Effects of management on soil fertility under pasture. 3. Changes in total soil nitrogen, carbon, phosphorus and exchangeable cations

Abstract

Soil samples from pasture plots which received differential superphosphate, nitrogen and grazing treatments for five years were analysed chemically. Changes in total soil nitrogen were studied annually on different sections of each plot. Changes in carbon, phosphorus and cations were studied in the final samples. High grazing pressure by sheep tended to reduce the accumulation of organic nitrogen and carbon, but the effects were variable on different sections of the plots and it was uncertain (P > 0.05) that a real difference existed when averaged over the whole experimental area. The grazing treatments caused a concentration of soil nitrogen towards one end of each plot, particularly where high numbers of sheep were used to produce a high grazing pressure. Substantial losses of incoming nitrogen from fertilizer and clover occurred at both grazing pressures. Nitrogen accumulations during the drier years of the experiment increased at the higher rates of superphosphate but this effect was lost during the final wetter year. The accumulation of nitrogen on each plot appeared to depend on the initial soil nitrogen content at least as much as on the experimental treatment. Annual nitrogen changes were not constant and the overall accumulations appeared to be the net effect of gains and losses in good and bad clover years respectively. Organic phosphorus and exchangeable potassium increased to a similar extent under all treatments. Inorganic phosphorus was affected only by fertilizer additions. These results suggest that, in regions of variable rainfall patterns, the rate of nitrogen and organic matter accumulation under dryland pastures is difficult to control, and that seasonal rainfall and initial soil nitrogen content are dominant factors.