Abstract
Cation exchange equilibria involving Ca2 +-Na+ and Na+-Ca2 + ion pairs was investigated to calculate the Gapon exchange coefficient for sodication and desodication on different textured soils belonging to various landforms of Punjab, Pakistan. The soils were made homoionic with Ca2 + or Na+ as index cation in varying cationic concentrations (0, 2, 4, 8, 12, 16 and 20 mmol L-1 ) and then desorbed with other cation at the same respective electrolyte concentration. The Gapon exchange coefficient (KG) was determined for two reactions, i.e. sodication and desodication, designated as KGSand KG~' respectively. The values of KGSand KGDgradually increased with an increase in the concentration of Na" or Ca + as index cation, but the KGDvalues were found higher than that of the KGSshowing preferencial adsorption of Ca2 + over Na+, i.e. the rate of desodication (Na+-Ca2 + ) was more as compared with the sodication (Ca2+-Na +), possibly due to Na+ saturation of soils and greater hydrated size of Na+ The sodication and desodication rates in soils at various electrolyte concentrations observed in relation to various textured soils showed more sensitivity of fine textured soils to saline-sodic irrigation waters as compared to coarse textured soils particularly at higher concentrations of index cations. The values of KGSand KGDcorrelated statistically with CEC and clay percentage of soils, which indicates that reclamation of higher CEC saline-sodic soils are more efficient with the application of external Ca source (CaCI2 in this study) than the coarse textured soils (loamy sand soils). Keywords: Gapon exchange coefficient, sodication, desodication
