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 s urface horizons of agricultural soils to the extent that some soils represe nt a potential diffuse source of pollution to surface waters. The relations hips between equilibrium phosphorus concentration at zero sorption (EPCo) o f 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 EPCo could be predicted from an equation including soil test (Olsen) P, soil phosphate sorption index (PSI) and org anic 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 predicto r of EPCo (R-2 = 0.77; P<0.001) and readily desorbable (0.02 M KCl extracta ble) P (R2 = 0.73; P<0.001) across a range of soil types under arable havin g soil organic matter contents of <10%.