Soil phosphorus dynamics: Agronomic and environmental impacts

Phosphorus (P) related eutrophication of surface water is of increasing concern in many areas of the U.S. This concern has arisen where inputs of P in fertilizer and manure to agricultural systems have exceeded output in harvested crops for several years. As the loss of P in surface (r(2) 0.85 to 0.96) and subsurface drainage (r(2) 0.86) have Seen shown to be related to soil P content, measures to minimize nonpoint source losses of agricultural P are most effective when implemented at the source. To achieve this, identification of soils and management systems vulnerable to P loss and an understanding of the main factors controlling soil P bioavailability is essential. Soil P bioavailability is determined by reaction with hydrous oxides, amorphous and crystalline complexes of Al, Fe and Ca, and organic matter. The rate and extent of these reactions is influenced by soil management and drainage. In wetlands for example, Fe redox dominates P solubility, while organic P mineralization can contribute up to 25 kg P ha(-1) yr(-1) in temperate mineral soils and up to 160 kg P ha(-1) yr(-1) in organic soils. However, it is critical that we are able to reliably determine threshold soil P values at which agronomic issues become environmental concerns. Agronomic soil P tests may target problem soils on which environmental soil tests will assess P bioavailability and P sorption capacity. Clearly, soil P dynamics are of agronomic and environmental importance, influencing both crop productivity and eutrophication and thus, must be considered in developing effective management plans.