Ca²⁺ and Mg²⁺ adsorption by an allophanic and a humic Andisol

Measurements were made of Ca²⁺ and Mg²⁺ adsorption by an allophanic soil and a soil dominated by Al-humus complexes as a function of pH and ionic strength in a range from 21 to 150 mmol l^-1. Both Ca²⁺ and Mg²⁺ adsorption by the two soils increased as the pH of the solution increased. Ca²⁺ and Mg²⁺ adsorption depended also on the ionic strength and it increased as the ionic strength increased. Under comparable solution conditions, the amount of Ca²⁺ adsorbed by the humic soil was, on a mole basis, 1.5 to 2 times that of Mg²⁺. The allophanic soil preferred Ca²⁺ over M²⁺, with a smaller Ca²⁺ adsorption/Mg²⁺ adsorption molar ratio of 1.4. Plotting M²⁺ adsorption against pH-1/2pM (M = Ca or Mg; pM is the negative logarithm of M²⁺ activity) yielded a single smooth curve for each soil. Based on this observation and the constancy of the total adsorbed cationic charge in monovalent-divalent cation exchange reactions in Andisols reported in the literature, it was concluded that one M²⁺ ion adsorbs on these soils occupying two surface functional groups.