Sodium Adsorption Ratio (SAR) can be expressed as

Correct option is A

The correct answer is (A) Na⁺ / √[(Ca²⁺ + Mg²⁺)/2]

Explanation:
• The Sodium Adsorption Ratio (SAR) is a measure of the relative concentration of sodium ($Na^+$) ions to calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions in a soil solution or irrigation water.
• It is used to predict the potential for sodium to accumulate in the soil, which leads to soil dispersion and poor physical structure.
• The formula is expressed as: $$SAR = \frac{[Na^+]}{\sqrt{\frac{[Ca^{2+}] + [Mg^{2+}]}{2}}}$$ where concentrations are expressed in milliequivalents per liter (meq/L).
• A high SAR value indicates a high risk of developing sodic (alkali) soil conditions, which restricts water infiltration and aeration.

Information Booster:
• SAR values are used to classify irrigation water: Low ($<10$), Medium ($10-18$), High ($18-26$), and Very High ($>26$).
• Excessive sodium causes soil particles to repel each other (dispersion), which destroys soil aggregates.
• Gypsum ($CaSO_4 \cdot 2H_2O$) is added to soils with high SAR to replace sodium with calcium on the exchange sites.

Additional Knowledge:
• Option B & C: These are mathematically incorrect representations of the ratio used in soil science.
• Option D: This represents a reciprocal which does not define the sodium hazard level.