An agronomic test to manage phosphorus environmental risk from pastures with good grazing management

Management of phosphorus (P) environmental risk has been hampered by the lack of widely applicable threshold values for the soil properties that determine concentrations of P in runoff. This paper simplified the task of developing threshold values by restricting land-use to pastures and by using a single methodology based on simulated rainfall across 38 field sites (76 plots) that included a range of land-use intensities, and soils that varied widely in lithology and soil properties. An 'adjusted' soil-test P was determined from the measured Colwell-P minus the threshold P for agronomic response, which was estimated from P-buffering index (PBI). Concentrations of total P (TP) in runoff rose exponentially with rising 'adjusted' soil-test P, although only with pastures with high groundcover (r2=0.87), or if plots were protected from erosion with a cover of shade-cloth (r2=0.70). Concentrations of TP in runoff were low (