter consists of mineral nutrients. Plant residues are largely composed of slow-to-decompose cellulose, lignin, hemicellulose and fats, with smaller portions of easily decomposed proteins, sugars, and starch. Decomposition is an oxidation process by which organic compounds are eventually converted into carbon dioxide, water, and energy. Nutrient elements such as nitrogen, if part of the initial organic compounds, may be released into the soil as soluble mineral nutrients or assimilated into the cells of the decomposing organisms.

When organic materials are added to a soil, a burst of microbial activity and growth takes place as opportunist (r-strategist) microorganisms supplement the more stable autochthonous (k-strategist) population. In addition to carbon dioxide formation, residue decomposition results in the formation of soil humus and the release of nutrients by mineralization. The burst of microbial activity associated with fresh residues may also cause some depletion of old soil humus, a process known as the priming effect. In anaerobic soils, oxidation is incomplete, and products include methane, alcohols, and other organic compounds in addition to carbon dioxide.

Carbon to Nitrogen Ratio The ratio of carbon to nitrogen in organic residues exerts a major influence over the rate of decomposition and the balance between immobilization and mineralization. In material with a C/N ratio greater than 25, nitrogen is likely to be immobilized from the soil solution causing a period of nitrate depression. With a C/N ratio less than 20, the amount of mineral nitrogen in the soil solution is likely to be increased during decomposition. Because the C/N ratio of stable soil organic matter is fairly constant, the supply of available nitrogen exerts some control over the accumulation of additional soil organic matter from plant residues. Thus, although residues with relatively high C/N ratios decompose relatively slowly, in the long run, soil organic matter accumulation is enhanced if sufficient N is present to both stimulate greater plant production and stabilize more of the C in humus.

Litter Quality Rapid litter decomposition leads to ready nutrient cycling. The quality of litter in terms of its suitability for ready nutrient and carbon cycling is improved by a low content of lignin and/or polyphenols, as well as a narrow C/N ratio. The ratio of lignin to N is, therefore, a useful measure of 62 .

litter quality. Litter quality varies greatly among plant species, especially trees, and to a large degree determines the level of organic matter that will accumulate in a soil. Vegetation with low-quality litter favors slow decomposition and the accumulation of a high portion of the system C in the soil humus.

Humus-Genesis and Nature Soil organic matter includes all the organic components of a soil: 1) living plant and animal biomass, 2) plant and animal detritus, 3) dissolved organic biomolecules, and 4) a complex mixture of biomolecule agglomerations on particle surfaces. The first three make up the labile carbon pool. The fourth group is humus, which is protected from rapid breakdown and further release of carbon. Humus includes a largely amorphous and colloidal mixture of biomolecules and organic decomposition products no longer identifiable as plant tissues. New in situ analytical techniques have altered previous perceptions of the nature of humus as very large complex polymers synthesized by microorganisms.

Humus is now understood to be a protected or stabilized fraction of soil organic matter that consists largely of microbial cell walls, bits of partially decomposed plant tissue, black carbon and biomolecules.

Humus is often stabilized in soils by complexing with clays. Like clay, humus is colloidal. It has surface charges giving rise to a cation exchange capacity that is much higher, gram for gram, than for most clays. Humus also has a high water-holding capacity. Humus itself is not a plant nutrient, but it enhances soil fertility by stimulating the weathering of minerals and complexing with metallic ions.

Soil organic matter influences plants directly when plants take up growth stimulating components, and when plants are affected by allelopathy. In the latter process, chemicals derived from one plant influence, typically negatively but occasionally positively, the growth of another plant.

Allelopathic interactions are very specific and generally short-lived.

The indirect effects of organic matter in plants via changes in soil properties are extremely important. The soil properties benefited by increases in soil organic matter are physical, chemical, and biological in nature. The influences of organic matter are very far-reaching and complex.

Amount of Organic Matter in Soils The surface horizons of mineral soils contain from a trace (sandy, desert soils) to as much as 20 to 30% organic matter by weight. Most arable soils contain between 1 and 4%. Andisols and Mollisols are known for their organic-rich surface horizons. Lower horizons are generally lower in organic matter.

Soil organic matter content is determined by the balance between humification and oxidation of the plant residues that reach the soil. Greater plant productivity generally results in greater amounts of residues and hence greater soil organic matter. Only about 15% (for above ground plant residues) to 33% (for roots) of plant residues returned to the soil are converted to relatively stable soil humus. The remainder of the carbon in the residues is lost as carbon dioxide within the first year after application.

Natural forests and grasslands generally have more soil organic matter because of greater return of residues and slower oxidation of those residues. Tillage greatly accelerates the loss of soil organic matter.

The level of organic matter in a soil is a function of many environmental factors. Factors that are associated with higher soil organic matter include: (1) low temperature and high rainfall; (2) lush natural vegetation (for a given climate, grasslands accumulate more and deeper organic matter in the profile); (3) fine texture; (4) restricted drainage; (5) little or no tillage; (6) rotations that include sod crops; and (7) high soil fertility and especially nitrogen availability.

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Management of Soil Organic Matter Soil management should aim to improve both the quantity and quality of soil organic matter.

In terms of management and function, soil organic matter can be divided into active, slow, and passive fractions. Most of the benefits derive from the active fraction, and it is that fraction which is most challenging to maintain or increase. Sufficient active organic matter requires the continual addition of organic residues as well as measures to limit oxidation losses since the active fraction is the first to be lost. Some useful organic matter management principles include: (1) continual additions with green manures, cover crops, animal manures, etc.; (2) goals in keeping with environmental factors; (3) adequate nitrogen for organic matter accumulation; (4) high plant production with return of roots and as much shoot material as possible; (5) minimum tillage; and (6) perennial vegetation.

Soils and the Greenhouse Effect Soil management to conserve and accumulate soil organic matter can play a major role in combating the greenhouse effect, which is caused by increased atmospheric concentrations of carbon dioxide, methane, CFCs, ozone, and nitrous oxide. Soils are a major source of all but the CFCs and ozone. Methane and nitrous oxide production occurs mainly in anaerobic soils such as rice paddies and marshes, while carbon dioxide is produced in all soils. Conservation tillage and restoration of natural ecosystems on a wide scale have the potential to increase soil organic matter sufficiently to significantly ameliorate the greenhouse effect. High levels of inorganic nitrogen fertilizer appear to reduce the capacity of soils to oxidize methane.

Organic Soils (Histosols) Soils with more than 20 to 30% organic matter (depending on clay content) are termed organic soils and are classified in the Histosols order in Soil Taxonomy. They form in marshy depressions where anaerobic conditions inhibit organic matter oxidation. Drainage for agricultural use results in subsidence as the soil is converted into carbon dioxide under oxidizing conditions. Organic soils have very low bulk density, high C/N ratios, and high water holding capacities (by weight). They are natural wetlands and should be preserved as such to limit atmospheric carbon additions and to preserve ecological wetland functions.

Organic Materials for Potting Media Peats mined from Histosols are commercially used for potting media and mulches. They are lightweight, hold much water, and have a high nutrient holding capacity (though often low in nutrient as found). Of the various types, fibrous peats are the most desirable for potting media. Alternatives are being explored to reduce peat mining, including composted materials and biochars.

Compost and Composting Composting is the practice of creating humus-like materials from raw residues outside the soil under conditions which provide sufficient nitrogen, moisture, air, and surface area for rapid, aerobic decomposition. Compost piles heat up to about 70oC, killing most weeds, seeds, and pathogens. The product, compost, is easier to handle and more stable than the raw materials. It can be used as a potting medium, a mulch, or soil amendment. Composts often hold nutrients which are released as further decomposition takes place.

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Model Answers to Study Questions 1. See Section 11.1 and Figure 11.3.

Global standing vegetation contains about 620 Pg of carbon (mainly as organic compounds) and the atmosphere contains about 790 Pg (as CO2). In comparison, the world’s soils have stored about 3–4,000 Pg of carbon (mainly as soil humus). These figures emphasize the potential for widespread changes in soil organic matter levels to affect the global carbon balance, and possibly rates of climate change.

2. See Section 11.3.

Physically, larger particles of mulch will decompose more slowly than smaller ones; for example, wood chips will last longer than an equal weight of sawdust. Chemical properties that slow decomposition include high levels of lignin and polyphenols, and a high C/N ration.

3. See Section 11.3 and Figure 11.6.

High levels of carbon, especially if in relatively easily metabolized forms as starch and cellulose, stimulate rapid growth of soil microorganisms, which in turn requires large amounts of nitrogen for synthesis of proteins and other essential cellular compounds. During the period of rapid growth when microbes are rapidly decomposing such material, the microbial use of nitrogen will depress the level of soluble nitrogen, often to nearly zero. During this period, plants growing in the soil will suffer from nitrogen deficiency. It is, therefore, best to wait until this period has passed before establishing such plants.

4. See Section 11.4, especially Figure 11.9.

Soil contains living biomass, detritus, bits of charred plant tissue, dead microbial cell walls, and molecules such as polysaccharides, organic acids, glycoproteins, allelochemicals, lignin and other identifiable chemical compounds of plant or microbial origin.

5. See Sections 11.1, 11.4, etc.

Reasons to include litter in soil organic matter: the soil profile includes the O horizons, and the Oi consists mainly of plant litter. Also, such litter serves as food for many soil organisms.

Reason to exclude plant litter from soil organic matter: The particle size of undecomposed litter is often larger than the 2 mm limit for the “fine Earth fraction” that is commonly considered in soil investigations. Raw plant litter may not have been altered very much from its state as vegetation.

6. See Sections 11.5 and 11.6.

Active fraction influences mainly microbial activity, macroaggregate stability and infiltration rate, nutrient mineralization rates. The passive fraction influences mainly water holding capacity, cation exchange capacity, and microaggregation.

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7. See Table 11.1 footnote, Section 11.3, text footnote 4.

All organic materials, by definition, contain carbon (C). Soil organic matter contains from 40 to 60% C, depending on the degree of decomposition, etc. The traditionally, soil organic C content is multiplied by 1.72 to convert into soil organic matter. This assumes that soil organic matter contains 58% C (1/0.58 = 1.72). Studies suggest that 2.0 is a more appropriate factor in most cases. Since C is what is actually measured by most methods and organic matter is merely estimated based on a “fudge factor”, it is scientifically more appropriate in most cases to report soil organic C than soil organic matter.

8. See Sections 11.6-11.8.

When a soil under natural forest or grassland vegetation is converted to cropland, soil organic matter content declines because losses of C increase while gains may generally increase less or even drop off. Losses of C are due to increased erosion, oxidation (largely a result of tillage), and removal of plant material in harvests.

9. Char refers to carbon that has been heated with inadequate oxygen to allow burning and the production of ashes—a process called pyrolysis. Char is a blackened C material, often still retaining the physical structure of the original plant cells. Although some char decomposes, most is very resistant to decay and remains unchanged in the soil for long periods. Fire is a natural process in most ecosystems and some soil have accumulated large amounts of char. Char has a very porous open structure with a great deal of surface area with many charged groups and therefore contributes to soil water and cation holding capacities.

10. See Section 11.10.

Soil processes produce several gases which contribute to the greenhouse effect: carbon dioxide (from organic matter decomposition and respiration), nitrogen oxides (from denitrification in wet soils), and methane from waterlogged soils. Reduced tillage can reduce the carbon dioxide emissions while avoiding the over-use of nitrogen fertilizers and increasing the amount of drainage for rice paddies and poorly drained fields can help reduce the emissions of the other two gases.

The soil may also provide some negative feedback to slow the greenhouse effect if the higher carbon dioxide content in the atmosphere results in more plant growth, especially below ground, and more C accumulation in the soil, thus removing more carbon dioxide from the air. Similarly, rising sea level may result in the formation of more coastal histosols (marsh soils) in which much C is sequestered slowing global warming (unless these marshes have such low redox potentials that they emit much methane!).

11. See Section 11.11 and Box 11.5.

Because Histosols formed under wetland conditions, their high organic matter content is relatively stable only in a wet, low-oxygen environment. Improvement of aeration for better crop growth also improves aeration for faster microbial decomposition of organic matter. Therefore, once a Histosol is drained for agricultural use, organic matter is lost much faster than it is gained, so the soil disappears, the soil surface sinking at a rate of a few mm to a few cm per year. Agricultural use of Histosols that involves substantially lowering the water table will inevitably lead to the disappearance of the soil itself. Production of crops requiring good aeration is therefore not sustainable on these soils in the long run.

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Chapter 12. Nutrient Cycles and Soil Fertility Overview Soils are at the very hub of the biogeochemical cycles that transform, transport and renew the supplies of the mineral nutrients so essential for the growth of terrestrial plants. As each nutrient moves cycles through the soil, a given atom may appear in many different chemical forms, each with its own properties, behaviors and consequences for soil development and for the ecosystem. For some elements, such as nitrogen and sulfur, the cycles are exceedingly complex involving many different biologically-mediated transformations and movement into and out of the soil as solid particles, in solution, and as gases. The cycling of phosphorus also involves a fascinating set of complex interactions among chemical and biological processes. For calcium, magnesium and potassium, the weathering of minerals and cation exchange reactions dominate the cycles. For the micronutrients, iron and manganese, mobility and bioavailability are controlled mainly by soil pH, redox potential and reactions with soluble organic compounds. These cycles impact not only soil fertility, but also the health of aquatic ecosystems and the health and survival of humans on Earth.

Nitrogen and sulfur are essential macronutrients which participate in complex geochemical cycling. Many processes in both cycles occur in the soil environment and involve microbial transformations. The plant availability of both elements is largely dependent on the breakdown of organic residues. Both elements can have significant impacts on environmental quality, depending on the forms, including gaseous forms, in which they are lost from soils.

Influence of Nitrogen on Plant Growth and Development Nitrogen is a major component of all proteins, including the myriad of enzymes that govern biological reactions. It is also central to the chlorophyll molecule. Healthy foliage generally contains 2.5 to 4.0% N, the higher values typical of legumes. Deficiency of N causes chlorosis (especially of the older leaves), spindly plants, and low protein contents. An excessive or imbalanced supply of N can cause excessive foliage at the expense of seed or flower production, top-heavy, lodging-prone plants and increased susceptibility to certain plant diseases and pests. Plants take up N principally in the form of nitrate and ammonium ions, an equal mixture usually giving best results. However, research shows that plants also take up soluble organic nitrogen (SON).

