Predicting yield responsiveness and phosphorus fertilizer requirements of soybeans from soil tests

Abstract

Field experiments with rain-fed Bragg soybeans (Glycine max) were carried out at 30 sites in central south-eastern Queensland over six years. The sites covered a wide range of soil types and P buffer capacities. Several rates of P fertilizer were band-applied in 0.9 m rows at planting. Relative grain yield at each site was correlated with 0.5 M NaHC03 extractable (PB) and 0.01 M CaCl2 extractable (PC) phosphorus. Mitscherlich equations of best fit indicated that PC was better correlated (r2 = 0.85) with relative grain yield than PB (r2 = 0.59). At 90% maximum grain yield, the 'critical' PC value was 43 µg P kg-1. The Mitscherlich curvature coefficient, c, was correlated (r2 = 0.49, P < 0.001) with the amount of P fertilizer required for 90% maximum grain yield, but not with soil P buffer capacity, as measured by a single point sorption index (r2 = 0.06). Factors other than the P sorbing ability of the soil were affecting the curvature coefficient, but could not be identified in this work. PC was sufficiently well correlated (r2 = 0.66, P < 0.001) with P fertilizer requirements to have use as a predictor of these requirements. The findings are compared with those obtained for other crops and P fertilizer application methods.