The comparative phosphate requirements of four annual pastures and two crops

Abstract

The levels of broadcast phosphate needed for 90% of maximum production by subterranean clover, Wimmera ryegrass at two levels of nitrogen, wheat and lupins are compared in a field experiment. Two seeding rates, one five times the other, were used to vary the stand densities and yields per unit area of the swards. A total of 180 kg nitrogen/ha was supplied to the wheat and high nitrogen grass plots; the low nitrogen grass plots received 30 kg/ha. Although both the high seeding rates and the high nitrogen applications gave up to double the yield per unit area, they did not significantly change the level of phosphate required for 90% of maximum yield by a given species or mixture. Grown as single species, lupins had the highest phosphorus requirement (202 kg/ha); wheat (118 kg/ha) and clover (107 kg/ha) had similar requirements; while grass with either a low nitrogen supply (58 kg/ha) or a high nitrogen supply (56 kg/ha) had the lowest requirement when measured during flowering. When clover and ryegrass were grown as a mixed sward, the phosphorus requirement (84 kg/ha) was close to the average of those for the pure clover and grass. Both the grass and clover responded to approximately the same level of phosphate when grown as a mixture. The amount of nitrogen fixed by the clover, either as a pure sward or when mixed with grass, increased with increasing phosphate application. We think that the phosphate level required by the grass when grown with clover rather than as a pure sward was an expression of this increased nitrogen supply and not a direct response to phosphate. The levels of phosphate required to produce 90% of the maximum grain yield in the lupin and ear yield in the wheat were similar to the phosphate requirement for 90% of maximum vegetative yield.