Abstract
Equilibrium phosphate concentration as a measure of the P needs of maize in 18 Alfisols in south-western Nigeria was evaluated by exhaustive cropping in a pot experiment. The critical equilibrium phosphate concentration (EPC) required to achieve 95% maximum dry matter (DM) yield varied from 0.08 mg L-1 in sandy clay soils to 0.15 mg L-1 in sandy soils. The EPC values were significantly but negatively related to clay (r = - 0.994, P<0.001) and Fe2O3 contents (r = -0.906, P < 0.001). The quantities of fertilizer P needed to attain a mean equilibrium solution concentration value of 0.12 mg L-1 (standard phosphate requirement, SPR) for optimum DM yield of maize in the pot experiment varied from 6.0 mg kg-1 to 32.0 mg kg-1. The SPR values were positively and significantly related to clay content (r = 0.792, P < 0.001) and Fe2O3 content (r = 0.734, P < 0.001). The values correlation with P uptake in the first crop (r = 0.854, P< 0.001) and after prolonged cropping (r = 0.696, P < 0.001). The use of SPR compared favourably with Bray-1, Olsen and Resin soil tests in the first crop but was superior to them after prolonged cropping.
