Sodium adsorption ratio (SAR) is a measure of the amount of sodium (Na) in water relative to the amount of calcium (Ca) and magnesium (Mg). It is used to assess the quality of water for irrigation and to manage soils that are affected by sodium.
Formula
SAR = [Na]/(([Ca]+[Mg])/2)1/2
Units
Milliequivalents of charge per liter
What it indicates
The relative amount of sodium ions to calcium and magnesium ions in water
How to determine
A soil test lab determines the SAR
What it means
Soils with SAR values of 13 or more are considered "sodic"
How to know if your water has a high SAR
Have your water lab tested
Soils with high SAR values may have the following characteristics:
Increased dispersion of clay particles and organic matter
Reduced aeration and saturated hydraulic conductivity
General degradation of soil structure
When the SAR is greater than 3, the water is considered sodic and can increase the exchangeable sodium percentage (ESP) of the soil
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