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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 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, avocado
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)