Origin and Distribution of Nitrogen Most of the N in terrestrial ecosystems is contained, not in the plant or animal biomass, but in the soil organic matter (if the atmosphere, with its 79% N2, is not considered). This atmospheric N is, however, the source (through N fixation) of most of the soil N. Soil organic matter typically contains about 5% N by weight, but most is in humus or clay-humus complexes that resist breakdown; therefore, only 2 to 3% of the N in the soil is likely to become available for plant use in a given year.

About 1 to 2% of soil N is typically in soluble, inorganic forms.

The Nitrogen Cycle: Immobilization-mineralization Most (>95%) of soil N is in organic forms that are protected from loss but not directly available to plants. As microbes attack soil organic matter, amine groups (R-NH2) are hydrolyzed to release simple ammonium (NH4+) ions, a process termed ammonification. The rate of this heterotrophic microbial process is greatly influenced by temperature and moisture. Much of the ammonium is eventually oxidized to nitrate. The overall process by which plant-available mineral nitrogen is released from organic compounds is termed mineralization. It is a major source of 67 .

nitrogen for plant uptake, even in fertilized agricultural soils. The reverse of mineralization is termed immobilization, a process by which microbes incorporate mineral nitrogen from the soil solution into the organic compounds in their cells. The balance between mineralization and immobilization largely determines the amount of available N in the soils at a particular time.

The Nitrogen Cycle: Fates of Ammonium Ammonium ions (positively charged cations) may become adsorbed on negatively charged clay surfaces by cation exchange. If certain 2:1 clays (e.g., vermiculite) are present, these ions may become trapped in interlayer "holes" in the crystal structure, a process known as ammonium fixation.

Such fixed ammonium is conserved but released for plants only very slowly.

Under conditions of relatively high pH and exposure to drying, ammonium ions may react with OH- ions to form ammonia gas, which can be lost to the atmosphere, the process known as ammonia volatilization. Such losses are most serious 1) in coarse textured soils, 2) from manure applied and left on the soil surface, and 3) under drying, high pH conditions. This type of N loss also occurs from rice paddies. In a reversal of the process, soils (and foliage) can absorb ammonia gas from polluted air, a process of localized significance to both soil fertility and air quality.

The Nitrogen Cycle: Nitrification In warm, moist, aerated soils ammonium ions in the soil are rapidly oxidized to nitrite (N02-)

by Nitrosomonas bacteria. Nitrobacter bacteria then oxidize the nitrite to nitrate (NO3-). The nitrite is toxic but generally does not accumulate because of its rapid conversion to nitrate. The complete reaction, called nitrification, releases acidity-causing H+ ions and energy (for the autotrophic nitrifying bacteria) while it consumes oxygen gas. The nitrate produced is readily available for plant uptake, as well as for loss from the soil by leaching and denitrification. Nitrate may also be immobilized by heterotrophic microbes when high C/N ratio organic materials are present. A sudden change in soil conditions, such as wetting a dry soil or tilling a previously undisturbed soil, results in a burst of nitrate production via microbial mineralization and nitrification.

The Nitrogen Cycle: The Nitrate Leaching Problem Negatively charged nitrate ions are not adsorbed by negatively charged humus and clay and therefore move through most soil profiles with the drainage water. This leaching of nitrate can contaminate groundwater and eventually surface waters. Water containing more than 45 ppm nitrate (10 ppm nitrate-nitrogen) is considered unfit as drinking water because of the danger of methemoglobinemia (Blue Baby Syndrome) in human infants and ruminant livestock. This potentially fatal condition is caused by the conversion of nitrate to nitrite in the victim’s stomach and the subsequent reduction in the oxygen-carrying capacity of the blood. Nitrate in water also contributes to eutrophication, especially in brackish estuaries. Soluble organic N, although much less studied, can also move down into the drainage water and thereby contribute to eutrophication.

Nitrogen Cycle: Denitrification Soil nitrate may be chemically reduced by denitrification to various nitrogen-containing gases which can then escape to the atmosphere. Under anaerobic, warm conditions, in the presence of a microbially available carbon source, anaerobic heterotrophic bacteria use the nitrogen in the nitrate ion as their electron acceptor, converting the nitrate first to nitrite, then to nitric oxide (NO), nitrous oxide (N20)and dinitrogen gases (N2). The N2 gas is harmless, but as the other gases eventually find their way to the upper atmosphere, they contribute to the destruction of the UV absorbing ozone layer.

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Denitrification is most significant in wetlands, rice paddies, and other hydric soils, but occurs sporadically on well drained soils as well. This process helps riparian wetlands protect streams from excessive nitrogen loading.

Biological Nitrogen Fixation Using the enzyme, nitrogenase, a few species of bacteria, actinomyces, and cyanobacteria convert N2 gas in the atmosphere to organically-combined amide groups, thus replenishing the supply of soil nitrogen depleted by plant harvest, leaching, erosional, and gaseous losses. Such biological nitrogen fixation ranks as one of the most important reactions in the biosphere. Specialized organisms and conditions are necessary because of the unique characteristics of the nitrogenase enzyme and its reactions.

Because of the high energy requirement of the reactions, most N fixation is carried out by a symbiotic combination of microbes and photosynthesizing plants. The best known of these symbioses are those involving Rhizobium or Bradyrhizobium bacteria and legume family plants. The reaction takes place in nodules that form on the roots of the legumes. Similar nodules also form on certain plants in association with Frankia actinomycetes. Both types of associations are capable of fixing up to several hundred kg of N per hectare. Much lower rates of fixation occur in certain free-living soil microorganisms.

Addition of N in Precipitation Lightning discharges and fires (including those in internal combustion engines) cause nitrogen oxides and nitrates to be formed in the atmosphere. Ammonia from various sites of volatilization also occurs. These N compounds undergo reactions in the atmosphere and become dissolved in the rain and snow, mostly as nitrates. The addition of N from these sources is typically 5-8 kg N/ha annually, but can be considerably greater if industrial, urban, or animal manure concentrations are located nearby. In natural ecosystems this source of N is nearly enough to offset losses. It can also be a significant source of N causing eutrophication in surface waters.

Reactions and Management of N Fertilizers Most fertilizers provide N as ammonium and nitrate which, like nitrate and ammonium ions from other sources, then participate in the N cycle processes. Three reactions of particular concern with regard to N fertilizers are: (1) osmotic effects of concentrated salts, (2) acidification resulting from nitrification of ammonium, and (3) localized sterilization from ammonia-producing fertilizers.

Key goals of N fertilizer management are (1) to maintain an adequate N supply in the soil, (2) to ensure that there is sufficient soluble N for optimal plant growth when the plant needs it, and (3) to minimize environmental damage by avoiding amounts of soluble N in excess of plant uptake, or at times of year when losses are likely to occur. Full consideration must be given to all sources of available nitrogen in the N cycle, before supplementing these with fertilizer. Some fertilizer technologies that may assist in proper N management include nitrification inhibitors and slow release fertilizer compounds.

Some agroforestry systems have the potential of greatly reducing nitrate losses from some tropical systems. Nitrates that have leached downward below the maximum rooting depth of a food crop such as corn can be absorbed by positively charged low activity clays and oxides in these tropical soils. Deep rooted companion tree crops absorb the nitrates, bring the N above ground in their leaves, which can be cut and spread over the food crop. This type of nutrient recycling can have significant effects on nitrate losses.

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Importance of Sulfur Sulfur is a constituent of several amino acids and vitamins found in both plants and animals, and thus is an important factor in determining the nutritional quality of foods. It is also found in numerous oils responsible for the flavors of certain vegetable such as cabbage and onions. Healthy foliage contains about 0.15 to 0.45% S, deficient plants becoming chlorotic and spindly, but not mainly on the older parts as is true for N. Sulfur deficiency in plants is becoming more common because of purer fertilizers, cleaner air (less S02), and higher rates of crop removal. Burning plant forestry and crop residues results in the loss by volatilization of most of the S contained.

Natural Sources of S Most soil sulfur is made available by the mineralization of soil organic matter (especially in the A horizons), by desorption of sulfate held by anion exchange sites (especially lower horizons of highly weathered soils), and by dissolution of sulfate minerals such as gypsum (especially in less weathered soils of drier regions). Absorption of sulfur dioxide gas from the atmosphere by both plant foliage and soils is another important source of sulfur for plants.

Various S-containing compounds in the atmosphere are subject to oxidation to sulfate and subsequently contribute sulfuric acid to both wet and dry deposition. This source of S and acidity is particularly important downwind from coal burning power plants and ore smelters. Sulfur added to soils by atmospheric deposition ranges from 30 to 75 kg S/ha near industrial sites to only 1 to 5 kg/ha in regions with unpolluted air. Sulfur from the atmosphere may be absorbed by plant foliage or adsorbed by the soil.

Sulfur oxides, along with those of nitrogen, are the source of the acids that caused “acid rain,” a widespread environmental problem thought to be responsible for killing fish in lakes, killing trees in forests, and depleting soils of their nutrients.

The S Cycle: Mineralization Mineralization of soil organic matter releases both sulfates and sulfides into the soil solution, although the sulfides are generally subject to rapid oxidation to sulfates. The principal form of S taken up by plants is the sulfate anion. Mineralization and immobilization of sulfur is controlled by environmental factors that affect microbial activity, much the same as for N. Soil organic matter generally contains about 8 to 10 times as much N as S.

Sulfur Oxidation In well-aerated soils, reduced forms of S (elemental So, hydrogen sulfide, etc.) are subject to oxidation by chemical and biochemical (microbial) reactions, which consume oxygen and produce sulfate and H+ ions. Every mole of S oxidized results in 2 moles of acidity (H+ ions). If iron sulfides are oxidized, subsequent hydrolysis of the iron also contributes to acidity. These reactions are facilitated by specialized bacteria such as Thiobacillus. Extreme levels of acidity can result in both affected soils and drainage waters. Pyritic mine spoils and coastal wetlands (especially cat clays) are known for their acid sulfate weathering when exposed to air.

Sulfate Retention and Exchange The sulfate anion is much more strongly adsorbed by soils than is the nitrate counterpart.

Highly weathered soils, with coatings of iron and aluminum oxides, and low activity clays relatively high in positively charged sites can adsorb large quantities of sulfate, usually in their lower horizons.

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Much of this adsorbed sulfate is protected from leaching losses, but is available to plants by anion exchanges processes.

Sulfur and Soil Fertility Maintenance In regions with clean (low sulfur dioxide) air and low S soils, maintaining adequate levels of available soil S is a worsening problem, which can be solved by careful recycling (not burning) of crop residues and animal manures, as well as by application of S-containing amendments.

Roles of Phosphorus in Plant Nutrition Phosphorus is an essential macronutrient for plants (and for all animals as well). Its many roles include cellular energy transfer via ATP, genetic inheritance via DNA, protein and enzyme synthesis via RNA, and cellular membrane function via phospholipids. Adequate P nutrition enhances many plant processes and hastens maturity.

Healthy leaves generally contain about 0.2 to 0.4 % on a dry weight basis. Phosphorus deficiency is not always accompanied by recognizable foliar symptoms, but typically plants are stunted with purplish coloring (especially on the older leaves).

Soil phosphorus can be problematic both when levels are too low and when they are too high.

P-depleted soils cannot support vigorous crops or protective revegetation with natural species. The result is erosion and degradation of poorly protected land. The problem of low soil P is three-fold: (1) the total P content of most soils is rather low compared to the long term needs of growing plants, (2) most of the P found in soils is in forms quite unavailable for plant uptake, and (3) soluble P added to soils rapidly is transformed into unavailable or fixed forms.

Phosphorus and Environmental Quality To overcome P-fixation, P fertilizer additions commonly have been much larger than plant removals, thus leading to a build-up of soil P in well-fertilized soils. Such high-P soils become non-point sources of nutrient pollution, with the potential to add much unwanted particulate-P and dissolvedP to waterways when water runs off the land, a process that results in cultural or accelerated eutrophication. Human activities that disturb the soils, such as logging, farming tillage, and construction, greatly increase the potential for P losses. Because the P-fixing capacity has been saturated by years of P fertilizer application, many intensively managed agricultural soils have large supplies of readily available P which can easily meet plant needs, but also is prone to polluting streams draining the watershed. Tillage, timber harvest, or other disturbances usually result in greatly elevated losses of P in the particulate form (adsorbed on sediment). Sediments eroded from a site usually contain much more P than the soil left behind, the enrichment ratio (P concentration in sediment/P concentration in soil) typically being as high as 2 or more.

The Phosphorus Cycle The soil solution is much more dilute (typically 0.001 to 1 mg/L) with respect to P than with respect to other macronutrients. The main P species in solution are HPO4-2 (in alkaline soils) and H2PO4- (in acid soils). Often the process limiting the uptake of P by plants is the movement of P to the surface of the plant root. Such movement to the root surfaces, whether by diffusion of phosphate ions or mass flow of the soil solution, is constrained by the strong fixation of the P by iron, aluminum and calcium compounds lining the soil pores in which the soluble P must flow. Fine root hairs growing in the rhizosphere assist in P uptake by increasing the soil volume exploited and by decreasing the 71 .

distance the P must travel to reach the root. But for many plants, especially in natural ecosystems, survival in low P soils depends on the transport of soluble P to plant roots through the hyphae of symbiotic mycorrhizal fungi.

Soil P exists either in organic forms or inorganic forms (mostly bound to Ca or to Fe or Al). In neutral to alkaline soils calcium-bound P is dominant, while in acid soils, most P is bound to iron or aluminum oxides. Both of these forms of P have extremely low solubility. Very little P is lost by leaching because of the strong affinity for P by soil iron and aluminum. Most P losses occur as P in removed plant and animal biomass, P adsorbed to eroded sediment, or P dissolved in runoff water.

Atmospheric deposition (principally on dust particles) generally contributes only enough to balance the very moderate leaching losses.

Organic P in Soils In many surface soils more than half of the total P is present in organic forms. The most common organic P-containing compounds are inositol phosphates, but not much is known about the chemical forms of much of the organic P in soils. Regardless of the specific compounds present, the P in organic compounds becomes available for plant use through release into the soil solution as inorganic phosphate ions following microbial mineralization of organic P. The balance between mineralization and immobilization of P by soil microbes depends largely on the C/P ratio of the organic materials present, mineralization occurring with ratios more narrow than 200/1. Mineralization of organic P compounds supplies an important portion of the plant P requirement in many ecosystems. The mineral P so released is often subject to fixation by calcium or iron and aluminum compounds before it can be accessed by plant roots.

