ZINC SORPTION BY ACID TROPICAL SOILS AS AFFECTED BY CULTIVATION

The sorption of zinc (Zn) by two acid tropical soils, Mazowe clay loam (kaolinitic, coarse, Rhodic Kandiustalf) and Bulawayo clay loam (coar se, kaolinitic, Lithic Rodustalf), was studied over a wide range of Zn solution concentrations. Samples of the two soils used in the experim ents were collected at both uncleared, uncultivated (virgin) sites and cultivated sites. The two virgin soils showed similar abilities to bi nd Zn. Mazowe soil (40 g organic matter kg(-1)) presented the highest affinity for Zn. Yet, Bulawayo soil (23.5 g organic matter kg(-1)) sor bed almost the same amount. Bulawayo soil had higher pH and Fe and Mn- oxide content than Mazowe soil. Once cultivated, the two soils behaved quite differently. After 50 years, Mazowe soil had lost 60% of its or ganic matter and effective cation exchange capacity (ECEC). In this so il, Zn sorption capacity had also been decreased by 60%. Clearing and 10 years under cultivation had affected neither the organic matter con tent nor the ECEC of Bulawayo soil. For this soil, Zn sorption was eve n higher in the cultivated soil, presumably due to an increase in the amount of Fe and Mn oxide from subsoiling. Zinc sorption was dependent upon pH, with retention dramatically increasing in the pH range 6-7. Sorption occurred at pH values below the point of zero charge (PZC), i ndicating that the sorption reaction can proceed even in the presence of electrostatic repulsion between the positively charged soil surface and the cation. In the two soils, the reversibility of the sorption r eaction was very low. More than 90% of the sorbed Zn was apparently st rongly bonded.