The Sodium Absorption Ratio (SAR) is a measure used in water chemistry and soil science to assess the suitability of water for irrigation purposes. It quantifies the potential risk of sodium accumulation in soil due to irrigation with water containing high levels of sodium.
SAR is calculated based on the concentration of sodium ions (Na⁺), calcium ions (Ca²⁺), and magnesium ions (Mg²⁺) in the water. The formula for calculating SAR is:
SAR = (Na⁺ / √(Ca²⁺ + Mg²⁺) ) * 100
The resulting value indicates the relative proportion of sodium to calcium and magnesium ions in the water. A high SAR value indicates a high sodium hazard, meaning the water has a high potential to negatively affect soil structure and permeability when used for irrigation. This can lead to soil degradation, reduced water infiltration, and impaired plant growth.
The allowable limit of the SAR value in South Africa is 5 (see DWS Irrigation Standard)
It’s important to note that the SAR value should be considered alongside other factors such as the salinity of the water (measured by electrical conductivity) and the specific requirements of the crops being irrigated. Consulting with experts in water management and soil science is recommended to ensure proper interpretation and management of irrigation water.