Determining environmentally sound soil phosphorus levels

ABSTRACT: Increased inputs of phosphorus (P) in agricultural runoff can accelerate freshwater eutrophication. This is of particular concern in areas of 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 recommendations for manure applications based on the potential for P loss in runoff, thereby creating an urgent need to assess the validity of the use of soil test P as an indicator of P loss in runoff. Several field studies have shown the dissolved P (DP) concentration of runoff is related to soil test P (r2 of 0.58–0.98) of surface soil (0 to 5 cm). These relationships can identify critical levels which can support a DP concentration of runoff. In this regard, critical soil test P levels established 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 solely to soil test P, due to variable site runoff and erosion potentials. Thus, an approach which integrates soil test P with estimates of potential runoff and erosion losses and local climatic, topographic, and agronomic factors is being developed for reliable, yet flexible recommendations of fertilizer and manure P management in environmentally sound production agriculture.