Influence of phosphate on zinc reaction in variable charge soils

The influence of phosphate (P) on the adsorption-desorption of zinc (Zn) and on Zn fractions was examined for four variable charge soils. The soils were two kaolinitic Alfisols and two Andisols containing high amount of amorphous materials. Zinc adsorption-desorption isotherms were produced using a batch protocol involving sequences of increasingly concentrated Zn solutions. The P concentrations were varied in the preparation of these isotherms to asses the effect of P on the isotherm's shape. Subsequently, Zn fractions were extracted sequentially with KNO3, NaOH, and HNO3+H2O. The results showed that both soil type and pH influence the effect of P on Zn reactions. In the kaolinitic Alfisols, P and Zn were adsorbed in similar amounts and the effect of increasing P rates on Zn adsorption-desorption reactions was negligible. On the contrary, in the allophanic Andisols, whose capacity to bind P exceeds more than 30 times their capacity to retain Zn, zinc adsorption was clearly enhanced with increasing P rates. In these soils, two distinct effects of P addition on Zn retention were observed. The first one arose from variations in soil-suspension pH and the concomitant changes in soil surface charge. The second effect was directly related to the amount of adsorbed P and was assumed to take place through increase Zn specific adsorption on organic matter and/or hydrous oxides. Phosphate adsorption on the surfaces restricted Zn desorption, which stresses once more the P induced increase of specific adsorption sites