Subtopic: Salt-Affected Soils and Land Reclamation
Question Statement:
Gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) is applied to reclaim alkali/sodic soils primarily to:
Options:
(A) Supply soluble $\text{Ca}^{2+}$ cations to displace exchangeable $\text{Na}^+$ from the soil exchange complex
(B) Lower the soil water table
(C) Increase soil sodium adsorption ratio (SAR)
(D) Kill soil earthworms
Answer Key:
A
Step-by-Step Solution & Explanation
Calcium ions in dissolved gypsum exchange with adsorbed sodium on clay micelle surfaces: $2\text{Na-Clay} + \text{CaSO}_4 \rightarrow \text{Ca-Clay}_2 + \text{Na}_2\text{SO}_4$. The released sodium sulfate is subsequently leached out.