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Relation Between pH_c and Sodium Adsorption Ratio (SAR) and an Alternative Method of Estimating SAR of Soil or Drainage Waters¹

ABSTRACT

The relation between the sodium adsorption ratio (SAR) and pH_c derived here predicts SAR of the drainage water (SAR_dw) better than existing empirical equations. Inaccuracies in SAR estimates from the new relationship can be anticipated by the extent to which the derived conditions are valid. Empirical equations provide no understanding of why they are sometimes inaccurate. Due to the relative insensitivity of SAR to calcium concentrations, SAR predictions of most drainage waters (where Ca HCO₃) are satisfactory when the Ca concentration of the drainage water is assumed to be constant at any fixed P_CO2. Prediction of SAR in drainage or soil water in the absence of gypsum precipitation requires knowledge of P_CO2, irrigation water composition, HCO₃/Ca ratio, and the leaching fraction. SAR_dw can be simply and accurately calculated from the derived equation and a table accounting for ionic strength and HCO₃/Ca ratio.

¹ Contribution from the U. S. Salinity Laboratory, USDA, Riverside, CA 92501. A version of this paper was presented at the WRCC-33 meeting, Land and Water Management for Control of Salinity in Western States, Fresno, Calif. 11 Sept. 1979.

² Geochemist.

Received for publication January 30, 1980. Accepted for publication December 26, 1980.