SOIL-PHOSPHORUS DYNAMICS - AGRONOMIC AND ENVIRONMENTAL IMPACTS

Phosphorus (P) related eutrophication of surface water is of increasin g concern in many areas of the U.S. This concern has arisen where inpu ts of P in fertilizer and manure to agricultural systems have exceeded output in harvested crops for several years. As the loss of P in surf ace (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 s ource losses of agricultural P are most effective when implemented at the source. To achieve this, identification of soils and management sy stems vulnerable to P loss and an understanding of the main factors co ntrolling soil P bioavailability is essential. Soil P bioavailability is determined by reaction with hydrous oxides, amorphous and crystalli ne 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 temperat e mineral soils and up to 160 kg P ha(-1) yr(-1) in organic soils. How ever, 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 environmenta l soil tests will assess P bioavailability and P sorption capacity. Cl early, soil P dynamics are of agronomic and environmental importance, influencing both crop productivity and eutrophication and thus, must b e considered in developing effective management plans.