MIND THE...GAP GOOD AGRICULTURAL PRACTICE TO MANAGE
GREENHOUSE GAS EMISSIONS IN CROP PRODUCTION
INTRODUCTION AND BACKGROUND
Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), together commonly referred to as 'greenhouse gases' (GHG), are produced as the result of natural microbial processes in the soil, with or without human influence. In addition, other gases are also produced as part of natural soil-plant-animal interactions, such as ammonia, nitrogen, oxygen etc. The issue to be addressed is the extent to which man's activities, in this context the essential activity of food production, influence these gaseous fluxes. This leaflet acknowledges the inevitable influence of the use of crop nutrients in agriculture on emissions of GHGs, and outlines a number of guidelines and actions for farmers which can help to minimize this influence. It is important to recognize at the outset that agriculture (with forestry) is probably unique among the commercial activities of man, in that it operates in a naturally GHG-producing environment. Furthermore, this activity is not an optional one, but is fundamental to the survival of the current and future population of the planet. Thus when evaluating the threats and opportunities associated with the emission of GHGs in the rural environment, it must be understood that only the anthropogenic component of these emissions has a potential for reduction. In addition, because agriculture is simply the refined and targeted management of natural processes, it is inevitable that it will generate GHGs in addition to the natural background production.
Natural emissions of GHGs can be large, as for example in the case of wetland and peat bogs, and these emissions cannot readily be altered. As will be shown in the following pages, however, the necessary knowledge and expertise is available to minimize the inevitable emissions from agriculture. This leaflet focuses on the best management of the use of fertilizing materials, from both organic and mineral sources and primarily those containing nitrogen, in order to minimize the production of the GHG nitrous oxide, N2O. At the outset it is necessary to state that the quantity of food required to feed today's global population cannot be produced without the input of additional nitrogen into the system. It has been authoritatively calculated that without this addition of nitrogen more than 48% of the world population would starve, not solely from a simple lack of food, but also because globally there would be insufficient active nitrogen to provide the (nitrogen-containing) protein required. Thus it is clear that the objective is to manage the nutrients so that their recovery and efficient use by plants is maximized and that losses, especially of nitrogen, are minimized. This approach will not only reduce potential loss of nitrogen as N2O from the agricultural system, but will also minimize losses by other routes. The development of guidelines for best practice in nutrient management on farm has been a focus of attention for the European Fertilizer Manufacturers Association (EFMA) for many years, and this leaflet is part of a continuing and integrated contribution to European agriculture and food production.
OBJECTIVE Achieving best practice in the use of crop nutrients is a goal in farming and in the agricultural support industry. It is clear that the results of minimizing losses and maximizing 'nutrient use efficiency' include not only better economic performance, but also reductions in losses from the system, including those as N2O. Nutrient use efficiency is the term used when judging the effective recovery and utilization of available nutrients: the crop output – yield and quality – per unit of nutrient input. The efficient use of an individual nutrient – which in the case of N2O emissions is clearly nitrogen – cannot be judged in isolation. Good crop growth requires an adequate and balanced availability of all sixteen essential nutrients. Some of these, such as carbon, hydrogen and oxygen, are supplied from air and water, and some (e.g. molybdenum) are normally available in adequate quantities in fertile soils. The main purpose of the addition of extra nutrients (usually in manures and fertilizers) is to ensure an adequate supply of the nutrients
which are normally limiting, principally nitrogen, phosphorus, potassium, magnesium and sulphur. If any one of the nutrients essential to plant growth is in deficient supply, it becomes the limiting factor and other nutrients which may be adequately available can become inefficiently used. Thus the principle of 'balanced nutrition', in which care is taken to ensure that there is no deficiency of any of the essential nutrients, is a fundamental requirement. The nutrient use efficiency of an individual nutrient, such as nitrogen, will not be optimal unless there is an adequate availability of the other essential nutrients. These absolute nutrient assessments are less easy with systems in which organic manures play a significant part, because the nutrients contained in these materials are more difficult to measure. Soil is a complex ecosystem of living and inert constituents, with microbial activity making an essential contribution to the fertility of soils in which good crops can be grown.
GHGs are generated by microbes living on the organic matter in soils and manures, and are the by-products of their natural activity. GHGs are not emitted directly from mineral fertilizer nitrogen as they are from the nitrogen in soil organic matter and manure, but only as a result of microbial transformations of this mineral nitrogen when it has been applied to the soil. Such transformations are natural and essential, but can be minimized by the optimization of nitrogen use efficiency. Thus, before determining fertilizer requirements, it is necessary to make an overall assessment of the soil type and condition and to identify factors other than nutrient supply that could potentially limit crop growth. The availability of nutrients in the soil and from any manures should be measured and assessed, so that the calculated needs of the crop can then be satisfied by additional fertilizer nutrient application as needed. This leaflet identifies the farm practices necessary to optimize integrated SOIL, CROP and NUTRIENT management.
