It is possible to estimate the sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) after determining Ca, Mg and Na concentrations in water (SOP# 835). The method has a detection limit of 0.1 for SAR and 1% for ESP.
U.S. Salinity Laboratory Staff. 1954. Choice of determinations and interpretation of data. pp. 25-26. In: L. A. Richards (ed.) Diagnosis and improvement of saline and alkali soils. USDA Agricultural Handbook 60. U.S. Government Printing Office, Washington, D.C.