Inorganic P in Soils The low concentration and immobility of P in the soil solution are due to the low solubility of P-containing minerals and the fixation or retention of P on the surfaces of soil particles. Most inorganic P compounds in soils fall into one of two groups: (1) those containing calcium (predominant in alkaline soils) and (2) those containing iron, aluminum or sometimes manganese (predominant in acid soils). Soluble inorganic P added to soils soon becomes fixed in one or the other group, the solubility of the fixed compounds decreasing with time.

Solubility of Inorganic P Compounds in Acid Soils The solubility and fixation of inorganic P compounds is controlled by a number of reactions.

A simple reaction is precipitation of iron or aluminum phosphate compounds. The solubility of the P in fresh precipitates is much greater than in aged precipitates.

Phosphate ions may also react with the surfaces of Fe and Al oxides and silicate clays in several ways. Attraction of the H2PO4- ion by simple anion exchange is somewhat reversible, and the P so held may become available to plants over time, especially with the addition of lime or organic matter.

Finally, phosphate ions may become structurally bound to the oxide or clay surface by replacing structural hydroxyls. This is an essentially irreversible reaction that leads to extremely low levels of availability for the P so fixed.

Inorganic P Availability at High pH Values In alkaline soils the availability of P is primarily determined by the solubility of the various Ca-phosphate compounds present. Solubility decreases in the order: monocalcium phosphate, 72 .

dicalcium phosphate, tricalcium phosphate, and the various forms of apatite (fluorapatite being least soluble). In alkaline soils reactions tend to convert the more soluble compounds into the less soluble ones.

Phosphorus Fixation Capacity of Soils Soils vary in the tenacity by which they hold onto, or fix, phosphorus. They also vary in their total capacity to fix P. The phosphorus fixing capacity of a soil can be thought of as the total number of site capable of adsorbing phosphate ions. Soils with the capacity to fix large amounts of phosphorus and hold the phosphate ion tightly are characterized by a very low equilibrium phosphorus concentration (EPC), the concentration of P remaining in solution after equilibrium has been approached. This soil characteristic can help predict P availability to plants as well as the release of P to surface runoff water.

A related soil property is the potential buffering capacity (PBC), defined as the change in the quantity factor (Q, the amount of labile P or P added) divided by the resulting change in the intensity factor (I, the solution P concentration).

Generally, soils with high amounts of clay, particularly iron and aluminum oxides or allophanic clays, are characterized by high P fixing capacity, low ECP, and high potential buffering capacity. P fixation tends to be lowest when soil pH is between 6.0 and 7.0. High soil organic matter tends to reduce P fixation and increase EPC by complexing with iron and aluminum and by masking fixation sites.

Practical Control of Phosphorus Availability Phosphorus availability can be optimized by adding enough soluble P to saturate most of the soil P fixing capacity. However, this approach is both expensive and can result in a soil so high in P that nutrient pollution in runoff water becomes a significant problem. Several means of optimizing P availability while using minimal amount of soluble P include: (1) band placement of fertilizer; (2) increasing P uptake by combining P and ammonium fertilizer; (3) using P-efficient plants that do not require high solution P concentrations; (4) increased cycling of organic P through application of animal and plant residues; (5) reduced P fixation by increasing soil organic matter content; (6) maintaining optimal soil pH; and (7) utilization and enhancement of mycorrhizal symbiosis.

Potassium in Plant Nutrition Plants contain nearly as much potassium as they do nitrogen (general 2.5 to 4.5% of dry leaf weight), but almost all of the potassium is in solution in the plant cell rather being a part of essential organic molecules. Potassium serves mainly osmotic functions and enzyme activation functions.

Deficient levels of potassium lead to low rates of many plant metabolic processes, low disease and drought tolerance, late maturity, and, in severe cases, can be diagnosed by the occurrence of necrotic margins on the older leaves.

The Potassium Cycle Potassium is supplied to the soil solution (and hence to plant roots) mainly by mineral weathering and by cation exchange on colloid surfaces. Organic matter mineralization has little effect as potassium readily leaches out of plant residues and so is not a structural component of soil humus.

Certain 2:1 type clays, especially vermiculite, can fix potassium ions in interlayer positions that become inaccessible to normal cation exchange and to root uptake. In some soils mineral weathering can supply potassium fast enough to maintain a sufficient supply of soluble and exchangeable K. In 73 .

other soils, however, continued removal of high potassium crops can deplete the available soil supply faster than natural weather can replenish it. Potassium is not lost from soils as gaseous forms, but both leaching and erosion losses can be substantial.

The Potassium Problem in Soil Fertility Easily exchangeable (i.e., readily available) potassium usually accounts for only a small fraction of the total soil potassium which is principally locked up in the structural framework of soil minerals. Because of the complimentary ion effect, leaching losses can be reduced by liming an acid soil. Although large amounts of potassium can be productively used by certain high potassium demanding crops (e.g., potato or alfalfa), some plants may take up potassium beyond the amount needed for optimum growth, a potentially wasteful phenomenon known as luxury consumption.

Forms and Availability of Potassium in Soils Potassium in soils is distributed principally among four forms, the availability of which varies as follows: 1. Potassium in primary mineral (e.g., micas, feldspars) structures is unavailable.

2. Potassium in non-exchangeable positions in secondary minerals (e.g., vermiculite or colloidal size mica) is only slowly available. This includes what is termed “fixed K.” 3. Potassium on the cation exchange sites of soil colloids is readily available to plants.

4. Potassium soluble in water is readily available and the immediate source of ions taken up by plants.

Only 1 to 2% of the total potassium in soils is in readily available forms at any one time, but the potassium continually distributes itself among the four forms listed above.

Factors Affecting Potassium Fixation in Soils The fixation of potassium in the interlayers of secondary minerals and its release to more available forms is controlled by four factors: 1. The nature of the soil colloids (especially the content of vermiculite and other 2:1 clays).

2. Wetting and drying cycles which cause the clay interlayers to become more exposed, both for the fixation and the release of K. 3. Freezing and thawing cycles, which cause effects similar to those of wetting and drying.

4. The presence of excess lime which decreases the percent potassium saturation as it increases the soil CEC by activating pH dependent charges, and also decreases the ratio of potassium to other cations (especially Ca and Mg) on the cation exchange complex.

Practical Aspects of Potassium Management Where crop harvest removes slowly and readily available forms of potassium more rapidly than mineral weathering can replace it, potassium-containing fertilizers may be needed to replenish the soil potassium reserve and to assure sufficient availability of potassium during periods of rapid plant uptake. Potassium should be applied close to the time of plant need, especially on soils with high potassium fixing tendencies, or very coarse textured soils with high potassium leaching potential.

Complete replacement of all potassium removed in harvest is rarely necessary as advantage should be taken of the considerable inherent potassium-supplying power of many soils. The return of animal manures and plants residues to the full extent possible can greatly reduce the need for fertilizer 74 .

additions, but in many parts of the world increased use of potassium-supplying fertilizers will be necessary as yields and therefore potassium removals increase.

Calcium and Magnesium as Plant Nutrients Calcium and magnesium are both macronutrients essential for all plants. The ability of a soil to supply these two elements to plants is intimately tied to soil acidity (and for agricultural soils, to liming). Plants generally use Ca in amounts second only to N and K. Calcium is the most plentiful cation on the exchange complex of nearly all soils that are not so acidic as to have a high aluminum saturation. For this reason, deficiencies of calcium are quite rare for most plants, except in very acid soils. When calcium deficiency does occur, it is usually associated with growing points (meristems) such as buds, fruits and root tips. Certain plants show Ca deficiency, even when the soil pH is adequately maintained by liming. Such deficiencies are often related to the transport of Ca in the plant.

Calcium in the soil is found mainly in three pools, calcium-containing minerals (such as calcite or plagioclase), calcium complexed with soil humus, and calcium on the clay and humus cation exchange complex. The cycling of calcium among these and other soil pools, and the gains and losses of calcium by such mechanisms as plant uptake, liming and leaching comprise the calcium cycle.

Significant losses of calcium and magnesium occur from the soil by leaching, crop removal and soil erosion. The total loss from all three causes in humid-region agricultural soils, expressed in the form of carbonates, approaches 1 Mg/ha per year.

The deficiency of Mg is much more common than that of Ca, at least when the soil pH is at an appropriate level. The most common symptom of Mg deficiency is interveinal chlorosis on the older leaves. The main source of plant-available Mg in most soils is the pool of exchangeable Mg on the clay-humus complex. As plants and leaching remove this Mg, the easily exchangeable pool is replenished by Mg weathered from minerals (such as dolomite, hornblende and serpentine). In some soils, replenishment also takes place from a pool of slowly available Mg in the interlayers of certain 2:1 clays. Variable amounts of Mg are made available by the breakdown of plant residues and soil organic matter.

Silicon in Soil-Plant Ecology Silicon has been proven to be an essential nutrient for only one genus of plant, Equisetum (horsetails). Yet silicon is taken up in considerable amounts by all plants and has repeatedly been shown to enhance growth and function in a wide range of plant species. Therefore, silicon is sometimes referred to as a beneficial element for plants or a quasi-essential nutrient. For humans and other animals, silicon is known to be an essential nutrient.

The main form of silicon present in the soil solution is H4SiO4 (orthosilicic acid) and this is the form taken up by plants. Silica is found mainly in primary rock-forming minerals and in secondary silicate clay minerals. These minerals weather, with the aid of microbes and plant roots, releasing silicic acid. Plants take up H4SiO4 and transport it into the stems and leaves where it is polymerized and deposited as silica (SiO2). This biogenic silica solidifies into the shape of the tissue cavities in which it is deposited forming phytoliths. These silica bodies are then deposited back to the soil with litter fall or washed into the soil with throughfall. They may slowly redissolve or remain unchanged for millennia, the variable reactivity indicated by the broken arrow from this pool. Considerable silicon is leached out of the soil–plant system as silicic acid and ends up in the oceans.

Even in sandy soils made up largely of silicon (but in the inert form of quartz), plants may respond positively to the addition of relatively soluble sources of this element such as calcium silicates or the mineral, Wollastonite.

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Micronutrients Micronutrients are elements essential for plant growth but are used in amounts smaller than for macronutrients. Generally, the number of atoms or the mass per unit plant dry matter is one or more orders of magnitude smaller than for the macronutrients with leaf concentrations ranging from about 300 mg/kg for Fe to about 1 mg/kg for Mo and Ni. The need for micronutrient management has become increasingly evident as plant removals, purity of macronutrient fertilizers, lab analysis sensitivity, and understanding of micronutrient roles in human nutrition all have increased.

Micronutrient Deficiency versus Toxicity For most micronutrients, the sufficiency range, soil concentrations suitable for optimum plant growth, are not as broad as for the macronutrients. Therefore toxicities, both to plants growing in soils and to humans and animals consuming the plant material, are often of concern with regard to micronutrients. Toxicities of micronutrients (mostly metals) and related non-nutrient heavy metals may be due to natural soil conditions (e.g., extreme acidity, poor aeration, high natural metal content) or to human activities such as contamination of soil with industrial wastes (e.g., Cu, Zn, Ni) or over application of micronutrient fertilizer (e.g., B, Mo).

Deficiencies of micronutrients are most common on acid, highly leached sandy soils (low total content of most micronutrients), organic soils (low total content and complexation with copper), soils of very high pH (except for Mo which is typically deficient at low pH), and soils that have been depleted by intensive cropping.

Roles of Micronutrients in Plants Most micronutrients function as components, or activators of various enzymes that control all aspects of plant physiology. The deficiency of many of the micronutrients produces characteristic symptoms on the plant which can be useful in making a diagnosis. A small dose of a needed micronutrient can often produce a dramatic increase in plant growth and vigor.

Source of Micronutrients Except for Fe and Mn which are typically found in concentrations of several 1,000 mg/kg of soil, the micronutrient content of most soils is seldom above 50 mg/kg. The primary source of the micronutrients is the primary mineral from which the original igneous rock was formed, but many micronutrients are also found in secondary silicates, in association with iron and manganese oxides, and as exchangeable cations. Organic forms are also important, especially for copper, nickel, and boron. Borate and molybdate undergo anion adsorption reactions. In the soil solution, iron, zinc, copper, manganese, cobalt, and nickel are found as simple cations, while boron, molybdenum, and chloride exist as dissolved anions.

Factors Influencing Availability of the Micronutrient Cations The cationic micronutrients (iron, manganese, zinc, copper, nickel, and cobalt) each undergo unique reactions in soils, but the availability of all is influenced by soil pH and oxidation state, their solubility generally increasing as pH is lowered and, for iron and manganese, as conditions become anaerobic. Overliming commonly leads to deficiencies of one or more of these micronutrients.

The cationic micronutrients react with certain organic molecules to form organo-metallic complexes called chelates. If these complexes are soluble, the chelation increases the availability of the micronutrient metal and protects it from precipitation reactions. A chelate is an organo-metallic 76 .

complex in which two or more active groups on a single organic ligand are coordinated with a single metallic cation. The chelating agents in soils may be natural compounds produced by decomposer microorganisms or excreted by roots, or they may be synthetic chelating compounds added to enhance micronutrient availability. An effective chelate is one for which the affinity between the organic compound and the metal is greater than competing affinities with other metals in the soil environment.

EDTA and EDDHA are examples of synthetic chelating agents that become less effective as the soil pH increases.

Mycorrhizal fungi acting symbiotically with higher plants have been shown not only to increase micronutrient uptake from deficient soils, but also to decrease the uptake of potentially toxic metals like copper and zinc from soils with unusually high levels of these micronutrients.

Factors Influencing the Availability of Anionic Micronutrients Chlorine, boron, and molybdenum each undergo unique reactions that govern their availability in soils. Chlorine is present in soils mainly as the chloride ion which is leaches quite freely with the drainage water. Chloride may accumulate to toxic levels under arid, especially irrigated conditions, but it is rarely deficient as large amounts are added to soils with precipitation and fertilizers.

Boron is present primarily in the form of undissociated boric acid. It is among the most commonly deficient of the micronutrients. It is most available in acid soils, but is also easily leached from acid sandy soils and so is often deficient on this type of soil. Lime-induced boron deficiency can occur above pH 7 as the adsorption of this element to both clay and humus increases as pH increases.

Molybdenum availability is mainly a function of soil pH, it being the one micronutrient which becomes most available at pH values above neutral, and becomes quite insoluble in acid soils. Liming acid soils usually alleviates Molybdenum deficiency. Molybdenum availability seems to be improved by the presence of phosphate ions, but decreased by sulfate ion.

