Indices of Phosphorus Loss Potential from Ontario Agricultural Soils to Surface Waters
Phosphorus (P) loss from agricultural soils has been identified as one of the major causes of eutrophication of surface waters. This study was conducted to evaluate the suitability of various measures of soil P as indicators of risk potential for P loss from agricultural soils to surface waters. To fulfill the research objective, soil samples were collected from six major soil series in southern Ontario, and were subjected to simulated precipitation and to leaching. Relationships between various soil P measures and dissolved reactive P (DRP) concentration in surface runoff and subsurface flow were assessed. Amongst the selected soil test P (STP) and the estimates of degree of P saturation (DPS), DPSM3-2 [Mehlich-3 P/(Mehlich-3 Al + Fe)], DPSM3-3 (Mehlich-3 P/Mehlich-3 Al), and water extractable P (WEP) had the highest correlation with DRP concentration in surface runoff and leachate across all six soil series. The Fe-oxide coated filter paper P (FeO-P) method gave the second best predictor of DRP concentration through a split-line linear model. The Olsen P test was significantly correlated to DRP losses in runoff and leachate but it was generally not as strongly correlated to DRP losses as were other soil P measures. Given that soil WEP concentration can represent risk of soil P loss, a study with a greater range of soils (n=391) suggested that DPSM3-2 and DPSM3-3 tended to overestimate P losses from alkaline soils, especially when soils had high DPSM3-2 or DPSM3-3. In comparison, soil FeO-P and DPSOl-b [Olsen P/(Olsen P + P sorption index)] each were significantly correlated to DRP concentrations in surface runoff, subsurface water and soil WEP concentration, and showed reasonable accuracy. Compared to STP and routine DPS, a detailed soil DPS estimated from P sorption isotherm (DPSsorp) and P buffering capacity (PBC0), could provide reliable predictions of runoff DRP concentration across different soil types. Within each soil type, runoff DRP concentration increased linearly with increasing DPSsorp following a common slope of approximately 1.79, while the common change point was at a PBC0 value of approximately 0.29 L mg-1. A unit change in the PBC0 value resulted in a much greater change in runoff DRP concentration below the change point than above the change point.