Phosphorus sorption parameters of soils and their relationships with soil test phosphorus

Understanding phosphorus (P) sorption characteristics of soils and their relationships with soil test P (STP) may aid the prediction of runoff soluble P concentration. This study was conducted to provide such information for soils in Ontario, Canada. A total of 60 surface soil samples were collected from six major soil series in Ontario. Phosphorous sorption behaviors were characterized by a modified Langmuir equation and summarized by four sorption parameters: P sorption maximum (Qmax), P sorption strength (PSS), equilibrium P concentration at zero net sorption (EPC0), and initially sorbed P (Q0). Soil PSS, EPC0, and Q0 could be predicted by saturated iron-oxide strip P, Olsen P, water extractable P, and Mehlich-3 P, with PSS following a power function (r2 = 0.93-0.95), EPC0 following a quadratic function (r2 = 0.42-0.82), and Q0 following a linear function (r2 = 0.57-0.79). We identified a critical PSS value (i.e., 0.39 L mg, corresponding to 30 mg Olsen P kg-1) above which soils had low surface runoff DRP and EPC0 concentration (<0.1 mg-1 L-1). The relationship between Olsen P and EPC0 confirmed that the soil with Olsen P < 30 mg kg-1 would lead to EPC0 concentration < 0.1mg L-1. The findings suggest that soils with Olsen P > 30 mg P kg-1 may cause significant runoff soluble P loss, and thus necessary mitigation measures may need to be adopted on confirmation with assessment of a site P index that integrates source factors with the transport factors (e.g., soil hydrological conductivity, slope, precipitation, and distance from the water source).