Soil testing to predict dissolved reactive phosphorus loss in surface runoff from organic soils

Phosphorus loss from surface runoff contributes to eutrophication of surface water, a problem that is often severe from polders with organic soils where agricultural production is intensive. A soil P test is essential to predict the potential for P losses to precisely conduct environmental risk assessment and to efficiently develop and evaluate beneficial management practices. This study evaluated the possibility of using the environmental and agronomic soil P tests, soil P sorption index (PSI), and degree of soil P saturation (DPS), which are used for mineral soils, to predict surface runoff dissolved reactive P (DRP) from organic soils. Forty-four soils from eight subgroups representative of organic lands across Ontario were selected to provide a wide range of soil test P (STP) within each category. A surface runoff study was conducted following the U.S. National Phosphorus Research Project protocol. Flow-weighted mean runoff DRP concentration (DRP30) was linearly related to soil water- and CaCl2-extractable P concentrations but with data distribution patterns that inefficiently represented the soil variability in P release potentials. The runoff DRP30was significantly related to Bray-1 P and FeO-extractable P concentrations in split-line models, each with a change point, but not to Mehlich-3 P and Olsen P. All DPS values calculated based on STP and their derived PSIs were closely related to runoff DRP30in either a linear or a split-line model. The DPS values expressed as Bray-1 P/(PSI + Bray-1 P) and FeO P/(PSI + FeO P) showed the highest correlation with runoff DRP30and thus can be recommended as environmental risk indicators of surface runoff DRP from organic soils.