THE FATE AND TRANSFORMATIONS OF ZINC ADDED TO SOILS

A new sequential extraction procedure to remove specifically adsorbed forms of trace metals and easily reducible manganese (Mn) oxide fracti ons was used to study the fate and transformations of zinc (Zn) added to soils. Most of the endogenous Zn in field soils (75-87%) was found to exist in a residual fraction which is considered to be silicates, w hile the Zn added as a fertiliser in the field soils was found predomi nantly in an EDTA-extractable fraction and in association with iron (a luminium) [Fe (Al)] and Mn oxides. The Zn recently added to soils was found to be more in the reactive forms (water-soluble plus exchangeabl e and EDTA-extractable Zn) than the Zn added to field soils in associa tion with long-term Zn application. With time, the EDTA-extractable Zn transformed into the unreactive forms (Zn associated with Fe (Al) and Mn oxides). The processes could be described by a diffusion equation. The apparent diffusion rate coefficients were found to be in the orde r of 10(-10)-10(-11)/s. The diffusion activation energy (E-a) was foun d to be 67 kJ/mol. The diffusion of Zn cations into microporous solids is probably a rate-limiting process. The transformation of reactive Z n into unreactive Zn was enhanced by elevated temperatures and by dryi ng and rewetting. The drying and rewetting effect at relatively high t emperature may be important in the processes which lead to decreases i n the availability of Zn to plants.