Determination of Cation Exchange Capacity on Low to Highly Calcareous Soils

Four different methods were evaluated for determining the soil cation exchange capacities (CEC) of calcareous soils with calcium carbonate contents ranging from less than 100 g kg−1 to over 900 g kg−1, and for acid soils with 100 to 600 g kg−1 of added CaCO3. The results showed that for calcareous soils, linear correlations existed between measurements obtained by any two of the four methods, either with or without CaCO3 removal. CEC values were decreased by the removal of carbonate from calcareous soils when the calculation was based on soil sample actual weight (i.e., weight adjusted for either the removal or the addition of CaCO3). The removal of CaCO3 from calcareous soils caused the CEC to decrease by 13.4% to 22.4%, while the addition of CaCO3 to acid soils caused the CEC to increase by 2.8% to 16.9%. The results indicate that with existing methods, the interference of CaCO3 on CEC was unavoidable, even though the solution pH for all four methods was 8.0 or 8.2. The modified LiEDTA and lithium (Li) formate methods produced low CEC values because of the low affinity of Li to the soil colloids. The double saturating solution method (CaCl2‐CaCl2) seemed to produce fairly accurate measurements, but the overestimation with too many complex procedures made this method impractical. The method using NaOAc‐NaCl in 60% ethanol as the saturating solution (method 4) proved to be sufficiently accurate and simple for application to calcareous soils with a wide range of CaCO3 contents. Soils with such a wide range of CaCO3 in south Florida (10–90%) can be calculated from the following equation established to describe and predict CEC by method 4: