A phosphorus sorption index and its use to estimate leaching of dissolved phosphorus from agricultural soils in Ontario

• Three simple P sorption indices were evaluated to represent soil P sorption maximum.
• (Sorbed P + Olsen P) is the best estimate of P sorption maximum.
• Olsen P/(sorbed P + Olsen P) improved the prediction of leachate P loss.
• Olsen P/(sorbed P + Olsen P) was independent on soil pH for soil P loss prediction.

Compared with soil testing phosphorus (STP), such as Olsen P, the degree of P saturation (DPS) generally improves the risk prediction for soil P loss. This study was conducted to assess various P sorption indices (PSI) to indicate P sorption maximum (Qmax) of soils, and their derived DPS indices as indicators of dissolved reactive P (DRP) concentration in soil leachate. A total of 236 intact soil columns were collected for leaching experiments from six major soil series in Ontario. By conducting the single-point isotherm, PSI-a of a given soil was determined as the amount of P sorbed by the soil during 24-h shaking in the 60 mg P L− 1 solution, PSI-b as the quotient of PSI-a/log C, where C is the solution P concentration after 24-h shaking, and PSI-c as the sum of PSI-a and Olsen P concentration. Among the tested PSIs, PSI-c gave the best prediction of Qmax. Compared to Olsen P, soil DPS-1 (i.e. the quotient of Olsen P/PSI-c) improved the prediction of leachate DRP concentration. Moreover, soil pH did not impact the relationship of DPS-1 vs. soluble soil P loss. Based on the results from conditional probability analyses, Ontario soils were grouped into no risk, low risk, medium risk, and high risk categories. Considering that Olsen P is the current agronomic STP in Ontario and PSI-c can be quickly determined, soil DPS-1 can be used to identify leaching P loss and/or combined with site hydrology and P management practices for a more comprehensive soil P loss assessment.