Determining environmentally sound soil phosphorus levels

TLDR

Increased phosphorus inputs from agriculture accelerate freshwater eutrophication, especially in intensive farming areas where soil P has risen to environmentally concerning levels. The study aims to evaluate whether soil test phosphorus can reliably indicate phosphorus loss in runoff to guide manure application recommendations. The authors develop an integrated model combining soil test P, runoff and erosion estimates, and local climatic, topographic, and agronomic factors to produce flexible fertilizer and manure P recommendations. Field studies show that soil test P correlates with dissolved P in runoff, supporting state‑established critical levels of 75–200 mg kg⁻¹, but runoff losses also depend on site‑specific runoff and erosion, indicating soil test P alone is insufficient.

Abstract

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.