Soil phosphorus concentrations to minimise potential P loss to surface waters in Southland

Losses of soil-derived P in overland flow induced by artificial rainfall were measured from six pastoral soils under a range of soil Olsen P concentrations. These soils were selected as being typical of the major soil groups that cover much of lowland Southland. Our objectives were to establish the magnitude and patterns of soil P release to overland flow under a wide range of soil Olsen P concentrations, established by amendment with either P fertiliser or dairy manure. The incorporation of superphosphate or manure into soils increased the soil Olsen P concentration as well as the concentration of P fractions in overland flow. There appeared to be no difference in the trend of P loss relative to Olsen P regardless of the form of P amendment. The magnitude of P loss appeared to be influenced by soil pedological origin, with lower dissolved reactive P (DRP) concentrations measured in overland flow from the Brown soils, compared to the less weathered Recent and Pallic soils. Regression analyses indicated that DRP concentrations in overland flow exceeded 0.02 mg DRP litre(-1) when soil Olsen P values were greater than 51, 24, 10, 9, 5, and 20 mg kg(-1) for the Woodlands, Waikiwi, Mataura, Northope, Pukemutu, and Waikoikoi soils, respectively. Given that eutrophication is likely to be encouraged when DRP concentrations in overland flow exceed this value of 0.02 mg litre(-1), we suggest that soil Olsen P concentrations should be kept at or as close to the agronomic optimum as possible to minimise potential soil-derived P losses and still maintain pasture productivity. Economic analysis indicated that significant savings could be made by keeping to the agronomic optimum, avoiding the unnecessary increase in soil Olsen P concentration and consequent increased environmental risk from P loss.