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An attempt was made to introduce a membrane ultrafiltration technique into the measurement of soil cation exchange capacity (CEC). The CEC data from four soil samples obtained using the simplified method yielded identical results to those obtained using conventional methods. The potential for automation of the method was discussed. Using the copper (Cu2+) ion as a substitute resulted in higher CEC values than obtained using sodium (Na+) ion as a substitute. The degree of increase is positively correlated to the concentration of organic material present in the soil. The difference in stability constants of complexes between soil organic matter (humic acids and fulvic acids) and metal ions accounted for the differences in the soil CEC values measured at the same reaction time with different metal ions as substitutes. In fact, the incompleteness of the substitution reaction was the ultimate reason for the difference in measured CEC values. This result raises the question regarding the reliability of conventional methods for the measurement of soil CEC, especially when applied to soils rich in organic matter. [ABSTRACT FROM PUBLISHER]