Measuring the cation exchange capacity of forest soils

Abstract

Various methods have been proposed for assessing the cation exchange capacity (CEC) of soils. A pH 7 buffered ammonium acetate solution is the most widely used method but its use for forest soils has been questioned as it may result in an overestimation of CEC in soils with pH dependent charges. The objective of this study was to determine the most appropriate method for measuring the natural CEC of forest soils using variations of 3 common methods. These methods included: (1) pH 7 buffered ammonium acetate (2) unbuffered 1.0, 0.5, and 0.1 M ammonium chloride, and (3) the compulsive exchange method using the original method and with magnesium chloride. The CEC of three reference materials (kaolinite, vermiculite and humified organic matter) was predetermined and used for comparison. In addition, samples from the major genetic horizons of 8 soils from Washington State, USA, the North Island, New Zealand, and Indonesia were analyzed for CEC. Results showed that the compulsive exchange method resulted in consistently low CEC values, and did not reflect changes in soil organic matter or pH. The pH 7 buffered ammonium acetate resulted in artificially high CECs in soils high in organic matter and low pH, undoubtedly due to increases in pH dependent charges. Although the 0.5 and 0.1 M NH₄Cl would work well for many forest soils, there may be insufficient NH₄ ⁺ to completely displace all cations and fill all exchange sites in soils with high CEC. The unbuffered 1.0 M NH₄Cl saturation solution is recommended for forest soils.

Acknowledgments

This research was supported by the USDA-FS PNW Research Station, Wenatchee Laboratory and the New Zealand Forest Research Institute.