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Uptake and Use of Water, Minerals, and Light
are needed to promote vigorous shoot development, whereas modest proportions of potassium are required for root formation. To encourage enlargement of the food-storage roots of root crops, the ratio of these two elements may need to be changed in favor of potassium. At the time when a plant achieves a ripeness-to-flower (chapter 7), increased phosphorus and potassium, relative to nitrogen, promote the development of reproductive organs. Some species fail to flower when, at vegetative maturity, too much nitrogen is available, with the plant’s metabolic energies being directed into shoot and root growth. Such relationships between nitrogen, phosphorus, and potassium and the development of specific organs underlie the reason for printing three numbers, the N-P-K ratio, on packages of fertilizer. For lawn grasses and most houseplants, recommended fertilizers have a proportionately high nitrogen content to promote leaf growth (20–5–5, being one example: 20 parts nitrogen to 5 each of phosphorus and potassium). Whereas a product having a 0–10–10 ratio is a typical formulation designed for flower and fruit set. A fertilizer for root crops may have an N-P-K ratio of 2–12–10, and an all-purpose mix, 5–10–5.
SOILS
The soil is one of the most important parts of a plant’s environment, the medium in which roots are anchored and from which nutrients, water, and oxygen are obtained. Soil is a complex mixture of inorganic materials, derived from the erosion of rock, and organic matter or humus—the decomposed remains of plants and animals. The inorganic fraction is divided into three classes, defined by particle size (1 mm = 0.04 inch): Sand particles have a diameter between 0.02 and 2 mm, silt ranges from 0.002 to 0.02 mm, and clay particles are smaller than 0.002 mm. Mixtures of sand, silt, and clay are called loams; a sandy-loam, for example, contains proportionately more sand. In humus-loams various proportions of organic matter are mixed with the other components. The proportions of each material determine the water-holding capacity of a particular soil. Water-holding capacity (or field capacity) is defined as the water content of a thoroughly wetted soil after surplus water has drained off by gravity. Sandy soils retain little water, whereas the addition of humus increases the water-holding capacity, the moisture being held in tiny spaces (capillary spaces) within and between the organic particles. Capillary water is the principal source of moisture for roots.
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