Need for Nutrient Balance The uptake and function of many micronutrients are influenced by the availability of other nutrients, both macro- and micro-. In some cases the effects are synergistic, that is plant growth improvement from the combination of nutrients together is greater than the sum of the growth responses to each individual nutrient. For instance, copper utilization is favored by adequate manganese, while, in turn, some plants need both zinc and phosphorus for optimum use of manganese.

Negative interactions, or antagonisms, also occur. For example, manganese and iron can interfere with each other’s uptake and utilization, as can zinc and copper.

Soil Management and Micronutrient Needs Although each micronutrient requires a specific management practice, micronutrient deficiencies are most common on leached sandy soils, mucks, and soils having very high or very low pH values. In very acid soils, toxicity of manganese and iron , as well as deficiency of molybdenum (and the macronutrient phosphorus) are to be expected. Calcareous and high pH soils are prone to deficiency of iron, manganese, copper, and zinc, while molybdenum may be toxic. Maintenance of soil pH in the range of 6.0 to 7.0 usually avoids most, but not all, pH- associated micronutrient problems.

Flooding and poor soil aeration can increase manganese and iron availability, even to the point of toxicity in some soils.

Micronutrient deficiencies are unlikely to be a problem where soil organic matter is maintained by liberal additions of plant and animal residues and manures. These organic amendments improve micronutrient availability because they contain significant quantities of these nutrients and because they form chelating agents that improve the availability of the supplies already in the soil.

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Where inorganic micronutrient supplements are needed, they are most effectively applied either as chelates to the soil or as foliar sprays. However, in some cases, especially where soil pH is conducive to good availability, addition of somewhat larger amounts of inexpensive, inorganic salts may be most economical. Where a micronutrient deficiency has been positively identified, the economic returns from a very modest investment in proper application of small quantities of the needed micronutrient can be very handsome, indeed.

Model Answers to Study Questions 1. See Sections 12.1 and 9.1.

Both are correct, but over different time frames. In the short term the urea is likely to raise the soil pH as it will quickly convert to ammonium carbonate by the action of microbial enzymes in the soil. The released ammonium will react with water (in equilibrium with ammonia gas) to form the strong base, ammonium hydroxide (see Eq. 13.3). Later, nitrification of the ammonium will produce hydrogen ions that acidify the soil. If much of the nitrate produced is not taken up by the grass, the net effect will be acidification.

2. See Section 12.1, especially Box 12.1.

First calculate the amount of organic matter per ha using the 3% SOM given (0.03 g/g) and the assumed 2 × 106 kg of dry soil/ha (see Box 13.1) for the upper 15 cm of a loamy soil. This gives 60,000 kg of SOM/ha. If we assume this SOM is about 5% N, there is 3,000 kg N/ha in the organic matter. Next we estimate the per cent of the SOM that turns over releasing its N in a year.

For a medium coarse textured soil like a sandy loam this is likely to be about 3% (depending also on the climatic region and soil wetness). Using the 3% we get 0.03 × 3,000 = 90 kg N/ha mineralized in one year.

3. Sections 12.1 and 12.2.

Both N and S compounds deposited on agricultural land might be viewed as beneficial because they would help meet the nutrient needs of crops and could the use of fertilizers to be reduced accordingly. The amounts of N and S deposited are rarely greater than the annual removal of these nutrients in harvested crops. However, the situation for land in natural vegetation is quite different.

Forests are especially susceptible to damage from deposition of N and S. The amounts deposited come on top of these release and cycled by soil processes and usually cause an overload of these nutrients in the system. The deposition also causes acidification with long term highly detrimental results to soils, trees and drainage waters.

Benefits of added N and S: Can erase the deficiencies of these essential nutrients that are widespread (especially N) in domesticated and wild plants throughout the world. Can encourage the maintenance or even increase of soil organic matter of which N and S are essential constituents.

Negative effects of too much N and S: Oxides of both N and S emitted to the atmosphere from industrial, transport, domestic, agricultural and natural sources return to the Earth in acid rain, and damage plants (especially native species), erode buildings and acidify soils. Excess nitrates leach from soils into streams, lakes and other water bodies (even into drinking water), stimulating eutrophication and posing potential human health problems.

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4. See Section 12.1 and Figure 11.6.

As the microbes begin to digest the wheat straw which has a C/N ratio of perhaps 105/1 (assuming about 42% C as per Section 12.2 and given 0.4%N) they multiply very rapidly, and in doing so must absorb the soluble soil N to meet their body tissue requirements for N. After a 75% loss of C by respiration, they will assimilate about 210 kg of C/ha from the straw (2,000 × 0.42 = 840; 0.25 × 840 = 210), a process that will require about 210/8 = 26 kg N/ha (microbial tissue assumed to have a C/N = 8/1). Since the straw itself contains about 8 kg N/ha (0.004 × 2,000 = 8), the microbes will removed the remaining 18 kg of N (26 − 8=18) from the soil solution, depleting most of the 25 kg N/ha originally present there.

5. See Section 12.1 (and Section 13.4).

Excess nitrates coming from mountainous stock piles of manure near concentrated animal production “factories” can contaminate groundwater, resulting in undesirable eutrophication and human health problems as the nitrates move into underground water supplies, streams, and downstream lakes and ponds. Usually, the manure is eventually spread on nearby lands and the rates applied are often 5–10 times those that good nutrient management would dictate. Smaller animal production units are more likely to be able to spread animal wastes on fields at rates in harmony with good crop management practices. In any case, protection of the environment may require that animal wastes be handled under the same kinds of public guidelines in force for any other potential pollutant.

6. See Section 12.1.

N losses from forested areas are generally much smaller than those from agricultural areas for two basic reasons: (1) Total additions of N (from natural and human sources) to forest lands are small compared to those applied to agricultural lands. (2) Plant uptake of N throughout the year, and at significant soil depths is greater in forested areas than in cultivated areas. Consequently, nitrate loss from forests is generally quite low.

7. See Section 12.1 and Figure 12.2, especially the blue arrows which represent anaerobic processes.

Compared to an upland soil, a wetland soil would process the N in leaf litter quite differently: initial decay and ammonification—the release of NH4+ from the amine groups of protein—would proceed, but more slowly. The ammonium would be more subject to volatilization if the pH was 7 or above, but nitrification would be inhibited by the lack of gaseous oxygen, so little nitrate would form. Any nitrate that did form would be immediately denitrified and thus reduced to N2 or NOx gases and lost from the soil. Also, anammox reaction could oxidize both ammonium and nitrites to NOx gases.

8. See Sections 12.2 and 9.6. a. Acid rain has been acidified in the atmosphere by strong acids such as HNO3 and H2SO4 that have been formed from the reaction with water of oxides of nitrogen and sulfur that have been released into the atmosphere from internal combustion engines, and from other industrial, domestic, agricultural and natural sources. The pH of acid rain may be as low as 4 or lower compared to the range of 5.6 to 67 for normal humid region rainfall. The acidity (especially acid cations nitrate and sulfate) lower soil pH, mobilize Al, replace and deplete Ca in receiving soils, especially those with sandy textures and low buffering capacity. b. Acid rain is most damaging in forested areas since limestone cannot be easily and economically applied to combat the acidity in forests.

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9. See Section 12.1 and Figures 12.2 and 12.19.

Use of a nitrification inhibitor would reduce leaching and denitrification losses, giving greater likelihood that the fertilizer will be more fully used. In soils where leaching losses are anticipated, such as on sandy soils and in rainy weather patterns, retaining more of the fertility being applied will have value. Similarly, in waterlogged soils, such an inhibitor would limit denitrification losses.

10. The level of any nutrient in a soil is determined by the balance between the inputs to and outputs from the soil. While the inputs of nutrients into agricultural soils greatly exceed those into forest soils, their removal from agricultural soils by accident or design greatly exceed those from forested lands. Crop removal, leaching and denitrification losses are far greater in cultivated areas.

The result is often a decline in total nutrient contents of agricultural soils and a stable or increasing level in forested soils.

11. See Section 12.2.

Three major trends have significantly increased the incidence of sulfur deficiencies on agricultural soils: a. Environmental policies have drastically reduced the emissions of SO2 into the atmosphere from fossil coal and oil, simultaneously reducing the quantities of S-deposition. b. Lower cost, more highly concentrated nitrogen and phosphorus fertilizers have replaced lowanalysis fertilizers that contained significant quantities of sulfur-bearing impurities, thereby decreasing the amount of S being applied to agricultural soils. c. Increased crop yields have resulted in larger rates of removal of sulfur from soils around the world.

12. See Section 12.1, especially Figure 12.10.

Riparian forests reduce groundwater nitrates by two processes: a. uptake of some of the groundwater nitrates by the forest trees, thereby reducing levels moving into streams. b. denitrification loss of nitrogen stimulated by organic compounds moving down from the forest litter, and by the anaerobic conditions prevalent in the soil near the stream.

13. See Section 12.2 and Figure 12.26.

Potential acid sulfate soils are commonly found in coastal wetlands rich in sulfur-bearing materials (e.g., iron sulfides and elemental S) which, if the soils are drained, are oxidized resulting in the formation of H2SO4. These soils may be extremely acid with pH values as low 1.5. Drained acid sulfate areas should be allowed to revert to their wetland status and not used for agriculture, whereupon reducing conditions will dramatically lower the acidity. Very large quantities of limestone must be applied to reduce the soil acidity of exposed acid sulfate soils.

14. See Sections 12.1, 12.3, 12.4 and Figures 12.11, 12.38, and 12.49.

Nitrogen fixation is the biological or chemical conversion of atmospheric nitrogen gas into NH3 or amino compounds usable by plants and microorganisms. Phosphorus fixation is the reaction of phosphate ions with the surfaces of Fe, Al oxides and silicate clays (acid soils) and with calcium 80 .

compounds (alkaline soils) to form quite insoluble and unavailable forms of this element.

Potassium fixation is non exchangeable K held in cavities in the interlayers of silicate minerals such as vermiculite and released only slowly to exchangeable forms.

15. See Sections 12.3, especially Figure 12.29. a. In Oxisols the P is fixed in insoluble forms by kaolinite clay and Fe, Al oxides, Al(OH)2H2PO4 being an example. b. In Aridisols the P is fixed in insoluble forms by calcium compounds, Ca3(PO4)2 being an example.

16. See Section 12.3 and Box 13.1. a. Eutrophication involves the enrichment of waters entering lakes and ponds with excessive N and P which stimulates the massive growth of unwanted aquatic plants, including algae. The algae utilize much of the dissolved O2 in the water for their own metabolism, leaving the water so devoid of O2 that fish and other oxygen-requiring organisms cannot survive. b. Excessive applications to soils of P-containing fertilizers, manures, and sewage sludges, and the transfer by runoff and erosion of the added P to lakes, ponds, etc. should be avoided.

17. See Sections 12.3 and 12.4 and Figures 12.39 and 12.50.

Since clay soils generally have higher P and K contents than sandy soils, everything else being equal, the ability of clay soils to sustain acceptable available P and K levels when the soluble nutrients are removed (buffering capacity) would be higher than that of sandy loams.

18. See Section 12.4 and Figure 12.46.

The exchangeable and soil solution forms of K were probably replenished by weathering and release of the non-exchangeable forms of this element in minerals like illite clay or mica.

19. See Section 12.3, Table 12.4, and Box 13.1.

Some soluble organic and mineral forms of P in soils move with the runoff and drainage waters into streams, lakes etc. Much more P is lost as part of the eroded fine sediment that is carried by the runoff water into downstream water bodies.

20. See Section 12.3.

Bringing a forested soil under cultivation (especially if tillage is used regularly) can be expected to result in a reduction of soil organic matter content and release of P into inorganic forms that crops will take up and that will subsequently be removed with harvest. If microbes decompose low P plant residues that consequently have a high C/P ratio, there is insufficient P to meet the microbial metabolic requirements, making it necessary for the microbes to take up P from the soil solution, and thereby depriving the plants needed P. This situation is the same as pertains for nitrogen deficiencies noted when plant materials with high C/N ratios are added to soils.

On the other hand, if P fertilizers are used, as is common in most modern crop farming, the total P content of the soil may increase over time, especially if more total P is applied than is removed in harvests. In no-till agriculture, the P is likely to accumulate in the surface few cm of soil.

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21. See Sections 12.1 and 12.3.

For a farm growing corn, soybeans, and wheat in a rotation, nitrogen fertilizer could be managed according to the four R’s in ways such as these: (a) Right source. Use slow-release fertilizers or fertilizers with compounds that inhibit urea hydrolysis or nitrification so as to keep the fertilizer in a form that is less mobile and delay the formation of highly mobile nitrate. (b) Right rate or amount. Use N fertilizer only as a supplement, if needed, after considering N from organic matter decomposition, from manure, from legume cover crop residues, etc. Apply only the amount that research shows is necessary for economic optimum performance assuming a realistic yield goal.

The N responses should be interpreted with a plateau type of equation so that no more N is be applied than what gives a statistically significant increase in profitability. (c) Right time. The N should be applied as close as possible to the time when the crop needs to take it up. Nitrogen should not be applied mainly at the beginning of the season or, even worse, far ahead of the season such as fall application. Instead, only a small amount of N should be applied at planting time with most of the N applied in one or two applications during the season just before the crop reaches the exponential growth phase. By that time it is possible to know the probable trajectory of both crop growth and nitrogen availability for that soil and season. (d) Right placement. The N fertilizer should be injected or soaked a few cm into the soil so that it doesn’t volatilize into the atmosphere or run off the surface. It should be applied near the crop plants so that its availability to weeds is minimized while availability to the crop is maximized.

Similar considerations can be applied to P fertilizer. (a) Right source. Phosphorus fertilizer should be applied from a source that will be available during the time frame that the crop needs it. (b) Right rate. If animal manures are applied, it is highly unlikely that P fertilizer is also needed. Cover crops can bring P up from deep in the profile and make it available near the soil surface. Mycorrhizal fungi should be encouraged to also assist plants in utilizing existing stores of P in the soil. Fertilizer should only be used as a supplement to such processes. The rate of application should be only enough to provide maximum profitability. If test strip without P application do not show statistically lower performance, then P is probably is not needed. Many agricultural soils have accumulated enough P to grow profitable crops for years or even decades without further P applications. (c) Right time. The most effective time to add small amounts of phosphorus to fertile soils is usually at planting time.

Phosphorus is much less mobile in the soil than N and so timing is less critical for losses. (d) Right placement. Fertilizer P should be added in concentrated bands in which the fertilizer will not be as susceptible to P fixation reactions in the soil. These bands should be a few centimeters away from the crop row and up to 20 cm deep. Broadcasting P on the surface of the soil is inefficient and makes it likely that much P will be lost in surface runoff causing eutrophication of natural waters.

