A Simple Method for Estimating Cation Exchange Capacity from Water Vapor Sorption.

Knowledge of soil cation exchange capacity (CEC) is crucial for soil fertility considerations, sorption and release of polar and non-polar compounds, engineering applications, and other biogeochemical processes. Standard procedures such as the ammonium acetate or the BaCl₂ compulsive exchange methods are expensive and laborious, and in previous studies the CEC has been predicted from parameters such as particle size distribution and organic matter content, specific surface area, clay content, and from Atterberg limits. In this study, relative humidity (RH)-dependent pedotransfer functions (PTFs) for estimating CEC from soil water content considering hysteresis have been developed based on 203 differently-textured soils. Furthermore, we compared the performance of the new PTFs with existing PTFs that predict CEC from clay content, organic carbon, soil pH and specific surface area. The new PTFs accurately predicted CEC, with RMSEs ranging from 2.4 to 2.9, for 35 independent soils (CEC range from 7 to 74 cmol₍₊₎ kg⁻¹). Commonly applied PTFs predicted CEC reasonably well with RMSEs ranging from 4.1 to 10. There was no impact of clay mineralogy or organic matter content on the performance of the new PTFs, thus a simple measure of soil water content at an arbitrary relative humidity can provide reasonably accurate CEC estimates for large scale studies.