Soil Tests as Risk Indicators for Leaching of Dissolved Phosphorus from Agricultural Soils in Ontario

Abstract

Phosphorus movement in subsurface flow from agricultural soils can be a significant pathway contributing to eutrophication of surface waters. Our study aimed to evaluate several environmental and agronomic soil P tests as indicators of dissolved reactive P (DRP) concentrations in soil‐column leachate from Ontario soils. Undisturbed soil columns were collected from six major soil series, with 10 sites of each to quantitatively cover a wide range of soil test P (STP) or degree of P saturation (DPS). Split‐line models described the relationships (P < 0.001) between leachate DRP concentrations and the values of ln(STP) and ln(DPS), with a greater slope observed above the change points than below them. Among the tested soil P measures, water‐extractable P (WEP), Mehlich‐3 P/(Mehlich‐3 Al + Fe) (DPSM3‐1), and Mehlich‐3 P/Mehlich‐3 Al (DPSM3‐2) had the strongest overall relationships with leachate DRP concentration. Ontario soils were grouped into no‐risk, low‐risk, medium‐risk, and high‐risk categories based on the conditional probability of yielding leachate DRP > 0.1 mg P L−1 at a given STP as measured by WEP and Olsen P or a given DPS as measured by DPSM3‐1 and DPSM3‐2. While the Olsen P test is most commonly used for agronomic calibration in Ontario, DPSM3‐2 provided the best indicator of leachate DRP concentration from Ontario soils. Regardless of the test method used, these numeric criteria could be combined with site hydrology and P management practices for a more comprehensive soil P loss assessment.