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

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.