Ammonium‐calcium exchange equilibria in soils and weathered pumices that differ in cationexchange materials

SUMMARY The retention of NH 4 + and Ca 2 + on soil and weathered pumice samples containing constant and/or variable charge components was measured in different NH 4 CI‐CaCl 2 solutions. The NH 4 +/Ca 2 + selectivity of each sample was evaluated using a quotient of the partition of NH 4 + on the exchange sites and in the solution relative to that of Ca 2 +. It increased with decreasing pH and increasing NH 4 Cl‐CaCl 2 concentration for a given equivalent fraction of NH 4 + in the solution. These effects were quantitatively explained in terms of the changes of NH 4 + and Ca 2 + concentrations in the solution and in the diffuse double‐layer of the ion‐exchange material as predicted by the law of mass action and the electric double layer theory. The NH 4 +/Ca 2 + selectivity of different exchange materials showed a similar variation among their exchange sites and increased in the order humus, allophane and imogolite (Si/Al ratio 0.5) < allophane (Si/Al ratio 1.0), montmorillonite < vermiculite, illite < halloysite. The origin of negative charge, the steric features around the exchange sites and clay‐humus interaction are suggested as being important in determining the NH 4 +/Ca 2 + selectivity.