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 (EPCo) 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 EPCo 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 EPCo (R-2 = 0.77; P<0.001) and readily desorbable (0.02 M KCl extractable) P (R2 = 0.73; P<0.001) across a range of soil types under arable having soil organic matter contents of <10%.