DETERMINING ENVIRONMENTALLY SOUND SOIL-PHOSPHORUS LEVELS

Increased inputs of phosphorus (P) in agricultural runoff can accelera te freshwater eutrophication. This is of particular concern in areas o f intensive crop and livestock farming, where soil P has increased to levels that are of environmental rather than agronomic concern. Thus, many states have been forced to consider the development of recommenda tions for manure applications based on the potential for P loss in run off, thereby creating an urgent need to assess the validity of the we of soil test P as an indicator of P loss in runoff. Several field stud ies have shown the dissolved P (DP) concentration of runoff is related to soil test P (r(2) of 0.58-0.98) of surface soil (0 to 5 cm). There relationships can identify critical levels which can support a DP con centration of runoff: In this regard critical soil test. P levels esta blished by several states, ranging from 75 to 200 mg kg(-1), appear to be realistic However, amounts of P lost in runoff are not related sol ely to soil test P, due to variable site runoff and erosion potentials . Thus, an approach which integrates sail test P with estimates of pot ential runoff and erosion losses and local climatic, topographic, and agronomic factors is being developed for reliable, yet flexible recomm endations of fertilizer and manure P management in environmentally sou nd production agriculture.