Calculate ordinary sodium adsorption ratio from Na, Ca and Mg in meq/L or mg/L, or reverse-solve sodium for a target SAR while holding calcium and magnesium fixed. Review the ion conversions; a target calculation is arithmetic, not a water-treatment recommendation.

Enter dissolved ion concentrations from the same water sample.

SAR alone does not establish irrigation suitability or soil risk. Interpretation also needs EC/salinity, soil and field context.

Related Calculators

Sodium Adsorption Ratio Formula

The following equation calculates ordinary sodium adsorption ratio using ion concentrations in meq/L, with calcium plus magnesium greater than zero. Convert mg/L of each named ion to meq/L first; values reported as a salt or as CaCO3 require a different conversion.

SAR = (Na⁺) / (√((Ca²⁺ + Mg²⁺) / 2))
  • Where SAR is the Sodium Adsorption Ratio
  • Na+ is the sodium concentration (meq/L)
  • Ca2+ is the calcium concentration (meq/L)
  • Mg2+ is the magnesium concentration (meq/L)

To calculate the Sodium Adsorption Ratio, divide the sodium concentration by the square root of half the sum of the calcium and magnesium concentrations.

What is a Sodium Adsorption Ratio?

Definition:

Sodium adsorption ratio (SAR) compares sodium with calcium and magnesium in water. Assess irrigation infiltration effects together with electrical conductivity or salinity, soil properties and site conditions; SAR alone does not establish water suitability or a universal hazard class. This ordinary SAR calculation does not model calcium precipitation or the separate adjusted-SAR methods.

How to Calculate Sodium Adsorption Ratio?

Example Problem:

The following example outlines the steps and information needed to calculate the Sodium Adsorption Ratio.

First, determine the sodium concentration. In this example, it is 10 meq/L.

Next, determine the calcium concentration. In this example, it is 4 meq/L.

Then, determine the magnesium concentration. In this example, it is 2 meq/L.

Finally, calculate the SAR using the formula above:

SAR = Na+ / sqrt((Ca2+ + Mg2+)/2)

SAR = 10 / sqrt((4 + 2)/2)

SAR = 10 / sqrt(3) ≈ 5.7735

FAQ

Why is the Sodium Adsorption Ratio important?

High sodium relative to calcium and magnesium can contribute to soil dispersion and reduced infiltration. Interpret SAR with electrical conductivity or salinity and the relevant soil and crop context; a numerical SAR reference band alone is not a field recommendation.

How can high SAR affect soil health?

High SAR values can cause soil particles to disperse, adversely affecting soil structure and permeability, which in turn can impact plant growth and water retention.

Can SAR levels be modified in irrigation practices?

Blending water or applying calcium amendments may help in some conditions, but suitability and application rates require water and soil analysis. A lower source-water SAR does not by itself guarantee a suitable blend, and this calculator does not determine gypsum requirements or model treatment chemistry.