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 (<0.5mgL-1) where adjusted Colwell-P was less than zero, which on this scale is the threshold for agronomic response. Similar results were found for dissolved reactive P (DRP) in runoff, although for plots with soil P below the agronomic threshold, the concentrations of DRP were lower than for TP. We conclude that Colwell-P and soil PBI together provide a widely applicable test for environmental P risk from pastures with good ground cover. The same threshold values may be used for both agronomic and environmental purposes.