22. See Sections 12.5 and 12.6. a. Because Ca is relatively immobile in the plant – especially in the phloem, it become most deficient in growing points, newly expanding leaves, roots, fruits and other plant parts not readily supplied by the xylem transpiration stream. Because Mg is much more mobile and plants can move it from older to newer growing tissues, the deficiency symptoms of Mg occur mainly on older leaves.

23. See Section 12.7. a. Phytoliths are small particles, generally silt-sized. They form as biogenic silica which solidifies into the shape of the cavities into which it deposited. They retain their shape and silica bodies, and return to the soil with the death of the plant. The serve in some plants to make tissues more erect or resistant to insect and pathogen invasion..

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24. See Section 12.8 and Figures 12.55 and 12.57.

No. As shown in Figure 12.55 if too much of any given nutrient is added, it may become toxic, causing adverse physiological reactions within the plants that may lead to death.

25. See Section 12.8.

The Fe in highly alkaline soils is very insoluble and unavailable to the fruit trees. The presence of bicarbonates in irrigation waters may encourage removal of Ca from the soil solution (as CaCO3), thereby increasing the Sodium Adsorption Ratio (SAR) and the pH. This would even further reduce the availability of the Fe. The iron deficiency can be ameliorated by applying chelate forms of Fe, either to the soil or by spraying the tree foliage; or by applying elemental sulfur to the soil, which when it is oxidized by soil microbes, will form H2SO4 that will decrease the soil pH and increase the Fe availability.

26. See Section 12.8, especially Figure 12.66.

Chelates are organo-metallic complexes in which metallic ions are firmly bound by large organic compounds that protect these ions, including several micronutrients, from reactions in soils that may render these nutrients unavailable for plant absorption.

The chelates act as carriers that either release the nutrients at the root surface where they are absorbed, or that actually enter the root, release the nutrient inside, then return to the soil. Chelates are present naturally in soils, but several commercial synthetic chelates are available for application, either to the soil or to the plant foliage.

27. See Section 12.8.

Probably Mo for two reasons: a. Most other micronutrients would require a larger application rate. b. Mo is the only micronutrient that is rendered unavailable at low pH levels.

28. See Section 12.8.

Animals are affected by deficiencies and toxicities of Mo (a micronutrient for both animals and plants) and Se (a micronutrient for animals, but not for plants). In both cases, the toxicities are most apt to occur where the soils are neutral to alkaline, and deficiencies are more prominent in acid soils. Under acid conditions, the relatively unavailable selenite form of Se is present whereas the more soluble selenate dominates at neutral pH and above. At low pH, Mo is bound strongly to Fe, Al oxides and silicate clays, forms that break up at higher pH levels.

29. See Section 12.8.

By selecting or breeding for tolerance to deficiencies or toxicities of trace elements, plant breeders and geneticists can (a) provide farmers with cultivars that will produce well without having to change the soil, and (b) provide environmental engineers with plants that can remove unwanted chemicals from the soil.

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30. See Section 12.8, especially Figures 12.57 and 12.60.

Species of plants differ in their requirements for nutrients. What may be an adequate level of a given nutrient for one species may be an inadequate level for another. Beets, like most dicots, have a high requirement for boron, so that if this requirement is met, the boron level in the soil may become so high that it is toxic to oats, which like most monocots, requires less B and is more sensitive to B toxicity.

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Chapter 13. Practical Nutrient Management Overview Goals of Nutrient Management A balanced approach to nutrient management will address four broad goals: 1. Cost effective production of high-quality plants (in agriculture, forestry, and ornamental landscaping) and animals, 2. Maintenance or enhancement of soil health, from nutrient-cycling and supplying capacities, to physical properties, as well as biological functions and diversity, and the avoidance of chemical toxicities.

3. Efficient use and conservation of nutrient resources, especially non-renewable deposits such as phosphates. Imports of nutrient-rich feed for concentrations of livestock give some regions a surplus of nutrients, particularly N and P. 4. Protection of the environment beyond the soil, especially the quality of water above and below ground.

Nutrient Management Plans

Environmental Quality As regards nutrient management to help protect water quality, specific practices can be recommended for various circumstances. The goal of these practices is to minimize the loss of N and P from land to water below a total maximum daily load, reducing accelerated eutrophication. Nitrogen primarily moves to waterways leaching in the form of dissolved nitrate in drainage water.

Phosphorus mainly moves to waterways in the form of P sorbed to sediment particles eroded off the land surface.

1. Field, farm, and regional scale nutrient budgeting can help avoid importing more nutrients, especially N and P, than can be removed by intended exports. Inputs of N and P in feed, fertilizer, animal manures, crops residues and atmospheric deposition should all be taken into consideration.

Excess nutrients too often end up in sensitive water resources. Reduced nutrient inputs or improved recovery and recycling of by-products may be called for. Nutrient surplus (added minus harvest removal) can be an indicator of environmental and economic wastefulness.

2. Riparian buffer strips are an effective means of preventing nutrients and sediment in overland flow from leaving the field and reaching water courses. The necessary width of vegetated buffer may vary from about 6 to as much as 60 m.

3. Cover crops are crops planted in a field, but not harvested. Killing the cover crop allows recycling of nutrients for use by the main crop. They also filter nutrients and sediments from runoff as well as take up nitrate that might otherwise leach down to the groundwater.

4. Nutrient losses from cropland can be substantially reduced, and soil quality protected, by the use of various types of conservation tillage. Generally, the less the soil surface is disturbed, the lower the loss of nutrients in runoff.

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Cover Crops Protecting the soil from erosive forces, cover crops during the “off season” can increase biodiversity on the farm, and add to the soil organic matter. By taking up nutrients with extensive and deep roots, cover crops can also reduce leaching losses. Winter annual legumes may fix enough N to supply a significant part of the needs of nitrogen-responsive crops such as corn and tomatoes.

Recycling Nutrients through Animal Manures The United States farm animal population produces about 10 times as much organic wastes as does the human population. Environmental problems occur when animals are raised in such concentrated operations that it is uneconomic to apply all the manure at rates in keeping with crop uptake.

Nutrient Composition of Animal Manures Animals excrete, in their feces and urine, most of the mineral nutrients in their feed. Manure contents are highly variable, so rates of application should be based on data from lab analysis of manure samples. On a dry weight basis animal manures contain from 2 to 5% N, 0.5 to 2% P and 1 to 3% K. They also contain from 30 to more than 90% water, so transport of fresh manure is costly on a per unit of nutrient basis. Depending on the type of manure and storage conditions, about 30 to 50% of the organic N in manure is expected to become plant available in the year of application.

Storage, Treatment, and Management of Animal Manures Manure from animals raised on grazing land or in small concentrations is relatively easy to recycle on the land from which the animals’ feed was derived, thus maintaining reasonable nutrient balance. For larger concentrations of animals, manure storage and handling may be by: 1) daily spreading, 2) fermentation in storage piles, 3) aerobic liquid, or 4) anaerobic liquid. Nutrient losses are especially high from anaerobic lagoon and pile storage, largely because of denitrification. The land base for the use of this manure can be challenging to find without causing environmental pollution.

Three relatively new methods of manure treatment which aim to make the manure a commercially viable by-product are: 1) heat-drying and pelletizing, 2) pyrolizing and forming a biochar, 3) commercial composting, and 3) anaerobic digestion with biogas production.

Industrial and Municipal By-products Four main types of organic wastes that can be disposed of in the soil while at the same time improving soil properties are: 1) municipal garbage (especially after composting); 2) sewage (both treated sludge and effluent); 3) food processing wastes; and 4) timber industry wastes.

Irrigation of cropland and forests is an effective means of sewage effluent disposal and use, providing both water and nutrients to vegetation. Sewage sludge, the solid by-product of wastewater treatment, contains beneficial nutrients and organic matter, but can also contain harmful levels of heavy metals and other toxins. The toxic content of sewage sludge has been greatly reduced since the 1970s, so heavy metals are not as great as problem as they once were. However the use of this material on farmland continues to be closely regulated, the rates of application generally being limited by the need to avoid excess nitrogen application. Of the organic N in sludge, from 10% (for composted sludge) to 40% (for activated sludge) can be expected to be released in the year of application. Some cities are developing and marketing commercial fertilizers from their composted sludge, but most is applied as liquid materials.

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In general, nutrients from organic sources will produce the highest return and cause the least environmental damage if applied to soils that are relatively poorly supplied with N and P and which are supporting virtually year-round vegetation.

Integrated Recycling of Wastes may make efficient use of animal and human waste products through land application and other uses such as biogas production and aquiculture.

Inorganic Commercial Fertilizers Most fertilizers are inorganic salts that contain one or more plant nutrient elements. Most are readily soluble in water and make their nutrients available for plant uptake almost as soon as they are applied to the soil. Fertilizers contain a higher and more precisely defined content of nutrients than do most organic materials. The primary fertilizer elements are N, P and K, although some fertilizers also supply S, Mg and micronutrient elements. A given nutrient may be supplied by various carriers which differ with regard to salt hazard, acid-forming tendency, solubility and tendency to volatilize.

The fertilizer grade consists of three numbers that stand for the percentages of total N, available phosphoric acid (expressed as P2O5) and soluble potash (expressed as K2O). To convert to the elemental content from the latter two forms of scientifically incorrect commercial forms of expression, the following factors are used: % P = % P2O5 × 0.44

% K = % K2O × 0.83

The nutrients in inorganic fertilizers added to soils generally become involved in the soil’s biogeochemical nutrient cycles, only a small portion directly enters the plant. Fertilizer efficiency tends to decline as rates of application increase.

The Concept of the Limiting Factor “The law of the minimum” states that plant production can be no greater than that level allowed by the growth factor present in the lowest amount relative to the optimum amount for that factor. As a consequence, for example, if P is limiting, adding more N will do little good. Conversely, in this situation, adding more P may allow the plant to respond better to further additions of N, thus producing a positive interaction of or synergy.

Fertilizer Application Methods Broadcasting involves spreading the material evenly over the entire area to be treated and is usually convenient but not very efficient in terms of the amount of nutrient that must be applied. It may be useful if the goal is long-term increases in fertility. A form of broadcasting is fertigation or application of nutrients dissolved in irrigation water. Localized placement methods include banded starter applications of solid and liquid fertilizers, dribble-on applications, knife injection or point injection, drip fertigation, and perforation methods (the latter used mainly for individual trees). Foliar sprays are very effective for micronutrients in many situations, but have a limited capacity to supply macronutrients because of salt hazards.

Timing of Fertilizer Application Timing of fertilizer applications in the field is governed by four basic considerations: 1) making a sufficient amount of the nutrient available when the plant needs it; 2) avoiding excess availability, especially of N, before or after the principal periods of plant uptake (especially during environmentally sensitive periods of groundwater recharge); 3) making nutrients available when they 87 .

will strengthen, not weaken, long-season and perennial plants; and 4) feasibility of conducting field operations.

Diagnostic Tools and Methods The field diagnostician must use all available information, including field observations, plant tissue analyses, and soil tests, to make sound recommendations and solve the puzzling problems that commonly arise in complex plant-soil systems. Organized, recorded observations should be made on soil variation, and plant symptoms (both above and below ground), including spatial and temporal changes. Deficiencies may produce recognizable symptoms. Management histories should be obtained, and a sketch map drawn.

Plant Analysis and Tissue Testing If samples of a specific plant part (e.g., petioles, newly opened leaves, etc.) are taken at a specific time (e.g., a first flowering), and the tissue analyzed for nutrient element content, the results can be compared to values known to be in the sufficiency range or the critical range for the particular plant species. The balance among nutrients must also be considered in interpreting the tissue data. Samples in the field can be measured for leaf “greenness” using near infrared light.

Soil Analysis Obtaining a representative soil sample is one of the most critical steps in the soil testing process. The sample should be a composite of 15 to 20 subsamples taken throughout the area to be represented, but not from unusual spots that might bias the sample. The depth to sample varies with the type of plant-soil system, but standard soil testing usually calls for a depth of 15 to 20 cm or the depth of tillage. Nutrient availability various with time of year, so tracking soil fertility over time is best accomplished by taking samples at a consistent time of year. Special tests for available N require very specific timing. The chemical analysis of the soil determines only a portion of the total nutrient content that correlates well with what plants can use over a growing season. Soil pH and salinity measures are also routinely determined by soil testing. Soil test results are generally only semiquantitative, being used to class the soil as high, medium, or low in particular nutrients. A rating of "low" indicates a good chance that the addition of that nutrient will result in a positive plant response.

Next to obtaining the sample, the most difficult part of soil testing is interpreting the results and making a recommendation for fertilization bases on these interpretations. The sufficiency level criterion for recommendation has proved much more economic than maintenance or cation balance approaches. Interpretations of soil test results are more reliable for major crops and for P, K, and Mg, than for minor crops or forests, and for N, S, Zn, Fe, Mn, and other micronutrients.

Site Index Approach to P Management Industrial agriculture practices have led to the over fertilization of many soils with phosphorus to the point that P losses from agricultural land has become an important non-point source of water pollution. The risk that a particular field or site might present a serious P pollution problem depends on the amount of P available on the site and the likelihood of this P being transported from the site to a sensitive receiving water body. The P site index is a numerical approach to estimating the degree of pollution risk that integrated both site transport characteristics (slope, erodibility, proximity to a stream or lake, presence of buffer strips) and site P source characteristics (soil test P level, amount and method of P application, manure or other organic waste application).

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Field Calibrated Recommendations Sound recommendations are based on response curves from many site-years of field data, but the interpretation of these curves is subject to many uncertainties, including the question about which mathematical model is most appropriate. Some models tend to result in much higher fertilizer rate recommendations and accompanying environmental risks than do others, without much evidence that the higher rates are actually producing a benefit. Complex computer models are being developed to determine more accurate predictions with weather data and soil and plant processes. However, fertilizer nutrients will continue to be an essential factor in enabling the world to produce the food and fiber needed by a growing population with rising expectations.

Model Answers to Study Questions 1. See Section 13.2.

Trees and grasses in the riparian zone alongside the stream absorb nitrates from the groundwater as it moves laterally toward the stream. Also, anaerobic conditions probably prevail in the soil in the riparian zone, making the nitrates subject to denitrification, thereby removing them from the groundwater.

2. See Section 13.2.

You would want a non-legume, since it must obtain all of its N from the soil, thereby making it a better forager for this element. You also want a species that grows rapidly in fall and whose roots penetrate deeply early-on. Non-legumes such as brassicas and some grasses are generally more aggressive and deeply rooted during early growth stages than their legume counterparts.

