Soil phosphorus status and environmental risk of phosphorus run-off from pastures in south-eastern New South Wales

Soils were sampled from 136 sites representing the major soil types in the Sydney drinking water catchments to provide an overview of agronomic phosphorus requirement and phosphorus environmental risk for the various pastoral land uses, which together comprise 40% of the 15 700-km2 catchment area. Critical values for Colwell P (estimated from the phosphorus buffering index) were subtracted from the measured Colwell P to derive a ‘normalised’ Colwell P for each site, with negative values broadly signifying potential agronomic response and positive values warranting an assessment of environmental risk. Normalised Colwell P was above zero at approximately half the sites, and >50 mg kg–1 at 20% of sites that were considered to present a significant environmental risk, highlighting the need for more effective use of soil testing. Colwell P was highly variable within soil type and land use. No category could be singled out for special attention for management except for dairy and former dairy sites. Where dairy effluent must be applied to soils already high in P, a plan is needed to manage stormwater run-off. Pastures at approximately 20% of sites were ‘degraded’, where the priority should be to improve pasture management and reduce the risk of phosphorus loss in surface erosion.