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SAR hazard of irrigation

Sodium hazard of irrigation water

High sodium ions in water affects the permeability of soil and causes infiltration problems. This is because sodium when present in the soil in exchangeable form replaces calcium and magnesium adsorbed on the soil clays and causes dispersion of soil particles (i.e. if calcium and magnesium are the predominant cations adsorbed on the soil exchange complex, the soil tends to be easily cultivated and has a permeable & granular structure).

This dispersion results in breakdown of soil aggregates. The soil becomes hard and compact when dry and reduces infiltration rates of water and air into the soil affecting its structure.

This problem is also related with several factors such as the salinity rate (see below) and type of soil. For example sandy soils may not get damage so easy as other heavier soils when it is irrigated with a high SAR water.

Sodium & crops

High sodium concentrations become a problem when the infiltration rate is reduced to such a rate that the crop does not have enough water available or when the hydraulic conductivity of the soil profile is too low to provide adequate drainage.

Other problems to the crop caused by an excess of Na is the formation of crusting seed beds, temporary saturation of the surface soil, high pH and the increased potential for diseases, weeds, soil erosion, lack of oxygen and inadequate nutrient availability.

Recycled water can be a source of excess Na in the soil compared with other cations (Ca, K, Mg) and therefore it should be appropriately controlled.

Tolerance SAR of irrigation water Crop
Very sensitive 2-8 Fruits, nuts, citrus, avocat
Sensitive 8-18 Beans
Moderately tolerant 18-46 Clover, oats, rice
Tolerant 46-102 Wheat, barley, tomatoes, beets, tall wheat grass, crested grass

Source: Extracted from the Australian Water Quality Guidelines for Fresh & Marine Waters (ANZECC)

What is SAR?

The index used is the Sodium Adsorption Ratio (SAR) that expresses the relative activity of sodium ions in the exchange reactions with the soil. This ration measures the relative concentration of sodium to calcium and Magnesium.

SAR is defined by the following equation:

[x]: ion concentrations in meq/L
Na: Sodium
Ca: Calcium
Mg: Magnesium

SAR Hazard of irrigation water



None < 3.0 No restriction on the use of recycled water
Slight to moderate 3.0 - 9.0 From 3 to 6 care should be taken to sensitive crops.
From 6 to 8 gypsum should be used. Not sensitive crops.
Soils should be sampled and tested every 1 or 2 years to determine whether the water is causing a sodium increase
Acute > 9.0 Severe damage. Unsuitable

An adjusted SARadj value may be calculated for water with high carbonate and bicarbonate content. For example, when the irrigation water contains free lime (calcareous soil). High carbonate and bicarbonate present in water will cause the precipitation of the calcium and magnesium and increase the relative concentration of sodium increasing the SAR index.

The amount of sodium may be also indicated by the Residual Sodium Carbonate (RSC).

Relation between SAR & salinity index

At a given SAR the infiltration rate increases as salinity increases or the other way around. Therefore the SAR and ECi should be used in combination to evaluate potential problems.

SAR/Salinity Hazard of irrigation water

If SAR is: 0-3 3-6 6-12 12-20 20-40

and EC (dS/m) is:

None >0.7 >1.2 >1.9 >2.9 >5.0

Slight 0.7 1.2 1.9 2.9 5.0

Moderate 0.2 0.3 0.5 1.3 2.9


As seen in the table, for extremely low salinity irrigation water, even low SAR water should be avoided. High salinity water (EC1.50-3.00) with SAR's above 4 needs to be carefully managed. It is recommended that once a year the soils should be subject to testing in order to assess possible sodium problems.

The higher the salinity, the higher the SAR index in order to cause infiltration problems. On the other hand the lower the salinity, the greater the risk of infiltration problems independent of the SAR value.

Rainfall can reduce the soil salinity and consequently increase the SAR index and reduce water penetration into soils.

Solutions to SAR problems in soils

The following solutions apply for SAR problems in soils:

- Change irrigation sources

- Blend irrigation water with water lower in sodium levels

- Increase aerification

- Application of sulfur, gypsum, or sulfuric acid injection

See below different technology solutions:

Desalination with Reverse Osmosis

Desalination installations system design

Desalination pre-treatment

Membrane technology

Related pages:

- Salinity Hazard

- Sodium Hazard (Sodium Adsorption Ration or SAR)

- Carbonate & bicarbonates in relation with Ca & Mg

- Toxic ions hazard of irrigation water

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