The response of organic and conventionally grown wheat to superphosphate and reactive phosphate rock

In 1991 and 1992 wheat was sown on adjacent properties at Ardlethan in southern New South Wales to assess its response to superphosphate (SP) and reactive phosphate rock (RPR) under conventional and organic management systems. Five rates of P (0, 10, 20, 30, 40 kg/ha) were applied as SP and RPR. No response to RPR was recorded in terms of crop growth or grain yield. However, in both years and on both sites, SP significantly increased plant dry weight at tillering and anthesis, the number of tillers and spikes per plant and grain yield. Superphosphate also increased the P content of grain and the amount of P exported. SP (40 kg P/ha) increased the level of cadmium in the grain, although not beyond permitted maximum concentrations. The use of SP reduced the level of colonisation by vesicular-arbuscular mycorrhizal fungi (VAM). The data indicate that the choice of the organic farmer, not to use SP, incurs a large yield penalty.