SOLUBILITY CONTROL OF KCL EXTRACTABLE ALUMINUM IN SOILS WITH VARIABLECHARGE

Solubility and kinetic data indicated that concentrations of aluminum (Al) extracted with 1M KCl are determined by the solubility of a preci pitated Al(OH)(3) phase in soils dominated by variable charge minerals . Kinetic studies examining the release of Al on non-treated and KCl t reated residues indicated the precipitation of an acid-labile Al phase during the extraction procedure. The log ion activity products estima ted for the KCl extracts ranged between 8.1-8.6 for the reaction Al(OH )(3) + 3H(+) < = > Al3+ + 3H(2)O, which was similar to the solubility product of several Al(OH)(3) phases. The mechanism proposed for Al pre cipitation indicated that Al released by exchange with added K+ hydrol yzed and released H+ that was readily adsorbed on surfaces of variable charge minerals. The increased ionic strength of the extracting solut ion further increased the amount of H+ adsorbed to the variable charge surface and reduced the H+ concentration in the aqueous phase. Consum ption of H+ induced further hydrolysis of Al, resulting in supersatura tion of the extracting solution and formation of polynuclear hydroxy A l species. It was concluded that the 1M KCl extraction does not quanti tatively extract salt exchangeable Al from variable-charge soils.