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 sowing 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 sowing time in May-June which needed from 11.5 kg P/ha, on a site with a moderate soil phosphorus status, to 36 kg P/ha, on a site with a very low soil phosphorus status, to produce 90% of the site maximum yield. Crops sown in April had higher yields and even without applied phosphorus accumulated more phosphorus (kg/ha) than crops sown in June with 40 kg P/ha. Crops sown in April 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. Grain protein was not affected by sowing date or the amount of phosphorus applied at sowing. At 90% of maximum dry matter yield shoots had 0.14-0.20% phosphorus at growth stage 30. The critical concentration of phosphorus in grain at 90% maximum grain yield increased with yield from 0.19 to 0.25% phosphorus. 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 from the soil.