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.
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