Available water capacity is a measure of the soil's capacity to hold water in a form available to plants. It is a function of soil porosity, texture, structure, organic matter content, and salinity. Available soil water is estimated as the difference between soil water content at 1/3 or 1/10 bar tension (field capacity) and 15 bar tension (permanent wilting point). The available water capacity is generally expressed as the sum of available water in inches to a specified soil depth. Generally, this depth is 5 feet or the depth to a root-restricting layer, whichever is less. Available water capacity infers the capacity of a soil to store or retain soil water, liquid agricultural wastes, or mineralized agricultural waste solids in the soil solution. Applying agricultural wastes increases soil organic matter content, helps to stabilize soil structure, and enhances available water capacity.
Limitations for agricultural waste applications are slight if the available water capacity is more than 6.0 inches per 5 foot of soil depth, moderate if it is 3.0 to 6.0 inches, and severe if it is less than 3.0 inches. Soils for which the limitations are moderate have reduced plant growth potential, limited microbial activity, and low potential for retaining liquid and mineralized agricultural waste solids. Lower waste application rates diminish the potential for ground water contamination and help to alleviate agricultural waste overloading.
Soils that have severe limitations because of the available water capacity have low plant growth potential, very low potential for retaining liquid or mineralized agricultural waste solids, low microbial activity, and high potential for agricultural waste contamination of surface and ground water. Reducing waste application rates, splitting applications, and applying waste only during the growing season diminish potential for ground and surface water contamination and help prevent agricultural waste overloading.
The volume of liquid agricultural waste application should not exceed the available water capacity of the root zone or the soil moisture deficit at the time of application. Low rates and frequent applications of liquid agricultural wastes on soil that has low available water capacity or during periods of high soil moisture deficit can reduce potential for ground water contamination.
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