Evaluation of phosphorus in savannah soils of western nigeria under bush fallow systems
Ayodele, O.J.; Agboola, A.A.
Journal of Agricultural Science 101(2): 283-290
ISSN/ISBN: 0021-8596 DOI: 10.1017/s0021859600037576
Yield calibration of soil tests in western Nigeria savannah has been difficult due to low response to P and response to only low P rates despite low available P in the soils. A study on the distribution of P forms in 60 surface soils of the savannah derived from igneous and metamorphic rocks grouped into 11 classes was made. Average total P was 0.018% and organic P, which varied between 44 and 114 mg/kg, constituted 41% of total P. Active P components made up 40% of total inorganic P, the bulk of which was in the Fe-P fraction. All P forms except non-extractable P correlated significantly with organic matter. The amounts of P needed to increase available P by 1 mg/kg in the soils, an index of P fixation, were correlated with pH and the free oxides of Fe and Al. The direct role of Fe oxides and probable indirect effect of organic matter were indicated by regression analysis. There was minimal maize yield response to P application and this was obtained only at low P rates from which an optimum rate of 20 kg P/ha was suggested. An attempt to use P sorption capacity (PSC) for P recommendation was not successful probably because of the low PSC values. A quick P recommendation based on available P status, fertilizer factor of 3.0 mg/l and an optimum available P level derived for the relative yield vs. soil P calibration curve is suggested.