Determination of Cation Exchange Capacity from Soil Water Retention Curve

Abstract

Cation exchange capacity (CEC) has a significant influence on the physical and chemical behavior of soil. Quantification of the CEC is an essential yet challenging task. A new methodology for the determination of the CECs of soils by using the soil water retention curve (SWRC) in the extremely high suction range is presented. The methodology is based on a theoretical SWRC model that explicitly considers the contribution of the cation hydration on the matric potential. For a homoionic soil, the CEC governs the relationship among the type of exchangeable cations, matric potential, and the corresponding equilibrated soil water. Thus, the CECs of homoionic soils can be deduced from their SWRCs. For a natural soil with a multiple cation species, the CEC of bulk soil can be determined from the SWRC of the Na+-exchanged form of the soil. A suite of materials ranging from nonexpansive to expansive soils is used to demonstrate the validity of the proposed methodology. The CECs of these soils are also measured independently by using the ammonium acetate method for comparison purposes. The close match between the independently measured and the SWRC-based CEC values confirms the validity and illustrates the potential promises of the SWRC-based methodology in determining the CEC of soils.