Based on theorical considerations of ideal exchangers, the traditional Gapon exchange selectivity coefficient (KG) is generally not expected to remain constant for exchangeable sodium ratio (ESR) greater than 0.20. Therefore, the ESR-SAR (SAR=sodium adsorption ratio) relationship is only expected to be linear and predictable for this limited range of exchange sites. The purpose of this study is to demonstrate that for some soils and/or clay minerals, which are considered nonideal exchangers, the KG may remain constant for ESR values greater than 0.20. This information is important for the reclamation of sodic soil systems with ESR > 0.2 to near Na+ saturation. Experimental data in the literature is most often limited in the ESR range of 0 – 1. Experimental Na+-Ca2+ exchange data for each of a number of soils and clay minerals were plotted according to three linear transformations of the Gapon expression in order to determine if a particular soil or clay mineral exhibits a constant Gapon exchange selectivity coefficient (KG) for ESR values significantly greater than 0.20. The three linear transformations are represented by plots of (1) 1/ExNa vs. 1/SAR, (2) ExNa vs. ExNa/SAR and (3) SAR/ExNa vs. SAR (ExNa = exchangeable Na). From the three plots employed, the ExNa vs. ExNa/SAR and the SAR/ExNa vs. SAR were shown to be the most applicable in predicting a constant KG for ESR values greater than 0.20. It is also shown that some generally low CEC materials may exhibit a constant KG for ESR values significantly greater than 0.20. These data also infer that for a particular soil the linearity of the widely used ESR-SAR relationship introduced by the U.S. Salinity Laboratory Staff may not be limited to an ESR value of 0.20. Key words: Exchangeable sodium percentage, sodium adsorption ratio, thermodynamics of cation exchange, adsorption maxima, ion affinity constants.
