The sodium adsorption ratio (SAR) is the principal diagnostic tool used to determine the sodicity hazard of irrigation waters. Published values of the Gapon selectivity coefficient (Kg) relating exchangeable Na ratio (ESR) of the soil to SAR vary rather widely, but the underlying causes have not been well defined. The purpose of this study was to identify the key variables influencing Kg in Chernozemic soils (Haploborolls) in Saskatchewan, Canada. In the group of soils examined, Kg for Na-Ca exchange ranged from 0.0065 to 0.013 (mmol L-1)-0.5. It was negatively correlated with organic C (r = -0.90***), with pH (-0.90***) and with effective cation exchange capacity (CEC) (-0.95***). Raising the pH of a soil with a high (92 g/kg-1) organic C content from 5.2 to 7.3 using Ca(OH)2 decreased Kg from 0.009 to 0.0065 (mmol L-1)-0.5. Acidification of a soil containing 27 g of organic C kg-1 by long-term use of anhydrous ammonia from pH 5 to 4.1 resulted in a relatively small increase [from 0.0114 to 0.0126 (mmol L-1)-0.5] in Kg. Our results suggest that the magnitude of the pH effect on Kg may increase as the pH-dependent component of the CEC increases. Together, organic matter content and pH accounted for 98% of the variation in Kg. Choice of measurement conditions may influence the SAR-ESR relationship. Release of Ca and Mg by mineral weathering, which occurred when soils were shaken with solutions having high SAR (SAR >= 20) and low electrolyte concentration (<100 mmolc L-1), may interfere with Na-Ca selectivity measurements made by the traditional batch method. A flow-through technique, where the soil remains stationary relative to the mobile liquid phase, may offer advantages in soils containing weatherable minerals because cation release may be less than in the batch equilibration method.
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