Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 284

8.3.2.4 Sodium Adsorption Ratio (SAR)

It is another method for measuring sodicity. It is a relation between soluble sodium
and soluble divalent cations (normally Ca and Mg) in a soil-water solution, which
can be used to predict the exchangeable sodium fraction of soil equilibrated with a
given soil-water solution.
The concentration of sodium ions relative to calcium and magnesium ions in the
soil is called the sodium adsorption ratio (SAR). It is calculated as

Na +
Ca ++ +

SAR = (8.2)

Mg ++

2

where the concentrations are expressed in millequivalents per liter (meq/l) and
are obtained from a saturated paste soil extract. By definition, SAR has unit of
(meq/l) 1/2 , but following conventional usage, SAR values in the text will not include
this unit.
To convert ppm or mg/l into meq/l, divide the concentration value by the
respective equivalent weight. For Na + ,Ca 2+ , and Mg 2+ ; it is 23, 20, and 12.2,
respectively.
Soil water extracts with SAR values >13 are indicative of a soil with a sodium
problem. Even at SAR values >8, there are instances when relatively high concen-
trations of Na relative to Ca and Mg results in dispersion of clay particles, soil
structural breakdown, and soil pore blockage which reduces infiltration rates and
increases erosion potential.

8.3.2.5 Salinity

The amount of dissolved salts in soil or water. It is usually described as the concen-
tration of dissolved salt in milligrams per liter (mg/l) and measured as the capacity
of a solution to carry an electric current (EC).

8.3.2.6 Salinization

The accumulation of salts in the root zone and on the soil surface is termed
salinization. It may be a result of rising water tables and the upward movement
of saline groundwater. Salinization causes destruction or reduction of vegetation,
soil structure breakdown, erosion, silting up of streams, and pollution of water
resources.

8.3.2.7 Salinity and Osmotic Potential

Salinity is correlated to osmotic potential, which is the primary cause of plant dam-
age, and death. Osmotic potential causes dissolved constituents in soil to try to retain
water, so plants have to compete with salt for water. The presence of excessive salts