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

Abstract

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.