Inorganic pH-Dependent Charge Component of Cation Exchange Capacity of Some Acid Soils

Cation exchange capacity (CEC) measurements of the clay fraction of acid soils freed of organic matter were made by using electrometric “bottle titration” technique. Freshly prepared acid-clays indicated a fourth buffer range in addition to the well established three ranges due to H⁺, Al²⁺ and basic Al ions. The sum of these three species which were quantitatively exchanged by NaCl gave a measure of the isomorphous charge, and the non-extractable fourth range accounted for the pH-dependent charge component of CEC.

With aging, the fourth range remained unaffected but the magnitude of the pH-dependent acidity increased due to the partial extraction of the basic Al species as these got polymerised. The fourth range seems to be the prime source of pH-dependent CEC; basic-Al/Fe species (range III) may or may not contribute to the said acidity.

In analogy with earlier findings with mica and pure clay minerals, the fourth range has been attributed to exposed octahedral edges of phyllosilicate minerals. Amorphous material contributed very little to the pH-dependent acidity. These results revealed fundamental aspects of ion-exchange behaviour that help in understanding the complexity of soil systems.