A capacity factor as an alternative to soil test phosphorus in phosphorus risk assessment

Soil test phosphorus (P) concentrations (STP) are often used as measures of environmental P risk. However, a low STP is not valid justification for further P application because P sorption capacity may be low and P added could be lost to surface waters. The degree of P saturation (DPS) normalises extractable P using extractable Al and Fe as a surrogate for P sorption capacity, but like STP, fails to convey a magnitude of capacity. We propose the use of a DPS-based prediction of the remaining soil P storage capacity (SPSC) that would capture risks arising from previous loading as well as inherently low P sorption capacity. The SPSC is a direct estimate of the amount of P a soil can sorb before exceeding a threshold soil equilibrium concentration. In this paper, we demonstrate the applicability of the SPSC for a variety of sandy soils impacted by dairy and poultry manure additions. The SPSC provides a means to assess the capacity of a soil to retain additional P and hence is a more useful indicator of P-related environmental risk than STP or DPS measures alone.