ZINC SPECIATION IN SOIL SOLUTIONS OF VERTISOLS

Soil solutions were obtained by a centrifugation method from 14 unfert ilised and fertilised Vertisols. The soil solutions were analysed for all major cations and anions and organic carbon (C). Chemical speciati on of zinc (Zn) in the soil solutions calculated with the aid of the c omputer program GEOCHEM showed that Zn in tile soil solution exists ma inly as free Zn2+ ions in these soils. Complexation of total soluble Z n by organic and inorganic ligands constituted 40% and 50%, respective ly, of total soluble Zn in fertilised and unfertilised soil solutions. The organo-Zn complexes constituted <10% of the total soluble Zn. The inorganic Zn complexes, ZnHCO3+ and ZnCO3, constituted 60-75% of the total inorganic Zn complexes. The Zn complexes with SO42- and OH- were less than or equal to 5% each of the total inorganic species in unfer tilised soils; ZnSO40 complexes were more common in fertilised soils. The activities of Zn were extremely low (0.01-0.1 mu M) in unfertilise d soils and were inversely related to soil solution pH. The experiment ally determined solubility lines for Zn2+ in the soil solution were un dersaturated with respect to the solubility of any known mineral form of Zn. Zn2+ activity was mainly determined by adsorption-desorption re actions. The weak acid ion exchangers, Chelex-100 and Bio Rex-70, reta ined smaller amounts of Zn front the soil solutions than the strong ac id exchangers, AG 50W X2, AG 50W X4, and AG 50w X8. Soil solution pH s trongly affected Zn concentrations in soil solutions. The amount of to tal soluble Zn present as Zn2+ ions as calculated by GEOCHEM was highl y correlated with tile amount of soluble Zn retained by the cation exc hange resins. In the case of Chelex-100, these amounts were equal, con firming the usefulness of Chelex-100 to estimate Zn2+ ions.