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Fractionation of Cation Exchange Capacity for Assessing Soil and Water Sodicity¹

ABSTRACT

High vermiculite, arid zone soils whose cation exchange capacities (CEC) decrease more than 45% by K fixation have higher than usual regression coefficients for the relation between their exchangeable-sodium ratio (ESR) and sodium-adsorption ration (SAR). The regression coefficients are approximated by the equation: regression coefficient of ESR/SAR relation = (0.00043 x percentage decrease in CEC upon K fixation ) - 0.0045. For eight soils having a wide range of vermiculite contents, the exchangeable-sodium percentage required to cause a 25% reduction in hydraulic conductivity at a salt concentration of about 10 meq/liter was highly correlated (r = 0.95) with CEC after K fixation.

¹ Contribution from the US Salinity Laboratory, Soil and Water Conserv. Res. Div., ARS, USDA, Reiverside, Calif. 92502.

² Director and Soil Scientist, respectively, US Salinity Laboratory.

Received for publication July 23, 1971. Accepted for publication September 22, 1971.