Extractable phosphorus (Colwell) concentrations of soil after banding fertiliser with seed in relation to the critical phosphorus requirement of a wheat crop

The phosphorus (P)-balance efficiency of wheat crops in southern Australia is typically about 50% (i.e. about 2 units of P are applied in fertiliser to produce 1 unit of P in grain). We investigated the concentrations of Colwell extractable P in the surface soil (0-10 cm) of a red kandosol (pHCaCl2 4.42) within the planting row of a wheat crop sown on 31 May 2011 at Condobolin, NSW. Triple superphosphate had been banded at 0, 4, 8 or 20 kg P/ha with the wheat seed which was sown at 4.5 cm depth. The aim was to quantify the temporal mismatch between P availability in the band and the critical P requirement (Colwell P concentration corresponding to 95% of maximum growth rate) of the crop. The critical Colwell P concentration of topsoil in the crop row was initially low, but increased during tillering and was ~54 mg P/kg during grain filling. To achieve this concentration of Colwell P after anthesis it was necessary to apply ~15.3 kg P/ha at sowing. It was estimated that this would initially generate a Colwell P concentration of 80-90 mg P/kg and it was concluded that the Colwell P concentration of topsoil in the crop row (i.e. soil associated with the fertiliser band) would remain above the critical requirement of the crop for the first 126 days (~70%) of the crop's growth period.