SUMMARYDifferential heats and isotherms for K‐Ca exchange and CEC values show that simulated weathering by grinding and NaTPB treatment of the coarse clay fraction of a rice‐growing marsh soil exposed vermiculitic layers to reaction with ions in solution, and also created more montmorillonitic layers and uncharged clay. Calcium selectivity was also increased, especially at K saturations>50%. Changes in the CEC values of the fine clay, fine silt and coarse silt by such treatment, compared with that in the coarse clay fraction, were used to predict changes in the layer charges of their component phyllosilica