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

The relation between the sodium adsorption ratio (SAR) and pHc derived here predicts SAR of the drainage water (SARdw) 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 ≅ ½ HCO3) are satisfactory when the Ca concentration of the drainage water is assumed to be constant at any fixed PCO2. Prediction of SAR in drainage or soil water in the absence of gypsum precipitation requires knowledge of PCO2, irrigation water composition, HCO3/Ca ratio, and the leaching fraction. SARdw can be simply and accurately calculated from the derived equation and a table accounting for ionic strength and HCO3/Ca ratio.