White clover responses to phosphorus and sulphur on granitic soils

Responses in herbage yield, nitrogen uptake and phosphorus uptake by white clover pasture to phosphorus and sulphur application were measured on very phosphorus-deficient granitic soils in the Northern Tablelands of New South Wales. The factorial experiment with six rates of P and three of S was repeated over three years (1966, 1967 and 1968) and at three sites. Coefficients of variation of up to 48 per cent indicated considerable within-site variability in herbage yields. Variation among years was caused mainly by variations in incident rainfall. Very large and highly significant responses in all variables to phosphorus occurred at every harvest regardless of the amount of incident rainfall or applied sulphur. However phosphorus responses were greater with increasing rainfall and in the presence of sulphur. To achieve at least 85 per cent of the maximum recorded yield in any one year, 48 to 73 kg P ha-1, were required. Responses to phosphorus were 1000-2000 kg ha-1 with 150 mm rainfall and up to 3000-4500 kg ha-1 with 450 mm rainfall during the growing season. Responses to sulphur were much smaller and less consistent, and occurred only in the presence of applied phosphorus. They also increased with increasing rainfall. The intermediate rate of 28 kg S ha-1 gave the maximum yield of herbage. Sulphur had a larger effect on nitrogen and phosphorus uptake than on herbage yield.