SOIL MANAGEMENT Soil physical characteristics The soil is a living ecosystem. Its physical state, particularly the balance between mineral constituents (particles of sand, silt and clay), organic constituents (e.g. humus, soil microfauna and flora) and the pore space (containing the soil air and water) is critical to its biological life and ability to support good crop growth. For example a compacted soil will have a reduced volume of pore space, resulting in increasingly anaerobic conditions and leading to a greater likelihood of GHG emissions. KK Soil physical characteristics should be identified - e.g. soil type, depth and texture - noting, ideally on a farm map, any potential implications for cultivation and water management. Further information, such as the direction of surface and underlying drainage, is added to these farm and field maps. KK Soil is examined to identify any structural problems, which should be rectified. For example, compacted soils inhibit root growth and access to nutrients, potentially reducing nutrient use efficiency. KK
Cultivation methods should be chosen which limit physical impact on the soil and conserve soil water.
KK Any potential risk of soil erosion should be assessed, and where erosion occurs preventive measures adopted. For example, high risk crops (e.g. potatoes, maize) on high risk fields (e.g. sloping) are avoided. The introduction of contour cultivations and revised sowing dates may be considered.
Soil biology The soil must be managed to provide a suitable environment for the microorganisms that are essential to the carbon and nitrogen cycles, and to help maintain soil structure. Equally, this 'biological well-being' of the soil plays an important part in allowing crop plants to develop extensive and efficient root systems which are key to good nutrient use efficiency. The provision and recycling of carbon-containing organic matter to the soil provides the main nutritional substrate for the soil microorganisms. However they also utilize nitrogen and other nutrients, and the downside of their essential activity is the inevitable generation of GHGs. KK Soils should be managed to be well-aerated and to conserve the organic matter necessary for healthy microbial and plant growth. KK Rotations are planned to encourage nutrient and carbon recycling, and to maintain the balance of nutrients and the well-being of organisms in the soil. KK The soil is a living ecosystem and the natural nitrogen and carbon cycles depend upon its healthy activity, which is helped by the return of plant residues to the soil.
Soil chemistry In the context of crop production, the chemistry in the soil is mainly related to its reserves of the essential plant nutrients and the acidity of the soil as measured by pH. It also includes the mineralisation of organic-bound nutrients into the forms in which they are available to plants. Where natural reserves of nutrients are insufficient in the soil, additional nutrients are added and become available to plants in the same forms as are naturally present in the soil. KK Soil nitrogen availability is measured or estimated so that the total supply from different sources - soil, manure and fertilizers - can be calculated and matched as closely as possible with crop requirements. KK Concentrations of available soil reserves of phosphorus, potassium and magnesium should be analyzed every 3 to 5 years depending on crop rotation and soil type. Soil reserves are targeted to be at recommended levels, and nutrients added as required. KK An assessment of likely crop response to sulphur is made according to soil and crop type and geographical location; plant leaf/herbage samples or deep soil analysis can be used as a guide, and sulphur applications made where needed. KK The soil pH should be tested every 3 to 5 years and acidity corrected as necessary using liming materials with known neutralising values. Soil pH is maintained within a suitable range according to soil type and crop rotation to optimize nutrient availability.
CROP MANAGEMENT In seeking to optimize crop performance and nutrient use efficiency, and thereby to minimize GHG emissions, there are a number of factors relating to the management of the crop which can play an important part.
Optimal growing conditions KK Where possible, variations within the field are taken into account to reduce differences in crop performance; for example any variability in soil type. KK
Crops are ideally sown under good soil and weather conditions to ensure good establishment.
KK Crop rotations should be planned to minimize risks of pests and disease carry-over from season to season and to provide good growing conditions for each crop in the rotation.
Plant protection KK
Nutrient use efficiency is encouraged by protecting crops from pests and disease.
KK Weed infestations are controlled to support the efficient utilization of nutrients and of available water and light by the crop being grown.
Water availability KK Any likely changes in the availability of water to the growing crop should be anticipated, attempting to avoid nitrogen applications which may be poorly utilized if droughting occurs. KK Where irrigation is required, applications should be carefully scheduled and planned so that the water and nutrients are used most efficiently. The nutrient requirement of each specific crop is calculated, taking account of the expected yield, variety, growing conditions, irrigation opportunity, and fate of the crop residue.
