RESIDUAL VALUE OF SUPERPHOSPHATE AND QUEENSLAND ROCK PHOSPHATE MEASURED USING YIELDS OF SERRADELLA, BURR MEDIC AND SUBTERRANEAN CLOVER GROWN IN ROTATION WITH WHEAT AND BICARBONATE-EXTRACTABLE SOIL-PHOSPHORUS

The residual value of ordinary (single) superphosphate (SP) and Queensland (Duchess) apatite rock phosphate was measured in a pot experiment using yields of three legume species (serradella, Ornithopus compressus; medic, Medicago polymorpha var. brevispina; clover, Trifolium subterraneum) fertilized with mineral nitrogen (N) grown in rotation with wheat (Triticum aestivum). The plants were grown in a glasshouse in a sandy soil (pH in CaCl2 - 5.8) for 30 to 34 days. There were four crops: legume, wheat, legume, and wheat. The phosphate (P) requirement of the plant species was determined from the amount of P required to produce 90% of the maximum yield. Soil samples were taken before sowing each crop to measure bicarbonate-extractable soil P (soil test P) which was related to the yields of that crop. Relative to freshly-applied (FSP), the residual value of SP declined by about 60% between the first and second crop, and by a further 25% by the fourth crop. For crop 3, the residual value of SP varied depending on the legume species, being about 8% as effective as FSP for serradella, 30% for medic, and 53% for clover. For crop 1, rock phosphate was about 4% as effective as SP for serradella, 11% for medic, and 7% for clover. Relative to FSP, the residual value of rock phosphate declined by between 50 to 80% for the second crop (wheat) when it was about 3% as effective as FSP. It remained 1 to 2% as effective for all species for the third and fourth crops. Serradella required about 10 to 50% less P as FSP than medic or clover to produce 90% of the maximum yield. However, for crop 3 serradella required 200% more P as previously-applied superphosphate than medic and clover. For rock phosphate, for crop 1, serradella required 50% more P than medic and 35% more P than clover; corresponding values for crop 3 were 200% and 100% more P. The relationship at each harvest between yield and P concentration in tissue (internal efficiency of P use) generally differed for different P fertilizers and plant species. Critical P concentrations in dried tops for maximum yields were 0.6 to 1.0%P. As measured using soil test P, rock phosphate was only about 8% as effective as SP at crop 1. Relative to FSP, the residual value of SP declined to be about 50% as effective by crop 4. By contrast, the residual value of rock phosphate increased by about 60%, to be 13% as effective as FSP. The relationship between yield and soil test P generally differed: (i) for different P fertilizers, for the same plant species; (ii) for different plant species, for the same P fertilizer; and (iii) for different crops of the same species and P fertilizer.