SOIL CATION-EXCHANGE CAPACITY MEASUREMENTS USING ULTRAFILTRATION TECHNIQUES - COMPARISON OF DIFFERENT METAL-IONS AS SUBSTITUTES

An attempt was made to introduce a membrane ultrafiltration technique into the measurement of soil cation exchange capacity (CEC). The CEC d ata from four soil samples obtained using the simplified method yielde d 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 obtaine d using sodium (Na+) ion as a substitute. The degree of increase is po sitively correlated to the concentration of organic material present i n the soil. The difference in stability constants of complexes between soil organic matter (humic acids and fulvic acids) and metal ions acc ounted 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 fo r the difference in measured CEC values. This result raises the questi on regarding the reliability of conventional methods for the measureme nt of soil CEC, especially when applied to soils rich in organic matte r.