Effects of management on soil fertility under pasture. 2. Changes in nutrient availability

Abstract

The effects of five years of differential grazing and fertilizer treatments under pasture on the availability of soil nutrients were examined by annual glasshouse experiments on the surface soil and by a terminal field crop of barley. The glasshouse results showed that high grazing pressure increased nitrogen availability in the surface soil, especially after certain field fertilizer treatments. However, grazing pressure had no effect on phosphorus or sulphur availability, which increased only with superphosphate application. The barley crop responded to both nitrogen and phosphate and confirmed that high grazing pressure had caused increased nitrogen availability. There were interactions between grazing pressure and previous superphosphate treatments, such that greatest nitrogen uptake occurred after low superphosphate and high grazing pressure. To exploit the available nitrogen, which was present largely as nitrate in the 0-60 cm soil depth, additional phosphate was required by the barley. No grazing effects on either phosphorus or sulphur uptake were detected in the crop, but sulphate in the soil profile to 60 cm was reduced after high grazing pressure. Thus it appears that nitrogen availability under pasture was influenced by both grazing and fertilizer management, whereas phosphorus and sulphur availability were controlled directly only by fertilizer application.