COMPARATIVE RESPONSES OF ANNUAL PASTURE LEGUME SPECIES TO SUPERPHOSPHATE APPLICATIONS IN MEDIUM AND HIGH RAINFALL AREAS OF WESTERN-AUSTRALIA

The comparative phosphorus (P) requirement of different annual pasture legume species was measured in seven field experiments in south-western Australia. Superphosphate was applied once only, at the start of each experiment. The duration of the experiments was from one to three years. The amount of P required to produce 90% of the maximum yield of each legume was used to estimate the comparative P requirements of the legumes at each harvest. Ornithopus spp. (O. compressus, O. perpusillus and O. pinnatus) required less P than Trifolium subterraneum, the most widely sown pasture legume in Western Australia. The P requirements of Medicago polymorpha varied with soil type when compared to that of T. subterraneum. M. polymorpha required less P on a soil with a neutral pH value, but had a similar P requirement on a more acidic soil. M. murex, generally required more P than T. subterraneum. In some experiments, the comparative P requirement of the different legumes varied for different harvests. At each harvest in each experiment, the relationship between yield and P concentration in tissue (internal efficiency curves) usually differed for different legumes. Presumably different legumes take up P from the soil at different rates within each harvest, and utilize the absorbed P differently to produce herbage and seed. The exceptions were that similar internal efficiency curves were measured for O. compressus and T. subterraneum in one experiment, and three cultivars of O. compressus in another experiment.