Why Has Phosphorus Become A Problem?

Confined animal feeding operations (CAFOs) defined and classified by number and class of animals, continuous days of confinement per year, and potential for waste discharge onto streams, creeks, rivers, or other waters have become a primary method of production for most classes of livestock including the dairy industry in central Texas. Sustainable dairy, beef, swine, and poultry production in the USA depends in part upon the successful management of animal wastes to prevent or reduce negative environmental impacts.

Nitrate-nitrogen movement from land applications of commercial fertilizer or animal waste into ground water has been a concern for many years, but recent research has shown that high levels of phosphorus from animal waste applications can become a major problem in surface soils and runoff water (Sharpley et al., Daniel and Logan, Sharpley and Menzel to cite only a few). Animal waste has been recognized as beneficial for plant growth and crop production since earliest agriculture. Logically, animal waste application to forage and crop land should serve both to remove waste from the production facility and to provide recycling of plant nutrients. Typically, solid waste and lagoon effluent from dairies in central Texas are applied to Coastal bermudagrass (Cynodon dactylon L.Pers.) and to a lesser extent to sorghum-sudangrass hybrids and corn during late spring to early fall. During winter some lagoon effluent is used to irrigate wheat or other cereal crops grown for grazing or silage. Because of the concentration of animals and the large volume of waste generated, there is generally insufficient suitable land area within economically transportable distances to match waste nutrient loads with crop nutrient requirements, especially phosphorus.

Traditionally, it has been thought that phosphorus applied to soil for crop production would remain in place with little movement by water solubility. Inorganic phosphate fertilizers often were applied in excess of assessed need because cost was not prohibitive and the practice was viewed as "banking" phosphorus to insure that it would not be a limiting factor in crop growth. Fertilizer was generally incorporated into the soil during land preparation for crop production.

The use of animal waste from CAFOs as a nutrient source for crop and forage production presents new problems. Much of the forage production consists of perennial species requiring that waste application be largely limited to the soil surface without soil incorporation. Animal waste applications may reduce the sorption capacity of soils because of the humic acid residual of the organic matter. Animal waste provides significantly more phosphorus than nitrogen relative to plant requirement. Therefore, waste application based upon nitrogen requirement results in excess phosphorus application. For many years rates of animal waste application have been based upon nitrogen content of the waste or upon the need for disposal without regard to nutrient content. While animal waste is a plant nutrient source, its nutrient concentration is relatively low and transportation cost per unit of nitrogen or phosphorus is high relative to commercial fertilizer. High transportation cost, convenience of application, lack of regulation, and ignorance of the potential for problems has resulted in high soil phosphorus levels in many areas that have received large applications of animal waste.

High levels of soil phosphorus have recently been related to elevated amounts of phosphorus in runoff water from these soils. It has been established that soils differ in ability to "hold" or "sorb" phosphorus. Current research in the USA is concerned with relating soil test phosphorus for individual soils (determined by various agronomic soil test procedures) to phosphorus concentration in runoff water.

Phosphorus concentration is the limiting nutrient for algal growth in freshwater bodies. When phosphorus levels become eutrophic (enriched) above normal, an algal "bloom" occurs in which algal growth and abundance is greater than normal. This condition can result in oxygen depletion, which in turn can produce fish kills, and degradation of quality of drinking water produced from phosphorus-eutrophic sources. Eutrophic phosphorus levels in certain water bodies have been linked to high levels of phosphorus in soils of the watershed. And high levels of soil phosphorus have been related to high concentrations of animals. However, other sources of phosphorus such as municipal waste treatment plants and industrial discharge can contribute to eutrophic levels in water bodies.

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