= (Na*0.04350) / ((Ca*0.04990) + (Mg*0.08229))/2)^0.5
Where: Na, Ca, and Mg are measured concentrations in mg/L. *raw data and not the rounded values that are shown on your report are used in the calculation of the reported SAR value.
Here's an example calculation using the following values:
[Na] = 47.92610 mg/L
[Ca] = 10.78850 mg/L
[Mg] = 3.73798 mg/L
Sodium adsorption ratio (SAR) is a measure of the suitability of water for use in agricultural irrigation, calculated from the concentrations of Ca, Mg and Na dissolved in the water. When applied to soil analysis, SAR is a measure of the sodicity of soil determined from the Ca2+, Mg2+ and Na+ analysis of aqueous extract (soluble paste) from the dried soil sample. The SAR value reflects the ratio of a detrimental element - sodium - to the combination of beneficial elements - calcium and magnesium - in relation to known effects on soil dispensability. Sodium disperses the physical structure of soil, impeding soil permeability.
SAR is an indication of amount of Na+ relative to Ca2+ and Mg2+ in saturation paste extracts of soil. As SAR increases (If EC is not high), the structure of medium and fine textured soils degrades. SAR can also be used in the determination of the required amount of calcium amendment.
SAR is a ratio and therefore unitless. SAR is non-calculable if the concentrations of both calcium and magnesium are non-detect (division by zero), in which case detection limits for calcium and magnesium are used in the equation instead of concentration.
Optimum SAR values are < 4.0. A SAR > 15 is severe.