EQUILIBRIUM PHOSPHATE CONCENTRATION AS AN ESTIMATE OF PHOSPHATE NEEDSOF MAIZE IN SOME TROPICAL ALFISOLS

Equilibrium phosphate concentration as a measure of the P needs of mai ze in 18 Alfisols in south-western Nigeria was evaluated by exhaustive cropping in a pot experiment. The critical equilibrium phosphate conc entration (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 sand y soils. The EPC values were significantly but negatively related to c lay (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 s olution concentration value of 0.12 mg L(-1) (standard phosphate requi rement, SPR) for optimum DM yield of maize in the pot experiment varie d 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 correlated with P upt ake in the first crop (r = 0.854, P < 0.001) and after prolonged cropp ing (r = 0.696, P < 0.001). The use of SPR compared favourably with Br ay-1, Olsen, and Resin soil tests in the first crop but was superior t o them after prolonged cropping.