A critical evaluation of 1 m kci-extraction method for determining exchangeable al ions in variable charge soils

To explain why soils rich in variable charge materials often lack 1 M KCI-extractable Al ions, I proposed a mechanism, an “induced hydrolysis” of exchangeable Al ions in the presence of variable charge materials at high salt concentrations. I extracted exchangeable Al from mixtures of a montmorillonite saturated with Al and a synthetic goethite using a 1 M KCI solution. The amount of the extractable Al (per unit weight of the montmorillonite) decreased with increasing proportion of the goethite in the mixtures. I interpreted this experimental result as supporting the proposed mechanism, i.e., H⁺released in the hydrolysis of Al is adsorbed on the goethite in a concentrated KCI solution and this removal of H⁺induces further hydrolysis, resulting in the polymerization or precipitation of Al as hydroxides. I calculated the amount of monomeric, therefore extractable, Al as a function of the goethite content in the 1 M KCI-montmorillonite-goethite mixtures. The calculated amounts agreed well with the experimentally determined ones in the whole range of goethite content examined.