| dc.description.abstract | 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. | en |
