Superphosphate requirements of clover-ley farming. I. the effects of topdressing on productivity in the ley phase
Experiments were undertaken on four farms in southern New South Wales to assess the requirements for superphosphate of clover-ley farming soils. The sites were selected to provide contrasts in initial soil phosphate status (medium v. high) in both low and high average annual rainfall conditions. At each site a sheep-grazing experiment with factorial arrangement of three annual topdressing rates (nil, 125, and 250 kg superphosphate ha-1) and two sheep stocking rates (district average, twice district average) was imposed for the duration of a normal ley phase (4 years). The response to topdressing depended on the initial soil phosphate status and was modified by seasonal rainfall conditions. At the high phosphate sites topdressing had little effect on pasture production, but at the medium phosphate sites topdressing increased pasture availability, and caused rapid and profound changes in botanical conlposition; topdressing induced grass dominance while omission of topdressing led to clover dominance. The changes in botanical composition had important effects on forage quality. With normal seasonal conditions there was no overall response to topdressing in sheep performance, but in poor years the pasture changes conferred by topdressing were reflected in a general increase in the level of sheep productivity.