Zinc speciation in soil solutions of Vertisols

Abstract

Soil solutions were obtained by a centrifugation method from 14 unfertilised and fertilised Vertisols. The soil solutions were analysed for all major cations and anions and organic carbon (C). Chemical speciation of zinc (Zn) in the soil solutions calculated with the aid of the computer program GEOCHEM showed that Zn in tile soil solution exists mainly as free Zn²⁺ ions in these soils. Complexation of total soluble Zn by organic and inorganic ligands constituted 40% and 50%, respectively, 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 SO²₄^- and OH^- were less than or equal to 5% each of the total inorganic species in unfertilised soils; ZnSO^o₄ complexes were more common in fertilised soils.

The activities of Zn were extremely low (0.01–0.1 µM) in unfertilised soils and were inversely related to soil solution pH. The experimentally determined solubility lines for Zn²⁺ in the soil solution were undersaturated with respect to the solubility of any known mineral form of Zn. Zn²⁺ activity was mainly determined by adsorption-desorption reactions. The weak acid ion exchangers, Chelex-100 and Bio Rex-70, retained smaller amounts of Zn front the soil solutions than the strong acid exchangers, AG 50W X2, AG 50W X4, and AG 50w X8. Soil solution pH strongly affected Zn concentrations in soil solutions. The amount of total soluble Zn present as Zn²⁺ ions as calculated by GEOCHEM was highly correlated with tile amount of soluble Zn retained by the cation exchange resins. In the case of Chelex-100, these amounts were equal, confirming the usefulness of Chelex-100 to estimate Zn²⁺ ions.