3. See Section 13.2 and 13.3.

Clear cutting of forests where all trees are removed from large tracts of land commonly results in accelerated losses of nitrates, especially slash is pushed into windrows and if herbicides are used to suppress unwanted vegetation during reforestation. Slightly higher soil temperatures and moisture levels accelerate the decomposition of the leaves and branches, and of the microbial biomass remaining in and on the land. Few plants are available to take up the nitrates generated by the mineralization of N in plant residues, microbial biomass and soil organic matter, so the nitrates are removed in the runoff and drainage waters. Selective cutting practices that leave significant numbers of trees on the land at all times could help prevent excessive nitrate losses, but would require more frequent entry into the stand and therefore more frequent and extensive road construction, a major source of soil erosion and nutrient loss.

4. See Section 13.2 and 13.3.

Forest fires convert some N and S and a little P to gaseous forms that are lost into the atmosphere.

Elements such as Ca, Mg, K, and P are left in the ashes, increasing their short-term availability.

Forest fires may result in nutrient pollution or toxicities in two ways: • • •

When heavy rains follow a forest fire, the concentrations of soluble P (and to a lesser extent, soluble N) in the fire ash are susceptible to being carried by surface runoff into streams.

Streams could be polluted from N and P in fire retardants such as diammonium phosphate and ammonium sulfate that are applied in large quantities to fight the fires.

Toxic quantities of boron applied in borate fire retardants remain in the soil and constrain plant growth for years after the application.

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5. See Sections 13.2-13.3 and chapter 12.

Fertilizer Element

Resource Conservation Issues

Environmental Quality Issues

Nitrogen

Systems that obtain plant available N through biological N fixation conserve finite supplies of fossil fuels. Ready use of mineral nitrogen sometimes encourages mismanagement of organic matter and organic waste resources.

Excessive amounts or ill-timed applications of N can lead to leaching losses of nitrate that contaminate groundwater, and runoff and leaching losses that accelerate eutrophication.

Excessive N application also accelerates the N cycle and releases nitrogen oxide gases that damage the ozone layer.

Phosphorus

The supply of P in the world is very Phosphorus is rarely toxic, but it limited and there is no substitute addition to oligotrophic aquatic possible for living things. Therefore systems can set off accelerated every effort must be made to recycle it eutrophication causing serious water as efficiently as possible, avoid wastage quality and ecosystem degradation. of this vital resource and to minimize Every effort must be made to avoid the dispersal of usable concentrations buildup of excessive levels of P in of P. soils, and to minimize the transport of P from high P soils to sensitive water bodies.

Potassium

The useable supplies of K, while finite, are very much larger than those of P, and the production of K fertilizer uses far less energy than making N fertilizer.

Potassium is rarely toxic, and is not a limiting nutrient for most aquatic systems, so it is not a pollution issue unless in such high concentrations as to present a soluble salts problem.

Excessive use of K fertilizer can cause impair the nutritional quality of forages if it reduces their calcium and magnesium contents.

6. See Section 13.7.

It is best to first consider the nutrient supplied by a multi-nutrient fertilizer material and not by a single nutrient material, therefore in this case we begin with the P. From Box 13.3 we see that “1846-0” means that the material contains 20% P2O5 which is really 8.8% P. The manager needs 20 kg of P per ha for 10 ha = 20 × 10 = 200 kg of P needed. With 8.8% P in the diammonium phosphate (called DAP for short), this will require 200 kg P / (0.088 kg P/kg DAP) = 2,273 kg of DAP. The DAP also contains 18% N, so this amount of DAP will supply 2,273 kg DAP × (0.18 kg N/kg DAP) = 409 kg N. The 10 ha of turf requires 10 ha × 60 kg N/ha = 600 kg N. Therefore we still need to supply 600 – 409 = 191 kg N from the urea. Since the urea contains 45% N, we will need 191 kg N/(0.45 kg N/kg urea) = 424 kg of urea. In summary, 2,273 kg of DAP + 424 kg of urea will be blended and applied to the 10 ha of turf.

7. See Box 13.6.

From Box 13.6 we see that “20-20-10” means 20% N, 20% P2O5 and 10% K2O. Since the hypothetical P2O5 contains only 44% P, we multiply 20 × 0.44 to give 8.8% P in the fertilizer.

25 kg/bag × 0.088 kg P/kg = 2.2 kg P in the bag of fertilizer (equivalent to 5 kg P2O5).

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8. See Sections 13.6 and 13.7.

Organic and inorganic (i.e. mineral) fertilizers each have advantages and disadvantages. The advantages of organic fertilizers include: (1) the fact that most would be waste products if not used (resource conservation and nutrient recycling); (2) slow release characteristics that may avoid undue concentration of soluble nutrients early in the season before plant use peaks; (3) most contain sufficient micronutrients to alleviate any deficiencies of these elements that might not be recognized; (4) organic fertilizers add organic carbon which may provide many benefits (microbial activity, structural stability, water storage, CEC, etc.) that the supply of nutrients by themselves would not. The main disadvantages of organic sources are (1) low concentrations of nutrients require large bulk of material to be transported and handled; (2) release of nutrients into available forms is not precisely controlled or predicted; (3) materials are highly variable in composition so exact application rates of nutrients may not be known (4) nutrients may continue to be released after the crop season is complete, leading to a leaching hazard (5) some organic materials may carry unwanted impurities and/or pests such as human disease pathogens in sludge or manure and weed seeds in some animal or green manures. Among the advantages of mineral fertilizers are (1) easy and convenient to apply because of physical form and high concentrations of nutrients; (2) predictable rate of nutrient release in soil. Disadvantages of mineral fertilizers include (1) provide high concentrations of soluble nutrients which may be susceptible to leaching; (2) salt hazard may burn young plants; (3) heavy use may inhibit or reduce beneficial microbial activity such as nitrogen fixation, earthworms, etc.; (4) use depletes limited mineral resources, especially P; (5) manufacture requires large amounts of energy, especially for N fertilizer.

9. See Section 13.6 and Box 13.2. (a) The nitrogen availability in the organic farmer’s compost is 0.015 kg/kg N content x the 0.10 kg/kg availability in year 1 = 0.0015 kg available N/kg compost. Therefore to supply the needed 120 kg of available N, the amount of compost needed would be: 120 kg avail N/0.0015 kg avail N/kg compost = 80,000 kg compost (80 metric tons!). Similar calculations for the amount of compost needed to supply the 20 kg of available P needed show: 20 kg avail P/0.0011 kg avail N/kg compost = 18,181 kg compost (18 metric tons). If she uses the compost to supply all the N she needs, she will be applying more than 4 times as much P as needed and will soon cause an environmentally damaging buildup of P. (b) If her soil were already at the optimal level of soil test P, then she might have to turn to N-fixing cover crops to provide N without also adding more P. 10. See Section 13.8 and Figure 13.36.

Limiting factor concept: plant growth is constrained by the essential element (or other factor) that is most limiting. Other nutrients (or factors) will not be effectively utilized by plants so long as the availability of the most limiting factor is constrained. Furthermore, supplying excessive amounts of one element may depress the utilization of other elements. These facts stress the need to supply nutrients in a balance that satisfies the needs of the plants.

11. See Sections 13.2, 13.3, 13.12, and 13.13.

In forested areas, most of the time all of the plant residues are deposited on the land. Nutrients from deep down in the profile are recycled on to the soil surface by the trees. Except when the forests are harvested, forest systems provide an ideal system of recycling nutrients.

In agricultural areas, most of the plant biomass and the nutrients it contains is harvested and removed from the land. Most of these harvested nutrients do not find their way back to the land, thereby removing them from the nutrient cycle. Replacements for these lost nutrients are applied in manures and other organic wastes, lime and commercial fertilizers, not always at rates and times that coincide with plant needs.

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12. See Section 13.11 and Figure 13.40.

Earth-orbiting satellites make possible intensive site-specific soil sampling and testing to identify and map nutrient balances. The maps and computerized application equipment linked to the satellite system make it possible to modify fertilizer application rates as the applicator travels across the field, thereby more nearly meeting the specific nutrient needs of the plants.

13. See Section 13.11.

If soil tests are properly calibrated and correlated with the results of field trials, and are interpreted by experienced and knowledgeable scientists, they are valuable tools in determining how much, if any, fertilizer should be applied. The value of soil tests is limited by factors such as inadequate intensity and depth of soil sampling, and the difficulty of extracting from the soil in a short period of time nutrient levels that are related to nutrient uptake by plants over a whole growing season.

14. See Section 13.10.

Plant tissue analysis may be more advantageous than soil tests when the availability and sufficiency of nutrients involved cannot be reliably predicted from soil tests. For example, N and S availability are difficult to predict from soil test, but the optimal values and ratios are well know for many plants. Many micronutrients fall into this category, especially metals like Cu, Zn and Fe in acid soils. Another situation in which plant tissue can tell us more than soil samples is the case of mature trees that obtain nutrients from the whole profile, including soil layers much deep than normally sampled. In addition, the balance or ratios among nutrient elements in the plant tissue are often known to be important, but cannot be well predicted from the ratios of these elements extractable from the soil.

15. See Sections 13.10 to 13.13.

The plant-availability of N is very dependent on microbial processes that are, in turn, driven by soil moisture and temperature (which are weather dependent). The main processes that increase plant available N are 1) the mineralization if mineral N from soil organic materials and plant litter and 2) N-fixation by bacteria (often in association with legume plants). These addition to the available pool may be countered by losses of available N through leaching of nitrate and gaseous volatilization of ammonia and denitrification products. While soil organic matter or various fraction thereof may be related to total N and the potential release of available N, the balance among the above mentioned processes is highly weather dependent. Therefore soil test done ahead of crop planting are of little use in predicting how much N will be made available and how much fertilizer will be required to top that up. In-season soil tests (such as the PSNT) can help because much of the weather has already happened. Computer models can also be useful. Most often, the amount of N applied is first approximated by the amount that will be removed in the expected harvest.

For phosphorus the situation is quite different. Soil tests are reasonable good at predicting how much P plants will be able to obtain from a soil over the coming season. The appropriate soil test for a particular region and range of soil must be used. It is also advisable to leave several control strips without P fertilizer to test the assumption that P application is needed.

For both N and P, tissue test during the season or after harvest can determine if enough (or too much) N or P was applied and inform decisions made for the following year. Also for both N and P, water quality concerns and the risk of transport away from the field must be taken into consideration as loss of either to waterways can cause eutrophication.

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Chapter 14. Soil Erosion and Its Control Overview Soil erosion by wind and water accounts for about 85% of the global soil degradation that has occurred during the past half century. It is a worldwide problem that is more ominous today than at any time in history. Degradation of natural vegetation commonly ushers in soil degradation, and vice versa. The relationship between protective vegetation and soil susceptibility to erosion helps account for the naturally high rates of geologic erosion in semi-arid regions of sparse vegetation and few, but heavy rain storms. Natural or geologic erosion in humid regions is commonly much lower due to the protective effects of dense forest vegetation.

Every year billions of tons of topsoil from farmers’ fields, logged over forests and degraded rangelands finds its way to downstream waterways and reservoirs. The economic and social costs of removing and managing these losses rival the costs of upstream losses of soil productivity. Annual offsite losses in the United States between $12 and $40 billion have been estimated. Off-site damages include silting in of harbors and reservoirs, human lung damage from wind-eroded fine dust, as well as lost fish production and recreation from water turbidity and nutrient over-loading.

Soil erosion is serious in the United States but is even more serious in many other parts of the globe. Poverty and population pressures are forcing developing countries around the world to destroy the natural vegetation (forests and grasslands) on even their sloping and hilly lands to make room for food crops. Soil erosion is the inevitable result.

Soil-loss Tolerance Research suggests that some soil erosion can be tolerated without degrading soil quality.

Specific soil loss tolerance limits, known as T-values have been developed for different soils in the United States. At the present time, 11 Mg/ha is the maximum T value assigned for most soils, some having lower values. It is assumed that the productive capacity of a soil can be maintained if the soil loss is held to less than the soil’s T value. In the United States one third of the cropland is eroding at rates in excess of T, some 15% being eroded at twice this rate.

Mechanics of Water Erosion Erosion is a three step process: First soil particles undergo detachment by the impact of rain drops. Second, the detached soil is transported by flowing water, gravity, or splash. Third, the transported soil material, now sediment, is deposited at some place lower in elevation, where the slope is reduced, or is delivered to a major stream. The proportion delivered to a stream is termed the delivery ratio, and is commonly 0.1 to 0.5.

Types of Erosion There are three types of soil erosion by water: (a) sheet erosion, (b) rill erosion, and (c) gully erosion. Sheet erosion is uniform erosion from the entire soil surface and is abetted by the splash effects of raindrops. Rill erosion involves tiny channels (rills) in which water tends to concentrate as it runs off the soil. Gully erosion creates deep channels that cannot be erased by cultivation. Although gully erosion looks more catastrophic and is locally destructive, by far the most soil is lost by sheet and rill erosion.

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Universal Soil Loss Equation The major factors affecting soil erosion are included in the Universal soil-loss equation (USLE): A=RKLSCP, where A, the predicted loss, is the product of R = climatic erosivity

S = slope gradient or steepness

K = soil erodibility

C = cover and management

L = slope length

P = erosion control practices

The R factor considers the rainfall intensity and seasonal distribution that emphasizes the erosive effects of storms. The soil erodibility factor (K) indicates the inherent erodibility of the soil and emphasizes the soil’s infiltration capacity and structural stability. Approximate K factors for major soil series have been ascertained.

The topographic factor (LS) focuses on the length and steepness of slope with the steepness receiving greater emphasis. The influence on soil erosion of plant production systems, and especially soil cover, is considered in the cover and management factor (C). It is complemented by the support factor (P) that considers practices such as contour tillage, contour strip cropping, terrace systems, and grassed waterways. Mechanically constructed terraces are now being complemented by vegetative barriers or "living terraces" comprised of rows of perennial deep rooted grasses or shrubs planted on the contour across the slope.

The conventional USLE just described is now being replaced by a revised USLE (RUSLE), a computerized system that takes into account specific local climatic, soil, and management factors throughout the year. RUSLE is based on the same principles as those which underpin USLE, but it provides greater specificity, accuracy, and speed in ascertaining preferred options to reduce soil erosion. The Water Erosion Prediction Project (WEPP) is a still more complex, process-based computer model that may eventually replace the simpler empirical equation-based models.

