SOIL-PHOSPHORUS TESTS AND GRAIN-YIELD RESPONSIVENESS OF MAIZE (ZEA-MAYS) ON FERROSOLS

The grain yield response of maize (Zea mays) to various rates of appli ed phosphorus (P) was measured at each of 17 sites in the South Burnet t region of south-eastern Queensland. The soils at all sites were Ferr osols. Relative grain yield of the nil applied P treatment [100x (yiel d at nil applied P/maximum yield)] was related to Colwell (0.5 M NaHCO 3) extractable P (P-B), CaCl2-extractable P, and equilibrium P concent ration and P buffer capacity calculated from P sorption curves. Of the se P measurements, P-B was most highly correlated with relative grain yield (R-2 = 0.94 for a linear response and plateau fit, R-2 = 0.92 fo r a Mitscherlich fit), and the P-B values at 90% maximum grain yield w ere 20 mg/kg for the linear response and plateau model and 32 mg/kg fo r the Mitscherlich equation.