NUTRIENT MANAGEMENT This leaflet has so far highlighted the factors that precede the direct management of additional nutrient inputs, working towards maximizing nutrient use efficiency and thereby minimizing avoidable GHG emissions from soil and manures. As has been mentioned, the GHG most influenced by crop nutrients is nitrous oxide, N2O. It is clear that this GHG is related to nitrogen (N). However it is misleading to focus solely on inputs and transformations of nitrogen, because the efficiency of utilization of this nutrient is directly affected if any other nutrient is in limiting supply, as has been discussed earlier. Thus this section on nutrient management refers to all nutrients, integrating all sources, in what is referred to as 'Balanced fertilization'. The nutrient requirements of each of the arable crops being grown and of the pasture land on the farm is calculated, taking account of crop residues and the other factors identified above. Integrated nutrient management involves satisfying these requirements from the three available sources of nutrients: the soil, organic sources including manures, and mineral fertilizers. It also requires that these nutrients are applied in an optimal manner.
Satisfying crop requirements from available nutrient sources Soil availability: The estimation of the quantity of nutrients available to the crop from reserves in the soil has been described above in the section on 'Soil chemistry'. These estimates define the proportion of the crop nutrient requirement which can be satisfied from existing soil reserves, and that which must be supplied from organic and/or mineral nutrient sources. Organic sources: These include primarily farm manures, but also other organic domestic and industrial wastes which contain significant quantities of plant nutrients and which are suitable for use on farmed land. The nutrients provided by these sources will satisfy a further proportion of the crop nutrient requirement. The quantity of available nutrients per tonne or cubic metre of each manure type that is applied should be determined. KK From the application rate per hectare, the quantity of nutrients applied from organic sources can be calculated so that any final balance of fertilizer nutrients required to satisfy the crop nutrient requirement is known. KK The fertilizer value of organic manures is maximized, as far as possible, by applying them to the land at times when manure nitrogen can be efficiently recovered by the crop, and to the fields which would derive the most benefit (not on clover leys, for example). Mineral nutrients These sources are usually described as fertilizer products, and as such contain legally defined quantities of nutrients, the agronomic performance of which is well defined. KK Mineral nutrient application rates can be exactly calculated from the declared composition of the product. KK Differences between the behaviour of the various forms of mineral nitrogen sources are well known, and allow for best practice in the choice of product which is most likely to satisfy the precise need. Different nitrogen sources have potentially different, but known, transformation pathways. KK The potential nitrogen use efficiency of these mineral nutrient sources is high compared with that of manures and other organic sources and gives greater management opportunity for the minimization of GHG emissions.
Application All nutrients should be applied at the correct time and at the correct rate, to coincide with crop uptake and to match requirement. Organic sources The storage and application of organic nutrient sources requires considerable care and attention, because it is more difficult to achieve a high nutrient use efficiency of the nitrogen contained in these sources than for mineral fertilizer nitrogen. Furthermore the microbial activity that leads to GHG production takes place during storage and application of these products (in contrast to mineral nitrogen fertilizer), as well as from within the soil. KK Storage and handling of organic sources must be managed with care and understanding to minimize potential losses, principally of nitrogen. Manure should be spread evenly so that subsequent fertilizer applications can be accurately matched to the organic nutrients already applied. KK Application must be at known rates not exceeding the quantities of total manure nitrogen per year that are required by the crop and that are within the legal limits. More stringent restrictions apply in designated Nitrate Vulnerable Zones. KK The condition of spreading machinery should be maintained to a high standard and the spreader correctly set for the bout widths being applied. Mineral fertilizers Good quality mineral fertilizers are specifically formulated and manufactured to enable the accurate application of the required nutrients to crops, both in terms of rate and evenness of spread. Best practice in the application of mineral fertilizers takes advantage of these characteristics to achieve the objective of precisely matching the application to the identified crop requirements. Modern mineral fertilizer spreaders make use of the latest technology, including GPS location and variable rate spreading, to ensure that best practice objectives can be achieved. KK A good quality product should be used and the fertilizer spreader or liquid fertilizer applicator be wellmaintained. Spreader operators should be appropriately trained. KK Fertilizer should be evenly applied, with a targeted coefficient of variation of less than 10%, thus limiting the risk of crop and nutrient losses occurring due to uneven spreading. The spreading of fertilizer in very windy conditions should be avoided. KK The spreader should be correctly set and calibrated, and recalibrated as necessary when different application rates or products are used. In addition special calibration trays can be used to check the uniformity of the spread pattern of the fertilizer onto the land.
CONCLUSION Adoption of integrated Good Agricultural Practice will result in the minimization of emissions of the greenhouse gas nitrous oxide inherent in the agricultural activities which produce food for the European and world population. The European fertilizer industry is committed to supporting overall best practice, especially in fertilizer use on farm.
ÂŠ EFMA - KRAM/mc09
EFMA, European Fertilizer Manufacturers Association 4-6 avenue E. Van Nieuwenhuyse, B-1160 Brussels, email@example.com; +32 26753550 - Fax +32 26753961