Stocking rate and the superphosphate requirements of sown pasture on an acid soil

Abstract

The response by Phalaris-subterranean clover pasture to applied nitrogen, phosphorus, and sulphur was measured at two sites on the same soil. Both sites had the same fertilizer history. For the past fifteen years the stocking rate on one site had been maintained at not less than three times that of the other. Yield responses to phosphorus were obtained at both sites. The response at the lower stocking rate was greater than that at the high stocking rate. A similar response pattern was found for nitrogen, but the difference in response between the stocking rate treatments was not significant. The pastures did not respond to sulphur. The trend was for sulphur to depress the yield at both stocking rates. The evidence suggests that less superphosphate was needed to maintain pasture production at high stocking rates than at low ones. Available phosphorus, determined chemically, increased with grazing intensity. There was no evidence that a particular soil phosphorus fraction was associated with this increase, or the lower requirement for added fertilizer. The only significant difference between grazing treatments were increases in the iron, calcium, and organic forms at the higher grazing intensity. It is probable that the present pasture response, and hence the fertilizer requirement of the soil, was influenced more by the altered growth potential of the plants under grazing, than by the effect of grazing on the soil nutrient status.