Water Erosion Control Any practice that increases the infiltration of rainwater and reduces water runoff will reduce soil erosion. Vegetative cover should be provided on or near the soil surface. Close- growing crops such as grasses should be included in the crop rotation and mechanical support practices such as contour tillage and strip cropping should receive high priority on sloping fields. Most important, however, is the use of conservation tillage.

Conservation Tillage Conservation tillage systems that leave at least 30% of the crop residues on the soil surface have revolutionized erosion control efforts in the United States. These systems vary from those that merely reduce tillage to the no-till system which permits direct planting in the residue of the previous crop. Conservation tillage systems have been adopted on more than 50% of the nation’s cropland, and are used on even a higher proportion of cropland South America and Australia. In poorly drained soils, specific tillage systems such as ridge till must be utilized to maintain satisfactory crop yields while simultaneously gaining the benefits of erosion and water runoff control. Combined with federal programs to discourage the cultivation of highly erodible lands, conservation tillage helped reduce national average wind and water erosion losses from 3.13 to 2.13 Mg/ha/yr from 1982 to 1992. This 30% reduction in erosion is indeed noteworthy. Conservation tillage has significant and generally 94 .

favorable effects on soil physical properties. For example, aggregate stability, saturated hydraulic conductivity, and available water capacity are generally higher on no-till plots compared to conventionally tilled plots. Likewise, conservation tillage significantly increases the organic matter content of the upper few centimeters of soil over that found with conventional tillage. Higher soil microbial populations are also common with conservation tillage as are increases in earthworm activity. New techniques such as vertical or tillage have allowed farmers to mix the soil in a vertical, rather than horizontal, fashion. Turbo tillage does greatly increase the possibility of tillage erosion, but retains much of the residue coverage.

Tillage Erosion Tillage is the third great force behind erosion, after water and wind. Most types of tillage move soil more downhill than uphill. Tillage erosion can be calculated using the Tillage Erosion Risk Indicator model (TillERI), where Ai = Et × Et. Ai is the annual rate of soil movement downslope due to tillage erosion, Et is the erosivity of the tillage operations, and El is the erodibility of the landscape.

Sediment Control on Construction Sites Five basic steps are employed in controlling erosion and sedimentation on sites disturbed by construction and excavation. 1. When possible the main excavation is done during low-rainfall periods. 2. The site is divided into small areas graded in stages so that only a small part of the whole site is bared at any one time. 3. Disturbed areas are covered as soon as possible, using fast-growing vegetation and/or synthetic erosion blankets. 4. The flow of storm water runoff is controlled by channels protected with soft or hard armor. 5. Sediment in the runoff is trapped in sediment basins or behind silt fencing before the runoff is allowed to leave the site.

Wind Erosion One third of the soil moved annually by soil erosion in the USA is due to wind erosion. More than 60% of the soil moved by wind is on croplands producing essential food supplies. In six of the USA Great Plains states wind erosion exceeds water erosion. Wind erosion is equally serious in vast areas of arid and semiarid areas of Asia and Africa.

Wind erosion the processes of detachment, transportation and deposition. Detachment is enhanced by wind-borne particles striking and disrupting soil aggregates. Some finer particles are moved by winds in the atmosphere but maximum movement takes place near the surface by the process of saltation, the bouncing of particles along the soil surface. Rough soil surfaces and the presence of plant residues help control the wind erosion.

A wind erosion prediction equation (WEQ) is also in use: E = F(ICKLV) The predicted wind erosion (E) is a function of the following factors: I = soil erodibility

L = field width

K = soil-ridge-roughness

V = vegetative cover

C = climate

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The soil erodibility factor (I) relates to the properties of the soil and to site slope. The soilridge-roughness factor (K) considers the cloddiness and ridges of the soil surface. The climatic factor (C) focuses on wind velocity, soil temperature, and precipitation. The width of field factor (L) relates to the length of travel of the prevailing winds over the field. The vegetative cover (V) considers the degree of soil cover and the nature of that cover. This has been revised and has a more accurate computer-based prediction model called the revised wind erosion equation (RWEQ). More complex models are being developed with global cooperation by scientists.

Control of Wind Erosion Practices that keep the soil moist, leave the soil surface rough, and reduce the wind velocity are used to reduce wind erosion. Stubble mulch tillage leaves some organic residues on the soil surface and provides a rough, cloddy surface as well. Tillage and strip cropping at right angles to the prevailing winds are helpful. Shelter belt trees can reduce wind velocities.

Land Capability Classification The U.S. Department of Agriculture has developed a land capability classification system that is helpful in identifying appropriate soil erosion practices. Eight land capability classes are recognized, Class I being least susceptible to erosion and Class VIII most susceptible. Class I lands which make up only 3% of rural, non-federal lands, have few limitations on their use, although prudent practices to maintain or enhance soil productivity are recommended. Land Classes II to IV (43% of the rural, nonfederal lands) are also eligible for cultivation with increasing restrictions on their use that limit the choice of crops and rotations, and require conservation practices such as contour tillage, strip cropping, terraces, and waterways. Land Classes V to VII (41% of rural, non-federal lands) should not be cultivated. They can be used only for grazing, woodlands, or wildlife. In Land Class VIII, land use is restricted to recreation, wildlife, water supply or aesthetic purposes with no commercial use.

The conservation practices required for different land capabilities tell us what is needed to reduce soil erosion and to conserve soils. Such practices must become integral components of sound sustainable agricultural systems.

Model Answers to Study Questions 1. See Section 14.1, especially Figure 14.2.

Geologic erosion is a natural process that usually removes soil more slowly than new soil forms.

Accelerated erosion is a much more rapid process due to human disturbances. The difference between the two is greatest in humid regions where the high rainfall allows for the existence of dense, protective vegetation if the ecosystem is not disturbed by human activities.

2. See Section 14.2.

On-site damages (to soil being eroded): loss of soil depth, loss of nutrients, deterioration of surface structure and reduction of water infiltration, loss of water and nutrient holding capacity due to clay and humus loss, spread of plant diseases, physical barriers of gullies, sandblasting of plants, uprooting of plants, etc.

Off-site damages (to the environment): human lung damage from fine dust, clogging of drainage ditches, filling in of harbors, rivers, and reservoirs, eutrophication of natural waters, damages to aquatic organisms due to water turbidity, loss of fish spawning habitat, etc.

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3. See Section 14.3.

First, the impact of speeding raindrops detaches soil particles. Second, the detached particles are transported by gravity, splash, or running water. Third, the transported particles are deposited lower in the landscape.

4. See Section 14.3.

Only a part of the soil eroded from a site is transported all the way to the stream draining a watershed. The delivery ratio (commonly between 0.10 and 0.50) describes the proportion of eroded soil that is delivered to the stream to become part of its sediment load. The A in the USLE was based on the amount of soil transported out of a 22 m long experimental plot. While most of this soil does not reach a stream and is therefore not really lost from the field, its transport and deposition downslope adds little to the soil quality of the sediment-receiving site. Perhaps the USLE would be more accurately named USME (Universal Soil Movement Equation).

5. See Section 14.5.

In determining the effects on erosion, the distribution of rain over time is as, or more, important than the total annual amount of rain. Most erosion occurs during exceptionally high intensity storms that may occur only a few times a year or decade. Many gentle rains of long duration may result in more total annual rainfall, but very little erosion.

6. See Section 14.5.

Low erodibility (K factor) soil: high organic matter, low in fine sand and silt, well aggregated, non-expanding, iron-aluminum oxide -rich clays, high infiltration rate.

High (K factor) soil: low organic matter, weak structure, high in silt and very fine sands, low infiltration rate, expansive clays.

7. See Section 14.5, especially Box 14.1.

From map in Figure 14.7, R = 2400 [midpoint of range 1700 to 3100); from Table 14.2, K=0.063; from Figure 14.8a, LS=2.65; from Table 14.3, C=0.003; P=1 because no practice is applied.

Therefore, A = 2400 x 0.063 x 2.65 x 0.003 x 1 = 1.2 Mg/ha, a loss that some people might consider acceptable.

8. See Section 14.6.

The no-till system leaves the surface undisturbed except for a narrow slit for seeding. The advantages are that organic matter accumulation, bio pore development, surface cover and erosion control are maximized, while the system also uses less energy, time and labor than other systems.

Two disadvantages are the cooler soil temperature in spring and the greater dependence on chemical weed control. Compaction can also be a problem in some cases, and is difficult to overcome once allowed to occur.

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9. See Section 14.7, especially Figure 14.28.

As eroded sediment is deposited on the uphill side of a vegetative barrier (e.g., contour grass strip), a terrace of relatively level ground is formed while the grass stems and roots hold up the terrace “wall”.

10. See Section 14.9, Figure 14.26.

Leaf litter.

11. See Section 14.10.

Soil properties conducive to erosion by both wind and water: high content of fine sand and silt, low content of organic matter, smooth surface with little roughness. Soil properties conducive to erosion by wind but not to erosion by water: dry soil condition, level topography.

12. See Section 14.12.

K and V. To the extent that soil management may alter soil organic matter and soil structure, I might also be included.

13. Tilled land with hummocky hills (steep short slopes).

14. See Sections 14.3, 17,4, 14.13, and 14.14.

In the case of both tillage and water erosion, soil is moved downslope. Tillage erosion moves the soil downslope within the field itself and may deposit it as high quality soil. Water erosion may carry much of the eroded materials off-site, and the soil it deposits in the field downslope usually has lost its structure and much of its fertility so as to be more sediment than soil. Moving soil upslope and making changes to field management may suffice for the tillage-eroded field, as that soil may have retained its fertility and structure. With soil removed from the site, the water-eroded field would require more drastic efforts for reclamation. In areas with water erosion, the site may require repair of gullies and rills, as well as the addition of soil fertility to replace the eroded topsoil.

15. See Section 14.14.

Land in Capability Class IIw is less limited in its safe use than that in class IVe. Also, the principle limitation for the former is due to wetness (poor drainage, flooding, etc.) while that for the latter is due to erosion hazard (probably steep slope).

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Chapter 15. Soils and Chemical Pollution Overview Industrial, agricultural, and naturally occurring toxic chemicals are contaminating soils in ways that threaten harm to our soil ecosystems, our water resources, our homes, and our health.

Increasingly, soil scientists are being called upon to solve problems of soil and environmental contamination.

Toxic Organic Chemicals All manner of organic chemicals, from solvents to pesticides, find their way to soils, whether by accident or by intentional application. Undesirable effects include toxicity to soil organisms and movement to other parts of the environment such as the water, the food supply, or the atmosphere.

Artificially synthesized organic compounds unknown to nature are termed xenobiotics.

Industrial Organics Such industrial xenobiotics as petroleum derivatives, solvents and explosives cause serious soil contamination near industrial sites, transportation spills and military bases. Leaking underground storage tanks are another cause of soil contamination. Many of these compounds are difficult for microorganism to decompose, often because of their chlorinated structures.

Pesticides.

Pesticides are an important class of toxic xenobiotic organic compounds that are intentionally applied to extensive areas of soil in both rural and urban settings as a means to protect plants from deleterious organisms. In addition to intention application, much reaches the soil by accident.

Pesticides include insecticides, herbicides, fungicides, rodenticides, and nematocides. Hundreds of millions of kg of thousands of pesticide formulations are used annually in the United States alone. The benefits from pesticide use include contributions to public health, reliable food production, and reduced soil erosion (herbicides make possible several effective conservation tillage methods). The costs of pesticides, however, go beyond the purchase price of the chemicals, and include contamination of drinking water, wildlife toxicity, and public health impacts. Although a distinct minority, organic farmers in many countries have demonstrated that profitable, high quality production is not dependent on the use of pesticides.

Kinds of Pesticides The main groups of insecticides are chlorinated hydrocarbons, organo-phosphates, carbamates, and neonicotinoids. Carbamates are relatively biodegradable but are also mobile in the environment and are highly toxic to earthworms and beneficial insects. Most herbicides include triazine, substituted urea, and sulfonylurea compounds. Most are fairly biodegradable and of relatively low mammalian toxicity, but some are quite toxic to fish and aquatic vegetation. Some nematicides are especially dangerous and mobile in the environment. Neonicotinoids are a relatively new class of compounds, with chemical structures similar to that of nicotine, and they affect the central nervous system of insects.

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Behavior of Organic Compounds in Soils Once in the soils, organic compounds may move by five different pathways.

1) Some tend to volatilize into the atmosphere with little chemical change, thus becoming air pollutants and often returning to the ground elsewhere in precipitation.

2) Some organic compounds, especially those with charged sites and high molecular weights, are tightly adsorbed by the soil colloids, preventing their movement to water or air resources.

3) Compounds that are highly soluble in water and not strongly adsorbed by colloids are especially likely to leach down through the soil profile to the groundwater. Many pesticides have been found to contaminate drinking water wells, occasionally at levels high enough to suggest a health hazard.

4) Chemical reactions, often involving sunlight, may transform some compounds into products that may be more or less toxic than the original chemical.

5) Soil microbes are capable of metabolizing or biodegrading many, but apparently not all, organic compounds. Some compounds biodegrade much faster than others.

In addition, significant quantities of some compounds may be taken up by plants, there to be stored, passed on in the food chain or metabolized. Generally, chemicals that resist microbial degradation are more persistent in the soil than others and constitute a greater environmental threat.

The continued application of a chemical to a particular soil may stimulate growth of a population of microorganisms capable of metabolizing that compound, so that eventually persistence is decreased.

Effects of Pesticides on Soil Organisms Because pesticides are intentionally applied to vast areas of soil, there is much concern about deleterious effects of these compounds on the soil organisms so vital to organic matter turnover and nutrient cycling. The most deleterious effects are generally caused by fumigants which are widespectrum biocides. The effects of pesticides on soil fauna vary widely from chemical to chemical and among different species of soil animals. Insecticides, of course, are often toxic to Collembola and other beneficial soil insects. A few specific compounds are quite toxic to earthworms, and their use should carefully consider this fact. Carbamate insecticides and benomyl (a fungicide) are particularly detrimental. In some cases the toxicity may be more subtle and chronic than acute- for example changes in mating or burrowing behavior rather than mortality. The effects on microorganisms are not usually too serious, as some groups are benefited when others are inhibited, and overall, most effects seem to be only temporary. Still, care must be taken to be certain that compounds applied to soils do not interfere seriously with vital soil microbial processes.

