Fertilizer P transformations and P availability in hillslope soils of northern Ghana

Alfisols of the Savannas in northern Ghana have high base saturation and moderate P sorption capacities. Lateritic nodules are common, occurring in highly variable quantities at different landscape positions. Such nodules can have high P sorption capacities, and therefore effectiveness of P fertilisation may depend on landscape position. The objective of this study was to investigate the effectiveness of Togo rock phosphate (TRP), 50% acidulated TRP (PAPR) and single super phosphate (SSP) in providing and maintaining available P. Phosphorus supplying ability of soils from upper and lower slopes of three locations in northern Ghana was studied with and without fertilizer addition by repeated desorption with anion exchange resin membrane (AEM) burial in the laboratory and by measuring dry matter yield (DMY) and P uptake of sorghum for six successive greenhouse croppings. Transformations of the applied fertilizers were studied by sequential extraction. Phosphate desorbed with AEM after 38 days, the DMY and the P uptake of sorghum all followed the order SSP > PAPR > TRP = control. The relative agronomic effectiveness of the PAPR was 63% of SSP. Although half the applied TRP was 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 the AEM. In the nodule-rich upper slope soils, initial availability of added P was higher, but decreased more rapidly than in lower slope soils. We attributed this to an initial 'concentration' of fertilizer in the smaller volume of soil fines followed by slow sorption into the nodules. 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 untransformed apatite.