Soil Testing to Predict Phosphorus Leaching

Abstract

Subsurface pathways can play an important role in agricultural phosphorus (P) losses that can decrease surface water quality. This study evaluated agronomic and environmental soil tests for predicting P losses in water leaching from undisturbed soils. Intact soil columns were collected for five soil types that had a wide range in soil test P. The columns were leached with deionized water, the leachate analyzed for dissolved reactive phosphorus (DRP), and the soils analyzed for water‐soluble phosphorus (WSP), 0.01 M CaCl₂ P (CaCl₂–P), iron‐strip phosphorus (FeO‐P), and Mehlich‐1 and Mehlich‐3 extractable P, Al, and Fe. The Mehlich‐3 P saturation ratio (M3‐PSR) was calculated as the molar ratio of Mehlich‐3 extractable P/[Al + Fe]. Leachate DRP was frequently above concentrations associated with eutrophication. For the relationship between DRP in leachate and all of the soil tests used, a change point was determined, below which leachate DRP increased slowly per unit increase in soil test P, and above which leachate DRP increased rapidly. Environmental soil tests (WSP, CaCl₂–P, and FeO‐P) were slightly better at predicting leachate DRP than agronomic soil tests (Mehlich‐1 P, Mehlich‐3 P, and the M3‐PSR), although the M3‐PSR was as good as the environmental soil tests if two outliers were omitted. Our results support the development of Mehlich‐3 P and M3‐PSR categories for profitable agriculture and environmental protection; however, to most accurately characterize the risk of P loss from soil to water by leaching, soil P testing must be fully integrated with other site properties and P management practices.