GATE AG 2021 • Q31 NAT • 1 Mark

Section 5: Irrigation and Drainage Engineering

Agricultural Drainage

Subtopic: Irrigation and drainage water quality and reuse

Question Statement:

The observed concentrations of magnesium (Mg²⁺), sodium (Na⁺), and bicarbonate (HCO₃⁻) in saturated extract of a soil sample taken from the root zone are 5.68 meq·L⁻¹, 9.90 meq·L⁻¹, and 11.20 meq·L⁻¹, respectively. If the concentration ratio of HCO₃⁻/Ca²⁺ is 2.8, the sodium adsorption ratio is ______. [round off to 2 decimal places]

Official Answer Key:

4.50 to 4.50

Step-by-Step Solution & Explanation

Ca²⁺ = HCO₃⁻/2.8 = 11.20/2.8 = 4.0 meq/L. SAR = Na⁺ / √[(Ca²⁺+Mg²⁺)/2] = 9.90/√[(4.0+5.68)/2] = 9.90/√4.84 = 9.90/2.2 = 4.50.

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