Dr. Onyatta John Onam

Phosphate fertilizers are applied to correct phosphorus deficiency in acidic tropical soils of variable charge, which account for a large proportion of the world's arable land. They are commonly applied in bands or mixed with seeds. Within the vicinity of the phosphate fertilizer zone, the concentration of phosphate is high and can cause dissolution of soil minerals. Hence a study was conducted to investigate the degree to which phosphate may effect release of cadmium (Cd) from soil,rate of release, and formation of phosphate reaction products. Since phosphate fertilizers contain a wide range of Cd concentrations, perturbation of phosphate reaction product formation by Cd was also investigated. The surface soils used in this study were selected from tropical soils in Kenya varying widely in physicochemical properties. The study shows that 1M NH4HzP04 solution induced the release of Cd from natural soils and the soils treated with Idaho monoammonium phosphate (MAP)-fertilizer.The enhanced release of Cd by the phosphate was attributed to the combined effect of Cd introduced to the soils and the Cd released from the soils through the attack of protons and the complexation of phosphate. Phosphate-induced Cd release from natural soils and treated soils increased during the short reaction period of0.25 to 1h, then decreased with time,and tended to approach a plateau. Decrease in Cd concentration was apparently due to readsorption of the Cd released on the surface of the soil particles and/ or formation of sparingly soluble reaction products in the solution. The amounts of Cd released by NH4HzP04 increased with increments in concentration of NH4H2P04. XRD analysis showed that NH4-taranakite formed in the soils treated with 1M NH4H2P04 solution. However, taranakite did not form when monoammonium phosphate was spiked with Cd even at 6.4 x 10-3M Cd(CI0 4h, indicating that Cd perturbed taranakite formation. The study indicates that at the fertilizer granule-soil interface wherein concentration of orthophosphate is high, more Cd was mobilized compared to bulk soil. The study further shows that NH 4taranakite can form in acidic tropical soilsbut the formation may be perturbed if the MAP-fertilizer applied contains significantly high Cd as an impurity.