Measuring cation exchange capacity and total exchangeable bases in batch and flow experiments

Cation exchange capacity (CEC) recoveries from a leaching procedure utilizing intact soil columns were compared to extracts from disturbed soil samples to determine the influence of macropores and preferential flow on ion exchange. Eleven soils representing eight soil series with a variety of morphological and physicochemical characteristics were used in the study. Leachate was introduced into duplicate undisturbed soil columns following the 1 M NH₄OAc, pH 7.0 procedure at a weight to volume ratio equivalent to that used for the disturbed soil samples. Effluents from disturbed and undisturbed samples were collected and analyzed for CEC and total extractable bases. Average CEC values for the intact columns were 49.1% lower than those measured by routine analysis. Regressional analysis indicated a significant difference between the two methods (p < 0.1). Particle size distribution was identified through multiple linear regression analysis as the most influential physicochemical property contributing to the difference between methods. Soil columns with sandier textures displayed high recovery rates attributable to uniform porosity and low CEC. As the silt fraction increased, the difference between methods increased due to formation of macropores and associated preferential flow through the soil matrix. However, increased clay levels allowed more thorough hydration of the matrix which apparently restricted flow, increased residence time and promoted exposure of more exchange sites, thus resulting in intermediate recovery rates. These results suggest that routine CEC measurements based on batch extractions of disturbed soil samples may overestimate ion exchange interactions, and therefore, overestimate true contaminant sorption capacities of soils.