Marginal responses over six years by sorghum and sunflower to broadcast and banded phosphorus on a low P Vertisol, and changes in extractable soil phosphorus

Abstract

At Emerald in central Queensland, dryland sorghum and sunflower were grown on a cracking clay soil (Vertisol) for 4 years and sorghum was grown in the sixth year, after an 18-month fallow. The initial bicarbonate-extractable soil phosphorus (P) concentration was 6 mg/kg in the 0-20 cm horizon. High analysis superphosphate (19.2% P) was broadcast at 0-180 kg P/ha in the first year, and banded rates of 0-20 kg P/ha were superimposed on the broadcast rates of 0-60 kg P/ha each year except in the fifth (fallow) year. Sorghum dry matter (DM) yields at anthesis, summed over 5 seasons, increased linearly (P<0.05) with both methods of P application, but significant (P<0.05) treatment differences occurred only after the 18 months of fallow. Total sorghum grain yields after 5 seasons fitted a quadratic reponse curve (P<0.05) with both application methods. Sunflower DM yields at anthesis increased significantly (P<0.05) with broadcast fertiliser in the first season only, but neither the rate nor the method of application of fertiliser affected seed yields over 4 seasons. Seed P concentrations for both crops were either unaffected or increased with increasing rates of fertiliser. Fertiliser P may not be needed for sunflower, but we recommend banding about 70 kg P/ha with sorghum at sowing over 5 years to ensure maximum yields. Based on extractable P values, the relative effectiveness of initial broadcast and incorporated P applications exceeding 40 kg/ha declined over the first 4 years, but then tended to remain steady at about 0.67. There was no consistent trend in relative effectiveness with time at the lower rates of P application.