Phosphate fertiliser residues in wheat-growing soils of the Western Downs, Queensland

Abstract

Effects of repeated annual application and residual effects of past phosphorus (P) applications were studied at Billa Billa and The Gums, Queensland, on 2 Vertisols representative of a large cropping area in this region. Phosphorus was applied annually at 0, 4, 8, 12, and 25 kg/ha at sowing to each wheat crop between 1978 and 1988. Phosphorus was also applied at 25, 50, 100, 200, or 400 kg/ha as a once-only application, before wheat was sown in 1978, and was incorporated by tillage.

Pot experiments were conducted each year with soil (6 kg/pot) gathered from field plots in May before wheat sowing to determine the residual value (RV) of previously applied P fertiliser under standard conditions of frequent watering. In 1979 the RV was determined in a second series of pots with a water deficit to simulate conditions of plant moisture stress that were to dominate the field experiments. The RV of P fertiliser was reduced to one-third that determined under well-watered conditions. Low residual values (8–47%) determined in the field in 1979 for P rates 25 and 50 kg/ha applied in 1978 were probably due to reduced P uptake from dry topsoil throughout which P residues were incorporated by tillage. By contrast, P applied annually at a low rate (4 or 8 kg/ha) in a band close to seeds created profitable responses when evaluated over the sequence of successive wheat crops.

The bicarbonate-extractable soil test value (P_bf) on either soil fertilised annually for n years with P at a rate R (kg/ha· year) was described by the model

where P_bu is the soil test value (mg/kg) of unfertilised soil.