SAR hazard of irrigation
Sodium hazard of irrigation water
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.
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.
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
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.
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
- Sodium Hazard (Sodium Adsorption Ration or SAR)
- Carbonate & bicarbonates in relation with Ca & Mg