Nitrogen response of pasture grasses on duplex soils formed from granite in southern Queensland

Abstract

The response to fertilizer N of native pasture containing black spear grass (Heteropogon contortus) and of buffel grass (Cenchrus ciliaris cv. Biloela) was measured on granitic soils at the Narayen Research Station, in southern Queensland. Experiments were carried out in grazed and ungrazed areas. In 1968-69, which was as dry as any year on record for this district, neither pasture responded to N; in wetter years both did, the native pasture giving a lower yield than the sown buffel grass pasture. Grazed areas that received 168 kg N ha-1 year-1 as urea did not respond to further additions. Analysis of buffel grass yields over intervals of 8 to 16 weeks during the growing season indicated that 1. there was no growth or response to N unless the rainfall exceeded about 30 mm, 2. above 30 mm the response to near-optimum rates of N increased with rainfall, 3. buffel grass grown with such rates of N produced more than four times as much DM per unit rainfall as buffel grass without added N when 100 mm of rain fell. Examination of rainfall records suggested that there is a high probability of worthwhile responses to N on these soils at Narayen.Spear grass and other native grass plants were killed by high rates of N during the first, dry season. The results of a pot trial with Biloela buffel grass and a late flowering strain of spear grass showed that N increased drought damage in both grasses but spear grass was more susceptible than buffel grass.