Organic Phosphorus as a Predictor of Plant-Available Phosphorus in Soils of Southern Nigeria

Phosphorus mineralized during four periods of cropping with maize in the greenhouse Was the factor which correlated best (r = 0.90) with P uptake from 21 surface and four subsoil samples from southern Nigeria. Over the cropping periods the organic P decreased an average of 27 percent and the organic matter 37 percent. The best estimates of P uptake by the four 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-l test with organic P or organic matter improved the estimate of P uptake by the first crop but not by all four 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 one-half of which were in bush fallow and the other half in arable crops with cropping periods generally less than 5 years following clearing. Under these conditions, organic matter decreases rapidly for a few years resulting in considerable mineralization of organic P.