The sodium adsorption ratio (SAR) is an irrigation water quality parameter used in the management of sodium-affected soils. It is an indicator of the suitability of water for use in agricultural

irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...

, as determined from the concentrations of the main alkaline and earth alkaline cations present in the water. It is also a standard diagnostic parameter for the sodicity hazard of a

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

, as determined from analysis of pore water extracted from the soil. The formula for calculating the sodium adsorption ratio (SAR) is: :

\text = \frac

where

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

, and

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

concentrations are expressed in milliequivalents/liter. SAR allows assessment of the state of

flocculation In colloidal chemistry, flocculation is a process by which colloidal particles come out of Suspension (chemistry), suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The actio ...

or of dispersion of clay aggregates in a

. Sodium and potassium ions facilitate the dispersion of clay particles while calcium and magnesium promote their flocculation. The behaviour of clay aggregates influences the

soil structure In geotechnical engineering, soil structure describes the arrangement of the solid parts of the soil and of the Pore space in soil, pore space located between them. It is determined by how individual soil granules clump, bind together, and Soil a ...

and affects the permeability of the soil on which directly depends the water infiltration rate. It is important to accurately know the nature and the concentrations of cations at which the flocculation occurs: critical flocculation concentration (CFC). The SAR parameter is also used to determine the stability of

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exte ...

s in suspension in water. Although SAR is only one factor in determining the suitability of water for

, in general, the higher the sodium adsorption ratio, the less suitable the water is for irrigation. Irrigation using water with high sodium adsorption ratio may require soil amendments to prevent long-term damage to the soil. If irrigation water with a high SAR is applied to a soil for years, the sodium in the water can displace the calcium and magnesium in the soil. This will cause a decrease in the ability of the soil to form stable aggregates and a loss of

and

tilth Tilth is a physical condition of soil, especially in relation to its suitability for planting or growing a crop. Factors that determine tilth include the formation and stability of aggregated soil particles, moisture content, degree of aeration, ...

. This will also lead to a decrease in infiltration and permeability of the soil to water, leading to problems with crop production. Sandy soils will have less problems, but fine-textured soils will have severe problems if SAR is greater than 9. When SAR is less than 3, there will not be a problem. The concept of SAR addresses only the effects of sodium on the stability of soil aggregates. However, high K and Mg concentrations have also negative effects on soil permeability. The effect of potassium can be similarly treated by means of the potassium adsorption ratio (PAR). To take into account simultaneously all major cations present in water, a new irrigation water quality parameter was defined: the cation ratio of structural stability (CROSS), a generalization of SAR.

References

{{reflist

External links

New Mexico State University: ''Irrigation water classification systems''
PDF file retrieved 24 December 2008. Agricultural soil science Colloidal chemistry Colloids Soil chemistry Soil science Water quality indicators

See also

References

External links