A Review of the Development and Use of the Degree of Phosphorus Saturation

Abstract

The application of phosphorus (P) fertilizers in agricultural soils can result in P loss and water quality impairment. However, the amount of loss depends on the rate of P applied, P removed in plant uptake, and the sorption capacity and strength of the soil. Understanding soil sorption is critical for preventing P loss to water, especially in dissolved forms. Because sorption capacity is difficult to modify in practice, risk mitigation should focus on maintaining sufficient plant-available P for productive growth while avoiding excessive P accumulation that compromises soil sorption sites and increases the likelihood of P loss. Soil test P (STP) has been used to indicate thresholds, or change points, in plant available P above which the likelihood of P loss increases but does not assess sorption, so varies by soil type. The degree of P saturation (DPS) integrates information of P availability and sorption capacity, providing a useful indicator of the potential risk of P loss in agricultural soils. We assessed 163 studies for the variability in change points in STP and DPS. The data indicated that DPS-based change points showed lower variability than those from STP. Variation in change points was caused by soil chemical parameters: pH, organic matter, and the concentrations of Fe- and Al-hydrous oxides. Soil texture was also evaluated but was not identified as a significant predictor. To limit high P mobility, various mitigation methods can be used, especially amendments that improve the soil's ability to adsorb P. Through understanding the influence of these soil chemical parameters, both STP and DPS can be used to guide agronomic P thresholds optimized for crop growth and reduced P loss to water.