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 (Q(max)), P sorption strength (PSS), equilibrium P concentration at zero net sorption (EPC0), and initially sorbed P (Q(0)). Soil PSS, EPC0, and Q(0) could be predicted by saturated iron-oxide strip P, Olsen P, water extractable P, and Mehlich-3 P, with PSS following a power function (r(2) = 0.93-0.95), EPC0 following a quadratic function (r(2) = 0.42-0.82), and Q(0) following a linear function (r(2) = 0.57-0.79). We identified a critical PSS value (i.e., 0.39 L mg(-1), corresponding to 30 mg Olsen P kg(-1)) above which soils had low surface runoff DRP and EPC0 concentration (<0.1 mg 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 < 0.1 mg 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).