SAR is measured by

SAR is the Sodium adsorption ratio and is a measure of the amount of sodium (Na) relative to calcium (Ca) and magnesium (Mg) in the water extract from saturated soil paste. It is the ratio of the Na concentration divided by the square root of one-half of the Ca + Mg concentration.
SAR is measured by: $${\text{SAR}} = \frac{{\frac{{{\text{N}}{{\text{a}}^ + }}}{{\sqrt {{\text{C}}{{\text{a}}^{2 + }}} + {\text{M}}{{\text{g}}^{2 + }}}}}}{2}$$