sodium adsorption ratio

sodium adsorption ratio (SAR) The tendency for sodium cations to be adsorbed at cation-exchange sites in soil at the expense of other cations, calculated as the ratio of sodium to calcium and magnesium in the soil (as the amount of sodium divided by the square root of half the sum of the amounts of calcium and magnesium, where ion concentrations are given in milli-equivalents per litre). A low sodium content gives a low SAR value. In practice, allowance must be made for other reactions within the soil which do not involve sodium but affect concentrations of calcium and magnesium. The SAR value is most likely to be changed by irrigation water.