The recovery of fertilizer phosphorus by wheat, its agronomic efficiency, and their relationship to soil phosphorus

Phosphorus (P) uptake by wheat from both soil and fertilizer, the recovery of fertilizer P and its agronomic efficiency, and the fertilizer P required for maximum profit were measured in 55 wheat fertilizer experiments during 1986-89 in the northern and central wheat belts of New South Wales. Moisture conditions during crop growth had a dominant effect on all these parameters, whose values were generally highest in the wettest year (1988) and much lower in the driest year (1989). Lactate-extractable soil P was well correlated with each of these parameters, there being a different relationship for the 3 years of adequate rainfall (1986-88) from that for the very dry year: in general, the values of each parameter in the dry year were about half the values in the wetter years. Recovery of fertilizer P in the grain was very low and declined at increasing levels of soil P from about 13% at 1 mg/kg to 2% at 50 mg/kg in 1986-88. This indicates high potential levels of fertilizer P residues in this soil/climatic environment. Fertilizer P required for maximum profit was much better correlated (r2 = 0.50) with soil P than was the requirement for 90% of maximum yield (r2 = 0.30), and the latter did not differentiate between the moist and dry years. 0