Estimating Dissolved Phosphorus Concentrations in Sub-Surface Flow From Six Major Ontario Soil Series.

The movement of phosphorus (P) in subsurface flow can contribute to losses from agricultural land. Our study aimed to evaluate several environmental and agronomic soil P tests as indicators of dissolved reactive P (DRP) concentrations in subsurface flow from Ontario soils. The undisturbed soil columns were collected from six soil series that had a wide range of soil test P (STP) values, and were leached with double distilled water. The surface soils were analyzed for Olsen P (Olsen-P, Ontario’s agronomic STP), Mehlich-3 P (M3-P), Bray-1 P (Bray-P), Fe-oxide coated filter paper strip P (FeO-P), water extractable P (WEP), 0.01 M CaCl₂ extractable P (CaCl₂-P), and a single-point isotherm (PSI). The degree of P saturation (DPS) included DPS_M3-1 (M3-P/(M3-P+PSI)), DPS_M3-2 (M3-P/(M3-Al+M3-Fe)), DPS_M3-3 (M3-P/M3-Al), DPS_M3-4 (M3-P/M3-Ca), DPS_Ol (Olsen-P/(Olsen-P+PSI)), DPS_Bray (Bray-P/(Bray-P+PSI)), and DPS_FeO (FeO-P/(FeO-P + PSI)). Differences were observed between soil series and various P parameters.  No quantitative relationship was observed between concentrations of DRP in leachate from Brookston Clay soils and levels of various soil P tests, with leachate DRP concentrations typically low and < 0.1 mg P L^-1. For the other soil series, DRP concentration in leachate increased (usually in a non-linear manner) with increases in STP or DPS. Split-line models adequately described relationships between concentration of DRP in leachate water and WEP (r²=0.9174), DPS_M3-2 (r²=0.8821), DPS_M3-3 (r²=0.8766), FeO-P (r²=0.8409), DPS_Ol (r²=0.8394), DPS_M3-1 (r²=0.8376), DPS_FeO (r²=0.8308), DPS_Bray (r²=0.8146), or Olsen-P (r²=0.6255). Above the change points, rate of DRP concentration increase in leachate per unit increase of STP/DPS was far greater than below these change points. In contrast, soil M3-P, CaCl₂-P, and Bray-P followed quadratic relationships with leachate DRP concentrations. Of the various estimates of DPS, the DPS_M3-3 showed excellent promise for identifying the soils that are with high risk for P leaching losses.