Measurement of cation exchange capacity of organic-matter fractions from soils using a modified compulsive exchange method

This article describes a modified compulsive exchange method suitable for the determination of cation exchange capacity (CEC) over a pH range on solid, separated organic-matter fractions. The method is demonstrated to be a reproducible and relatively simple means of measuring CEC on fractions separated by physical means in conjunction with hydrofluoric acid (HF) treatment. By including a modification for measuring the residual magnesium (Mg2+) content by dilution and conductivity, the method is simplified to measure CEC versus pH using a single sample; the only laboratory instruments required are pH and conductivity meters and a balance. The lack of agreement between this method and the silver thiourea (AgTU) method for some fractions at low pH is attributed to the uncontrolled variation in solution ionic strength when acid is introduced to effect solution pH changes in the AgTU method.