Mineral nutrition of soybeans grown in the South Burnett region of south eastern Queensland. 2. Prediction of grain yield response to phosphorus with soil tests

Grain yield response of soybean (Glycine max cv. Bragg) to applied phosphorus was measured at 19 experimental sites in the South Burnett region. The soil phosphorus supply factors of quantity, intensity, buffer capacity and rate were estimated by various soil chemical tests, and relative yield [(yield at nil applied phosphorus/maximum yield) x 100] regressed against these tests. The equilibrium phosphorus concentration-the intensity measure-accounted for the greatest percentage variation in relative yield (80%) and at 90% maximum yield was 0.014 ¦g P/ml. Phosphorus extracted by 0.01 M CaCl2 was highly correlated with the equilibrium phosphorus concentration (r2=0.93) and accounted for 73% of the variation in relative yield. Soil levels of calcium chloride-extractable phosphorus were interpreted as follows: < 0.044 ¦g P/g, response to phosphorus probable; 0.044 ¦g P/g to 0.058 ¦g P/g, response uncertain; > 0.058 ¦g P/g, response unlikely