ORGANIC PHOSPHORUS AS A PREDICTOR OF PLANT-AVAILABLE PHOSPHORUS IN SOILS OF SOUTHERN NIGERIA

Abstract

P mineralized during 4 periods of cropping with maize in the greenhouse was the factor which correlated best (r = 0.90) with P uptake from 21 surface and 4 subsoil samples from S Nigeria. Over the cropping periods the organic P decreased an average of 27% and the organic matter 37%. The best estimates of P uptake by the 4 crops from analyses made before cropping were organic P (r = 0.80) and organic matter (r = 0.74). The Bray P-1 test was considerably lower (r = 0.52). Inclusion of the Bray P-1 test with organic P or organic matter improved the estimate of P uptake by the 1st crop but not by all 4 crops. The large contribution of organic P to the plant-available P is probably related to the fact that the samples were taken from sites 1/2 of which were in bush fallow and the other half in arable crops with cropping periods generally less than 5 yr following clearing. Under these conditions, organic matter decreases rapidly for a few years resulting in considerable mineralization of organic P.