The effect of topdressed superphosphate on changes in nitrogen:carbon:sulphur: phosphorus and pH on a red earth soil during a long term grazing experiment

Abstract

Increases in total nitrogen in a grazing experiment at Wagga Wagga (N.S.W.) over the years 1962-1966 were related to the levels of superphosphate applied (0-375 kg ha-1), the average rate of increase being 59 kg ha-1 year-1. The rate of change varied between years and was negative in a dry year. Total sulphur levels were not influenced by phosphate treatments (mean 150 ppm), but sulphate sulphur was only maintained by the highest superphosphate level. Total, aluminium, iron, calcium and 'available' phosphate increased with superphosphate levels. The level of aluminium-bound phosphate increased more rapidly than the other fractions. In the treatments without applied phosphate the aluminium bound phosphate declined more rapidly than the other fractions. Organic carbon was not influenced by treatments but showed an increase with time. The C:N ratio declined from 13 to 11.8 during the experiment. Soil pH declined by 0.18 pH units, but this was not affected by superphosphate levels. Stocking rates had no effect on these parameters of soil fertility.