Development of An Environmental Soil P Test for Ontario, Canada.

Phosphorus (P) index is a tool that is used to assess risk of soil P loss and to identify BMPs which mitigate contamination risk to water resources. Concerns exist on the scientific validity of the current Ontario P index (OPI), as little work has been done to develop, test, or validate the related factors under Ontario conditions. The objective of this study was to develop an environmental soil P test method (ESPT) for Ontario soils. Six soil series distributing in the major manure production areas of Ontario were selected to cover a great range of chemical and physical properties, with each soil series sampled for 10 sites to provide a range of soil test P levels. Runoff study was conducted using the rainfall simulation procedure, and the leaching study was performed using the undisturbed soil column approach. A total of 17 soil P tests and measurements for degree of soil P saturation (DPS) were evaluated by relating to P concentrations in surface runoff and leachate water. The relationships between soil test P and P concentrations in surface runoff and leaching water obtained from the 60 sites were further confirmed by using soil samples collected from largely diversified soil types and geographical locations across the province. Degree of P saturation determined using Mehlich-3 (M-3 P/M-3 Al) procedure (DPS_M3-A1) was commonly best related to both total dissolved and dissolved reactive P (DRP) in both surface runoff and leachate water. The calculated values of soil DPS_M3-A1 was 0.0517 and 0.0309, when the concentration of DRP was 0.05 mg P L^-1 in runoff and leachate, respectively. Compared to Olsen-P, the agronomic soil P test currently recommended in Ontario, DPS_M3-A1 improved effectiveness for prediction of DRP by 24% in surface runoff water and by 54% in leachate. The prediction effectiveness of DPS_M3-A1for DRP concentrations in surface runoff water can be further improved by inclusion of contents of silt and clay. The validation study proved the excellent relationship between DPS_M3-A1 and water extractable P, a soil test P that is believed best reflecting potentials for soil P loss. DPS_M3-A1 can be used as an ESPT in Ontario.