Solubility and Hydrolysis of Aluminum in Soil Solutions and Saturated‐paste Extracts

Three Coastal Plains soils that had exhibited different “critical” pH values for maximum cotton yields in field experiments were analyzed for Al solubility. Fertilized soil was used to verify critical pH by a root‐growth experiment in a growthchamber and to obtain displaced soil solutions (⅓‐bar moisture) and water extracts from saturated pastes. Extracts were also obtained from saturated pastes made with 0.01 M and 0.05 M KCl and with 0.0033 M and 0.0167 M CaCl 2 . Solutions were analyzed for pH, Ca, Mg, Mn, K, NH 4 + , Al, and SO 4 2‐ ; the remaining ions required for electrical neutrality were assumed to be Cl ‐ and NO 3 ‐ . A computer program, involving successive iterations of numerous equilibrium equations, was used to solve for ion activities and ion‐pair concentrations, including the Al‐ion species of Al 3+ , AlOH 2+ , Al 6 (OH) 15 3+ , and AlSO 4 + . The ion‐product of (Al 3+ ) (OH ‐ ) 3 was considerably larger and much more pH‐dependent with AlOH 2+ as the hydrolytic product than with Al 6 (OH) 15 3+ . Calculated (Al 3+ ) values were much lower for polymeric hydrolysis than for monomeric hydrolysis.