Comparison of the phosphate requirements of burr medic and yellow serradella with that of wheat in the low rainfall wheatbelt of Western Australia

Abstract

The responses of burr medic (Medicago polymorpha) and yellow serradella (Ornithopus compressus) to topdressed phosphate were compared with the response of wheat (Triticum aestivum) to drilled phosphate on a range of soils in the low rainfall wheatbelt of Western Australia. Species were compared firstly on the amount of applied phosphorus (P) required for 90% maximum yield at 3 harvests, and secondly on the ratio of their curvature coefficients from the Mitscherlich relationship between absolute yield and P applied. On 7 marginally acidic, medium-textured soils, the comparative requirements of burr medic (cvv. Serena or Santiago) and wheat (cv. Gutha) for applied P changed with harvest. At harvest 1 152-55 days after sowing (DAS)], burr medic generally had a larger curvature coefficient and required less applied P for 90% maximum yield than wheat. At harvest 2 (107-111 DAS) and for harvest 3 (seed yields), the curvature coefficients for burr medic were smaller than those for wheat, and burr medic generally required more applied P at 90% maximum yield. The average P requirement at 90% maximum yield of burr medic was 14 kg P/ha at harvest 1, 22 kg P/ha at harvest 2, and 19 kg P/ha for seed production. The average P requirement of wheat was 28 kg P/ha at harvest 1,11 kg P/ha at harvest 2, and 15 kg P/ha for seed production. On 3 acidic, light-textured soils, yellow serradella (cv. Madeira) generally had a lower requirement for applied P at 90% maximum yield, and a larger curvature coefficient, than wheat (cv. Gutha) at all harvests in each experiment. The requirements of yellow serradella and wheat also varied between harvests. The average P requirement at 90% maximum yield of yellow serradella was 16 kg P/ha at harvest 1 (52-55 DAS), 21 kg P/ha at harvest 2 (105-110 DAS), and close to 26 kg P/ha for seed production. The average P requirement of wheat was 29 kg P/ha at harvest 1,25 kg P/ha at harvest 2, and >33 kg P/ha for seed production. In most experiments burr medic had a larger absolute response to applied P at harvests 1 and 2 and a lower absolute response for seed production than wheat. Yellow serradella had a smaller absolute yield response to applied P than wheat over all 3 harvests. When converted to a percentage of maximum yield, burr medic and yellow serradella had a lower percentage response to applied P at harvest 1, and a higher percentage response at harvest 2 and for seed production than wheat.