Zinc (Zn) deficiency is believed to be a consequence of reactions taking place between soluble Zn and the soil solid phase. This study was carried out to obtain quantitative relationships between Zn in equilibrium solution and that retained by the soil solids in calcareous soils. Twenty calcareous soils (saturated paste pH 6.9-7.9; calcium carbonate equivalent 4.64-22.80%) from Tehran province, Iran were equilibrated with varying solution concentrations of Zn and the amounts removed from the solution were used to check the fit to five adsorption isotherms, namely, Freundlich, Langmuir, Temkin, Gunary, and two-surface Langmuir. Adsorption data of all soils showed statistically significant fit to the first four adsorption isotherms but only seven of the 20 soils tested showed fit to the two-surface Langmuir. Coefficients of the adsorption isotherms showed statistically significant relationship with some soil characteristics. Clay%, calcium carbonate equivalent%, and cation exchange capacity appeared to be the most influential soil characteristics with regard to Zn adsorption, whereas soil organic matter seemed to be of no importance under the conditions of this study.

Key Words: Freundlich; Langmuir; Two-surface Langmuir; Temkin; Gunary.