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Abstract
Adsorption and desorption studies are useful indices of quantity/intensity (Q/I) relationships for nutrient ions in soil. This study investigated zinc (Zn) adsorption from zinc sulfate (ZnSO4·7H2O) and the desorption of the adsorbed Zn with 0.1 M HCl in the surface (Ap) and subsoil (Bt2) horizons of a Rhodic Kandiustult. Graded concentrations of Zn, from 0 to 154 μM Zn g−1 soil were used for the studies. Zinc adsorption in the soil increased with concentration. The Bt2 horizon adsorbed more Zn than the Ap horizon (P<0.05). At the highest level of added Zn, the concentration of Zn in equilibrium solution could not reach the critical available Zn level of at least 15.4 μM, due to the low concentrations used. Zinc adsorption isotherms were resolved into two parts, part 1 terminating at 25 μM level of added Zn in the Ap horizon and at 62 μM level in the Bt2 horizon. Only part 1 of the isotherm of the Ap horizon adjusted to the Langmuir equation (P<0.01), while both parts fitted the Freundlich isotherm (P<0.05 and P<0.01). Zinc desorption isotherms were also split in two parts with part 1 terminating at 31 μM level of added Zn in both horizons of the soil. The resolution of both adsorption and desorption isotherms into two parts either of which covered similar concentration ranges strongly suggests that more than one mechanism was responsible for Zn adsorption in the soil. The total amount of Zn desorbed was similar between the two horizons of the soil. Only 31% and 29% of total adsorbed Zn (0.1 M HCl extractable Zn+added Zn) was desorbed by three sequential extractions in the Ap and Bt2 horizons respectively, but 151% and 127% of added Zn was desorbed in the two horizons of the soil. A maximum of two extractions, which yielded 122% and 105% of added Zn in the Ap and Bt2 horizons, respectively, are sufficient to remove all the added Zn in the soil.