The exchangeable sodium percentage (ESP) and the sodium adsorption ratio (SAR) are calculated after determining Ca, Mg and Na concentrations in a saturation extract (SOP# 235).
U.S. Salinity Laboratory Staff. 1954. Choice of determinations and interpretation of data. p. 26. In: L. A. Richards (ed.) Diagnosis and improvement of saline and alkali soils. USDA Agric. Handb. 60. U.S. Government Printing Office, Washington, D.C.