Sodium Absorption Ratio

Magnesium is also an important component in evaluating the sodium absorption ratio (SAR) of irrigation waters and soil extracts. The SAR is calculated as

SAR = (Na₊) / √(Ca₂₊ + Mg₂₊)/2

In this equation, the concentrations of sodium (Na₊), calcium (Ca₂₊), and magnesium (Mg₂₊) ions are expressed in meq L^-1. When concentrations of magnesium, calcium, or both elements are increased, relative to sodium, the SAR decreases. Many soils in arid climates are affected by SAR in that as the SAR increases, the permeability of the soil decreases since the sodium reacts with clay, causing soil particles to disperse resulting in reduced water penetration into the soil (208). In most cases, a soil is considered sodic when the SAR>13 (209). However, at lower SAR values, some crops may still be susceptible to the adverse effects of sodium on nutrient uptake rather than to the physiochemical effects on soil permeability.

TABLE 6.2

Primary and Secondary Minerals, Nonminerals, and Gems Containing Magnesium

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