GREENHOUSE EVALUATION OF AGRONOMIC POTENTIAL OF DIFFERENT SOURCES OF PHOSPHATE FERTILIZER IN A TYPICAL CONCRETIONARY SOIL OF NORTHERN GHANA

The concretionary soils of Northern Ghana, which are near neutral with respect to pH and which comprise mostly lateritic ferruginous nodules are known to sorb significant amounts of phosphate. Instead of import ed superphosphate, the use of less expensive indigenous Togo rock phos phate (PR) or partially acidulated (50%) Togo rock phosphate (PAPR-50) , are possible alternative phosphate fertilizer options for these soil s. The objective of this research was to evaluate the effectiveness of freshly-applied SSP, PR and PAPR-50, and the effectiveness of the res idues of these fertilizers in a glasshouse pot study. Laboratory studi es were also undertaken to study the transformation of these fertilize rs after their application to the concretionary ferruginous soils. In the greenhouse study, yield of dried tops and the P uptake by the tops of maize var. Dobidi (Zea mays) was used to measure fertilizer effect iveness. One level of P was applied for each fertilizer (26.4 kg P ha( -1)). Plants were grown for 28 days. After harvesting the first crop, subsequent cropping was carried out to evaluate the effects of the res idual P in the pots. The results showed that increases in dry matter y ield of shoot and total P uptake followed the trend SSP > PAPR-50 > PR > control. The relative agronomic effciency (RAE) of PAPR-50 was 58% that of commercial SSP in increasing growth of the crop, while that of PR was only 23%. The residual effect of either PAPR-50 or PR on dry m atter yield and total P uptake was found to be negligible compared wit h SSP, suggesting that apatitic P was poorly effective relative to SSP in the used soils. The P fractionation results confirmed that PR and PAPR-50 did not significantly increase any of the P fractions in eithe r the soil fines or nodules after the first crop. By contrast, applica tion of SSP increased all extractable Pi fractions, most of the P adde d being recovered from the nodules in forms associated with Fe (hydrox ide and residual Pi). It is concluded that, relative to SSP, the P fro m residues of PAPR-50 and PR are poorly effective in the soils studied for sustainable plant production.