Sodium Adsorption Ratio

The sodium adsorption ratio (SAR) indicates the amount of sodium present in soils, relative to calcium and magnesium. If the sodium fraction is too large, soil permeability may be low, and the movement of water through the soil may be restricted. SAR is important to plant growth because its magnitude is an indication of the availability of soil pore water to plant roots.

The sodium ion has a large radius of hydration. This means that its hydration sphere of bound water molecules is larger than most cations. When dissolved sodium is adsorbed into clay-bearing soils, it causes them to disperse and swell. This reduces the soil pore size and causes the permeability to water to be greatly reduced. If the sodium ion is replaced by ion exchange with cations having a smaller radius of hydration, the soil dispersion is reduced.

In most natural waters, Ca2+, Mg2+, and Na+ are by far the most abundant cations, so other cations can usually be neglected in their effect on soil dispersion. Also, Ca2+ and Mg2+, being doubly charged, are more tightly adsorbed to clay surfaces than sodium and, therefore, are preferentially adsorbed. They also have smaller radii of hydration and cause less soil dispersion. The relative amounts of sodium, calcium, and magnesium adsorbed to soil are proportional to the amounts dissolved in groundwater. By measuring the concentrations of sodium, calcium, and magnesium in water used for irrigation, the SAR can be calculated using Equation 6.22 and the risk of low soil permeability evaluated.

[Na+] = concentration of sodium in eq/L (water). [Ca2+] = concentration of calcium in eq/L (water). [Mg2+] = concentration of magnesium in eq/L (water).

What SAR Values Are Acceptable?

Acceptable SAR values for irrigation water depend on the particular water and soil characteristics and are often considered in conjunction with the specific conductivity of irrigation water. Since high conductivity means that the water contains high levels of dissolved solids, it generally will contain high levels of calcium and magnesium. In any case, it will contain many other cations that compete with sodium for soil adsorption sites. Thus, high conductivity limits the deleterious effects of sodium, as shown in Figure 6.11. It should be noted that water with specific conductivity greater than about 3000 ^ S/cm is of poor quality for irrigation regardless of the SAR value. When plant roots encounter water with a concentration of TDS that is too high, the osmotic pressure balance across the outer root membrane drives water from within the plant out into the soil, desiccating the plant.

1. Water having SAR values less than 3 will not diminish soil permeability when specific conductivity is greater than about 200 |j.S/cm.

2. Water having SAR values between 3 and 6 and specific conductivity between 500 and 2000 |iS/cm may require care with irrigation methods. Soil permeability may be diminished.

3. Waters having SAR values greater than 10 and specific conductivity less than 500 |j.S/cm will require considerable care with irrigation methods. Soil permeability may be too low to allow sufficient water to reach plant roots.


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