Effects of nitrogen and phosphorus fertilizers and gypsum on a Danthonia caespitosa-Stipa variabilis grassland. 2. Residual response

Abstract

The residual effects of nitrogen and phosphorus fertilizers applied over four consecutive years and gypsum applied once only, were assessed over five years on a Danthonia caespitosa-Stipa variabilis grassland, growing on the semi-arid Riverine Plain of south-eastern Australia. Totals of 155 and 31 0 kg ha-1 of N, 100 and 200 kg ha-1 of P, and 3.34 and 6.68 t ha-1 of CaS0,. 2H2O had been added. The increase in dry matter production in response to nitrogen ceased one year after the last application of nitrogen fertilizer. After five years, grassland which had received 200 kg ha-1 of phosphorus and 6.68 t ha-1 of gypsum yielded 6500 kg ha-1 of dry matter, compared with 1300 kg ha-1 in the absence of fertilizers and gypsum. Legume growth was still suppressed five years after nitrogen fertilizer had been applied, but legumes increased production greatly in response to the previous phosphorus and phosphorus-gypsum treatments. Nitrogen content of the herbage averaged 2.1 per cent four years after the previous high level phosphorus treatment, compared with 1.6 per cent with the control after four years. It was unaffected by previous nitrogen fertilizer. Phosphorus content was reduced for the first three years after nitrogen fertilizer was last added, but was increased by phosphorus fertilizer to 0.28 per cent, compared with 0.1 8 per cent on the control, four years after phosphorus fertilizer was last added. Soil moisture characteristics, and total soil nitrogen and carbon showed no differences between treatments. The Truog phosphorus in the soil under the high phosphorus level decreased from 103 p.p.m. to 53 p.p.m., compared with the unaltered control level of 7 p.p.m, over the five year period.