Simple phosphorus saturation index to estimate risk of dissolved P in runoff from arable soils

There is increasing evidence that phosphorus has been accumulating in the surface horizons of agricultural soils to the extent that some soils represent a potential diffuse source of pollution to surface waters. The relationships between equilibrium phosphorus concentration at zero sorption (EPC 0 ) of soil and a number of soil physicochemical variables were investigated in the surface layers of arable and grassland agricultural soils sampled from the Thame catchment, England. Soil EPC 0 could be predicted from an equation including soil test (Olsen) P, soil phosphate sorption index (PSI) and organic matter content (OM) (R 2 =0.88; P <0.001) across a range of soil types and land use. The simple index Olsen P/PSI was found to be a good predictor of EPC 0 (R 2 =0.77; P <0.001) and readily desorbable (0.02 m KCl extractable) P (R 2 =0.73; P <0.001) across a range of soil types under arable having soil organic matter contents of <10%.