Regional Vulnerability to Pesticide Leaching Highest vulnerability to the leaching of pesticides and nitrates into the groundwater occurs in regions with high rainfall, sandy soils and high usage of pesticides that are not adsorbed by soil colloids. In the United States such conditions are common in the southern Atlantic Coast area, in the corn belt, and in irrigated regions of the West where intensive vegetable and fruit production may occur. The degree to which a particular organic contaminant is a threat to groundwater depends on the compound’s persistence, water solubility, and tendency to become sorbed on colloids (the Kd and Koc of the compound).

Remediation of Soils Contaminated with Organic Chemicals Widespread, chronic soil contamination is best addressed at the source (in situ) by reduction or elimination of offending chemicals as far as possible. Acute localized cases of contamination (e.g., at industrial spills, waste sites, etc.) require some means of cleaning up the soil that is often 100 .

accomplished by ex situ treatment i.e. by removing the soil to treatment bins and treating it chemically and physically to remove pollutants. The soil may be heated to drive off volatile chemicals, and washed with water or ventilated with air to remove other compounds.

In Situ processes are preferred since it is generally much less expensive to treat the contaminated soil in place. Some organic surfactants, when injected into the soil, react with clays to form organo-clays that attract organic compounds from groundwater, holding them until they can be degraded. Also, two general types of bioremediation are used in situ to degrade or remove pollutants.

First, the growth of microbes known to metabolize the offending chemicals is encouraged by enhancing the physical and chemical environment in the soils. In some cases, the soil is inoculated with organisms known to readily metabolize the chemicals. Such organisms may be isolated from sites that have long histories of similar contamination. Sources of mineral nutrients may also be supplied. A successful example of these methods was the clean up of Alaska beaches after the oil spill. In that case, special fertilizer supplied the microbes with N and eliminated the factors that had constrained the rate of biodegradation of the oil.

A second example of bioremediation is phytoremediation wherein higher plants, often in combination with microorganisms, remove the pollutants from the soil, either in plant biomass or by degradation within the plant or in the biosphere around plant roots.

Contamination with Toxic lnorganic Substances Although such non-metallic inorganic substances as fluorine and boron contaminate soils in some instances, most inorganic contamination is due to metals, often collectively called "heavy metals." Many, such as arsenic, cadmium, chromium, lead, and some forms of mercury, are extremely toxic to humans and animals. Others, including several plant micronutrient elements such as copper, nickel, and zinc, principally cause problems of plant and fish toxicity. Soil becomes contaminated with these metals because of smelting fumes, industrial sewage sludge application, spills, and solid waste disposal. As elements, these toxic metals cannot degrade, and hence tend to accumulate in soils over the years.

Reactions of Inorganic Contaminants in Soils A small portion of metals applied to soils with sewage sludge are held as exchangeable ions.

Some are bound tightly with soil organic matter, and a considerable portion is associated with carbonate or oxide minerals in soils. Additionally, residual amounts are bound as sulfide and other very insoluble forms. Long-term use of arsenical insecticides has seriously contaminated soils in some orchards and vineyards, while soils near highways once accumulated large amounts of lead before this gasoline additive was banned. Boron contamination often results from irrigation water, or even excessive fertilizer application. Mercury from industrial processes contaminates many soils and may become highly toxic if transformed by microbes into methyl-mercury compounds. Chromium is a carcinogen widely used in industrial applications. In contaminated soil and water, reducing chromium from Cr(VI) to Cr(III) can eliminate its toxicity.

Prevention and Elimination of Inorganic Chemical Contamination Three principal approaches can be used to ameliorate inorganic contamination.

1) Through banning some materials (e.g., lead in gasoline) and greatly reducing the use of others, the inorganic loading to soils can be substantially decreased.

2) By immobilizing the toxins in the soil, contamination of water resources and food chains can be greatly reduced. This approach includes raising soil pH to reduce metal solubility. Draining wet 101 .

soils to oxidize certain metals (but not chromium) has the effect of decreasing solubility and plant uptake.

3) Bioremediation through phytoextraction by hyperaccumulating plants (phytoremediation) is showing some promise for removing certain metals from contaminated soils and making them recoverable when the harvest plant material is smelted.

Potential Hazards of Chemicals in Sewage Sludge About half of the sewage sludge produced by municipalities in the United States is applied to the soil. Concerns have been raised about the effects of heavy metals in land-applied sludge on such soil organisms as earthworms, which accumulate metals and pass them up the food chain (to birds, rodents, and their predators). Direct ingestion by cattle of sludge applied to forage or by children in areas where sludge has been applied to lawns and gardens is of some concern, in addition to food chain contamination through crop uptake.

Industrial sludges tend to contain far higher levels of harmful metals and organics than do municipal sludges. In fact, recent source reduction programs have resulted in marked reductions in the contents of metals in municipal sewage sludge, so that soil contamination and food chain effects of heavy metals is not nearly as great a problem as was the case in the past. Still, both federal and state regulations govern the application of sewage sludge to soils. The rates of application allowed are now limited more often by the nitrogen content rather than by the heavy metal content of the sludge.

Landfills Simple measures of reduced materials usage, greater materials recycling, and composting of many organic wastes could go a long way in minimizing the contamination of soils and groundwater from landfilled wastes and the formation of toxic and explosive gases from organic vaporization and anaerobic decomposition. Proper siting and designing of landfills also can help solve these problems.

There are two basic types of landfills: 1) Natural attenuation landfills are designed to minimize leachate production, but allow what leachate is produced to be decontaminated by such soil processes as biodegradation, adsorption, and precipitation.

2) Containment or secured landfills are less dependent on soil processes for protecting groundwater since they are supposed to be completely lined with leak-proof plastic and clay materials. They are designed to minimize rainfall infiltration and to collect whatever leachate does form and pipe it to a treatment system for decontamination or recirculation.

Soil as Organic Waste Disposal Sites The soil offers an environmentally safe way to dispose of, or even put to good use, many kinds of non-toxic organic wastes such as animal manures, food processing wastes, and wood wastes. If properly managed, these wastes can be assimilated by the soil ecosystem and soil properties improved in the process.

Radionuclides in Soil Products of nuclear fission, as well as naturally occurring radionuclides, can be found in soils.

Of the fission products, strontium-90 (1/2 life = 28 years) and cesium-137 (1/2 life = 30 years) are the most worrisome. However, because of soil cation exchange and buffering, plant uptake from the soil contributes little to food chain contamination. Fallout from nuclear tests and reactor accidents can, 102 .

however, directly contaminate plant foliage and thus find its way into the animal-human food chain.

The reactions of Sr and Cs in soil are closely related to those of Ca and K, respectively. These and other nuclides such as uranium and plutonium contaminate soils as a result of nuclear waste disposal.

The solubility and mobility of the various nuclides are quite variable. After the 2011 nuclear catastrophe at the Fukushima Daiichi nuclear power plant, soils were analyzed for radioactive Cesium to determine suitability for production of rice. Even with restricted land use, rice was measured at higher than acceptable levels, causing a widespread panic.

Radon Gas from Soils Radon, a radioactive gas generated by the radioactive decay of uranium in certain rocks and soils, is highly toxic to humans as a long-term carcinogen, similar in effect to tobacco smoke. Radon enters houses and buildings from the soil through cracks in the basement walls and foundations and in the piped in water. It can accumulate in the indoor air, especially when the house is tightly sealed, as in winter. Continual breathing of radon gas over a period of years is thought to cause lung cancer.

Radon does not react with the soil, but the problem of indoor radon is related to the type of soil parent material and the porosity and moisture content of the soil underlying building foundations. Gravelly soils are more likely to transfer radon to homes than finer textured, wetter soils.

Model Answers to Study Questions 1. See introduction and Sections 15.1-15.3 and also chapter 13.

Widespread and heavy use of insecticides, herbicides and fumigants on agricultural soils has lead to contamination of both surface and ground water. Chemical pesticides should be used as a last, rather than as a first, resort in managing pest effects in agroecosystems. Before resorting to the use of an insecticide or herbicide, every effort should be used to minimize the detrimental effects of insects and weeds by means of crop diversification, habitat establishment for beneficial insects, organic soil amendments, cultural practices to reduce weed competition, and selection of resistant crop cultivars. When pesticides are used, they should be chosen for low toxicity to nontarget organisms, low mobility in soils, and low persistence. Even then, the use of pesticides, especially insecticides and nematicides often has wide-ranging detrimental effects on the overall microbial and faunal communities so that the harm done, though not always obvious, may out weigh the benefits. Examples include insecticides that kill natural enemies of pests and fungicides that kill mycorrhizal fungi.

Applications of excessive quantities of animal manures and chemical fertilizers along with practices that permit excessive runoff and soil erosion, lead to excess nutrient levels in soils and in the groundwater, and water that runs off the land. Nutrient applications from all sources must be made and timed in such a way that they meet the needs of the plants without stimulating losses to the air and water. To take advantage of any excess nutrients remaining in the soil after the main crop is harvested, cover crops should be grown.

2. See Section 15.3.

Pesticides may vaporize, be adsorbed by soils, react with soil organic matter or other compounds in the soil, be decomposed by soil microbes, or be absorbed by plants. The only intended effect of pesticides is to destroy the pests, all the remaining effects being incidental and unwanted. We can prevent reactions of pesticides in soils by not using them. Where their use may be desirable such as to control weeds to aid in conservation tillage, we can take steps to keep them in the soil for short periods of time so microbes can destroy them, and so their movement into water and air can be prevented.

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3. See Section 15.8.

Sewage sludge may contain either or both organic and inorganic pollutants, but hazards from inorganic pollutants (primarily heavy metals) have received most attention. Reduction in the contents of these metals at the source of their input into the wastes has greatly lowered their content in the sludge. Even so, care must be taken to limit the rates of application of these wastes to the amounts permitted by regulation and to monitor their levels once they are added.

4. See Section 15.5.

Bioremediation is the biological process of removing and/or degrading organic and inorganic contaminants from soils. Both microbes and plants (independently or in combination) can remove or degrade the contaminants. In some cases selected microbes known to have the ability to degrade the contaminant are introduced into the soil, in others organisms currently in the soil can do the job. Bioremediation is commonly less expensive than physical and chemical processes, especially if the latter must be carried out ex situ. Also, the organisms will increase in numbers so long as there are sufficient quantities of the contaminant to be destroyed.

Unfortunately, bioremediation is commonly a slower process than most of the physical or chemical processes, a distinct disadvantage.

5. See Section 15.6.

Heavy metals react in the soil with organic matter, with carbonates and oxides of iron and manganese, and with sulfides to form compounds that are very insoluble and unavailable to plants.

Only if the very high sorption capacity of the soil for these elements is exceeded will their levels be high enough to encourage uptake by plants. However, the solubility of most of these metals increases substantially as soil pH is lowered below 6.0, therefore regulations require maintaining soil pH above 6.5 if sewage sludge has been applied.

6. See Section 15.9.

30 years ago landfills were unused (commonly swampy) areas into which wastes of all kinds were dumped and burned. The solid materials helped fill the land in, consequently the name landfill.

Little attempt was made to control the water moving through the wastes or off the site. There were few constraints on what could be put in the landfill which was only superficially managed. Today, regulations require that wastes be buried in carefully located and designed landfills. Special care is taken to reduce potential water pollution. Wastes are segregated to a degree to permit composting of easily decomposable materials separate from the more stable solids. Containment-type landfills for more hazardous wastes reduce or eliminate the movement of liquids from the site into the groundwater.

7. See Section 15.5 and Figure 15.11.

Organo-clays are products of the reaction of silicate clays with certain positively charged organic surfactants. These products attract rather than repel organic pollutants, making it possible for them to remove organic pollutants from groundwater and hold them until they can be degraded by soil microbes.

8. See Section 15.5, Figures 15.11, and 15.12.

The advantage of stabilization of organic pollutants by binding them to O.M. and silicate clays is that they are removed from ground water thus preventing them from moving into streams. Most 104 .

organic toxins are hydrophobic and so are not attracted to mineral surface, but instead are sorbed on or into soil organic coatings on soil clays. Toxic molecules may be secluded inside the organic matter, in nano pores so small that neither bacteria nor their enzyme molecules can enter to degrade or react with the toxin. Nanopore seclusion therefore preserves the toxin. However, it also isolates the toxin from contact with roots or microbes, so the biological activity may be low to non-existent so long as the toxin remains undisturbed inside the nanopore.

Disadvantage: Unless they are degraded in place by microbes, the pollutants are still in the soil, and some other means may have to be found to remove them.

9. See Section 15.10.

Only strontium 90 and cesium 137 are sufficiently long-lived to be of any importance in soils. The level of each of these nuclides in most soils is sufficiently low that uptake by plants currently presents no widespread hazard to animal and human life. However, in areas affected by nuclear accidents, these elements behave much like calcium and potassium, respectively. Soils with high amounts of micaceous clays can be expected to tightly sorb much of the cesium as they would potassium.

10. See Section 15.5, especially Figure 15.10.

The advantages of in situ remediation of soils are that it is relatively inexpensive, and it leaves the soil essentially in its natural state for future use. The advantages of ex situ remediation are that the soil can be removed, and then confined in a container where it can be heated to remove volatiles, and mixed with appropriate chemicals to dissolve and leach out the pollutants. Ex situ remediation generally takes less time than in situ methods.

11. See Sections 15.5 and 15.8, Figures 15.16 and 15.23.

Phytoremediation takes advantage of the capacity of selected plant species to accumulate both organic and inorganic pollutants. The pollutants are either accumulated unchanged or degraded by the plants or by microbes associated with the plant roots. If they accumulate, the plants can be harvested and the chemicals removed from the site. Phytoremediation is most useful where the level of contamination is not too high, is within the rooting depth of the plants in question, and is spread over a large area.

12. First, figure how much the Ni concentration must be lowered to meet the target: 800 mg/kg – 80 mg/kg = 720 mg/kg. Then figure how much Ni must be removed per ha to achieve this change in concentration: 720 mg Ni/kg soil × 2 × 106 kg soil/ha-15 cm deep [see footnote page 152] = 1.44 × 109 mg of Ni/ha = 1,440 kg Ni/ha. Then figure how much plant dry matter (DM) is removed in a year’s worth of harvests: 4,000 kg DM/ha-harvest × 2 harvest/yr = 8,000 kg DM ha-1 yr-1 and then how much Ni is contained in that DM harvest: 8,000 kg DM ha-1 yr-1 × 0.01 kg Ni/kg DM = 80 kg Ni ha-1 yr-1 removed. Finally figure how many years to achieve the 1,440 kg Ni to be removed: 1,440 kg Ni ha-1 / 80 kg Ni ha-1 yr-1 removed = 18 years.

105 .