Effect of sowing date on the uptake and utilisation of phosphorus by wheat(cv. Osprey) grown in central New South Wales

Experiments were sown at Cowra and Condobolin to determine the effect of so wing time (April-June) on the response to and utilisation of phosphorus by the wheat cultivar Osprey which requires vernalisation. Crops sown in April needed only 0-2 kg/ha of applied phosphorus to achieve 90% of the maximum yield for the site. This contrasts with crops sown at the traditional sowin g time in May-June which needed from 11.5 kg P/ha, on a site with a moderat e soil phosphorus status, to 36 kg P/ha, on a site with a very low soil pho sphorus status, to produce 90% of the site maximum yield. Crops sown in Apr il had higher yields and even without applied phosphorus accumulated more p hosphorus (kg/ha) than crops sown in June with 40 kg P/ha. Crops sown in Ap ril had a lower dry matter harvest index, a lower phosphorus harvest index, produced less grain per kilogram of phosphorus in the shoots at maturity, and had a higher grain phosphorus concentration than crops sown in June. Gr ain protein was not affected by sowing date or the amount of phosphorus app lied at sowing. At 90% of maximum dry matter yield shoots had 0.14-0.20% ph osphorus at growth stage 30. The critical concentration of phosphorus in gr ain at 90% maximum grain yield increased with yield from 0.19 to 0.25% phos phorus. Farmers who sow wheat in April can achieve optimum yields with lower inputs of fertiliser phosphorus. However, this benefit will be offset by a lower phosphorus-use efficiency and more rapid depletion of phosphorus reserves f rom the soil.