FERTILIZER P TRANSFORMATIONS AND P AVAILABILITY IN HILLSLOPE SOILS OFNORTHERN GHANA

Alfisols of the Savannas in northern Ghana have high base saturation a nd moderate P sorption capacities. Lateritic nodules are common, occur ring in highly variable quantities at different landscape positions. S uch nodules can have high P sorption capacities, and therefore effecti veness of P fertilisation may depend on landscape position. The object ive of this study was to investigate the effectiveness of Togo rock ph osphate (TRP), 50% acidulated TRP (PAPR) and single super phosphate (S SP) in providing and maintaining available P. Phosphorus supplying abi lity of soils from upper and lower slopes of three locations in northe rn Ghana was studied with and without fertilizer addition by repeated desorption with anion exchange resin membrane (AEM) burial in the labo ratory and by measuring dry matter yield (DMY) and P uptake of sorghum for six successive greenhouse croppings. Transformations of the appli ed fertilizers were studied by sequential extraction. Phosphate desorb ed with AEM after 38 days, the DMY and the P uptake of sorghum all fol lowed the order SSP > PAPR > TRP = control. The relative agronomic eff ectiveness of the PAPR was 63% of SSP. Although half the applied TRP w as transformed to other forms than acid extractable apatite, this did not liberate P to the AEM. Less than 10% of the SSP was absorbed by th e AEM. In the nodule-rich upper slope soils, initial availability of a dded P was higher, but decreased more rapidly than in lower slope soil s. We attributed this to an initial 'concentration' of fertilizer in t he smaller volume of soil fines followed by slow sorption into the nod ules. At the upper slope, much of the TRP and PAPR were transformed to less available forms, while at the lower slope more TRP remained as u ntransformed apatite.