Why are some soils more productive than others?
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Answer:
Soil productivity is defined as the capacity of a soil to produce a certain yield of agricultural crops or other plants using a defined set of management practices.
Soil productivity encompasses soil fertility plus the inherent and management-related factors affecting plant growth and development. It is generally measured in terms of inputs versus outputs, which for agronomic situations generally refers to water and/or nutrient input versus crop yield. The critical soil functions influencing productivity within any soil are those that provide physical support, a rooting medium with plant-available water, air for respiration, and essential nutrients. Humankind can also have a tremendous impact on soil productivity through its effects on the dynamic soil properties. Agricultural management decisions regarding tillage, fertilization, crop rotation, irrigation, and drainage are among the practices that can significantly affect soil productivity.
Soil productivity factors that are usually diminished by soil erosion include direct loss of soil fertility, loss of soil organic matter, deterioration of soil structure, and decreased water-supplying capacity (capacity to provide water to growing plants). The primary seat of fertility of many soils is the topsoil. Direct loss of soil fertility occurs when surface-applied fertilizers or available plant nutrients attached to soil particles are removed during runoff and erosion. Indirect loss of soil fertility occurs in the organic matter that is lost when topsoil erodes. Burwell et al. (1975) found that sediment transport accounted for more than 95% of the N and P lost and most of the K lost from fallow, continuous corn, and rotational corn treatments. A classic study in Missouri (Miller and Krusekopf, 1932) showed that nutrients removed by erosion, expressed as a percentage of the amounts removed by continuous com, were N, 55%; P, 90%; K, 605%; Ca, 550%; and Mg, 290%. A rotational cropping system of corn- wheat-clover markedly decreased soil erosion and reduced nutrient losses to 22, 36, 214, 212, and 97%, respectively, of the N, P, K, Ca, and Mg removed by the crops.
As these data illustrate, loss of soil organic matter by erosion causes a disproportionate loss of several nutrients. Organic matter is the most abundant indigenous source of N and some secondary nutrients and micronutrients, especially in strongly weathered soils. Generally, eroded soils have higher acidity and lime requirements than uneroded soils. Soil erosion often results in replacement of topsoil with more-acid subsoil, selective removal of the baseforming elements (K, Ca, and Mg) from the topsoil, and removal of applied lime before it reacts to neutralize soil acidity.
Soil fertility and lime requirement can be corrected so readily by soil amendments that it is difficult to claim soil productivity benefit through these parameters directly from controlling soil erosion. However, the added costs of maintaining productivity with purchased amendments is an obvious disadvantage. Furthermore, substantial productivity is often sacrificed before nutrient and lime deficiencies are discovered.
Deteriorating soil structure during soil erosion contributes to diminished productivity in at least two important ways—decreased infiltration and storage of available water and impaired root exploitation of the soil. Lower organic matter and higher clay contents associated with erosion make soil more susceptible to compaction. If it does not actually decrease the available water-holding capacity of the soil, compaction reduces infiltration and restricts plant root growth. Both decrease the water-supplying capacity of the soil and diminish the soil’s yield capacity.
Erosion decreases the soil’s water-supplying capacity in still another way. Accelerated erosion removes soil from the surface much more rapidly than the soil profile deepens. Thus, the rooting depth rapidly decreases in soils that are shallow to a root-restricting layer such as a bedrock or hardpan.