Native pasture response to nitrogen and sulfur under two cutting frequencies on a cracking clay soil in southern Queensland

Abstract

The responses to fertilizers containing nitrogen (N) and sulfur (S) and to cutting frequency of native pasture containing black speargrass (Heteropogon contortus), forest bluegrass (Bothriochloa bladhii), and Queensland bluegrass (Dichanthium sericeum) were measured on a cracking clay soil derived from basalt. A factorial layout included N (as urea) at 0, 28, 56, 112 and 224 kg ha-1 year-1, superphosphate at 0 and 250 kg ha-1 year-1 plus potassium chloride at 0 and 125 kg ha-1 every second year and mowing every 3 and 6 weeks in summer from 1963-64 to 1969-70 inclusive. In order to investigate the effect of a possible S deficiency a split-plot design comprising five N and two S levels (0 and 67 kg ha-1 year-1) was superimposed in 1968-69 on those plots that did not receive superphosphate, and were cut every 6 weeks. A relation fitted for the N treatments that received superphosphate and were cut at weekly intervals from 1965-66 to 1969-70 indicated that yield depended on summer rainfall and on N through an interaction with rainfall. At high N levels, fertilizer containing superphosphate increased yield in most seasons through an interaction with N. The split-plot experiment showed that S was the element in the mixed fertilizer application responsible for pasture response. More frequent cutting reduced pasture yield (except for the first two summers because of experimental error) by 24% and N yields by 6%. N at rates above 56 kg ha-1 year-1 initially increased black speargrass basal area and later reduced it and the basal area of forest bluegrass at the highest N rate. The basal area of 'other species' was increased at higher N rates.