DYNAMICS OF PHOSPHORUS FRACTIONS IN A SAVANNA ALFISOL UNDER CONTINUOUS CULTIVATION

We examined the dynamics of inorganic P (P-i) and organic P (P-o) pool s of a savanna Alfisol under continuous cultivation complemented with nitrogen (N) and phosphorus (P) fertilizers with or without cow manure (D), using a modified Hedley fractionation. Continuous cultivation wi thout P fertilizer decreased the concentration of P-i and P-o pools in cluding the residual P fraction compared with an unfertilized treatmen t with natural vegetation. Adding P fertilizers alone or in combinatio n with D (P, NPK, DS-P, D + NP and D + NPK) increased the concentratio n of labile P-i pools, but decreased the concentration of P-o pools an d the residual P fraction. The tendency of the residual P fraction to decrease along with P, pools suggested that the residual P fraction wa s largely P-o. This is consistent with the correlation between the res idual P fraction and total P-o (r = 0.74*) and the residual P fractio n and organic carbon content (r = 0.47*). Analysis of the relationshi ps between plant available P, estimated by resin-extractable P, and th e more stable P-i and P-o pools indicated that 85% of the variation in resin-P was attributable to the hydroxide extractable P-i (OH-P-i) an d HCl-P-i which acted as sinks for fertilizer-P. The contribution of t he residual P fraction to resin-P was indirect via bicarbonate-extract able P-o (HCO3-P-o) and OH-P-o. These results also clarify why attempt s to find P extraction methods which correlate well with response to P and to P uptake in the savanna have given inconsistent results.