Question

Assertion (A): Gypsum can be used to manage sub-soil acidity.
Reason (R): The \(\text{SO}_4^{2-}\) released by dissolution of gypsum increases level of Ca and reduces level of Al in both soil solution and exchange complex.
Choose the correct answer from the options given below:

• A. (A) is true but (R) is false.
• B. (A) is False but (R) is True.
• C. Both (A) and (R) are correct, and (R) is correct explanation of (A).
• D. Both (A) and (R) are correct but (R) is not correct explanation of (A).

Hint:

Unlike lime (\(\text{CaCO}_3\)), gypsum does not increase soil pH because it does not generate hydroxyl ions (\(\text{OH}^-\)). Instead, it mitigates acid-soil damage by detoxifying active aluminum through sulphate complexation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Subsoil acidity and aluminum (\(\text{Al}^{3+}\)) toxicity limit root penetration into deeper soil layers. Traditional liming agents (\(\text{CaCO}_3\)) have low solubility and cannot leach into the subsoil, whereas gypsum is more soluble.

Step 2: Detailed Explanation:
- Assertion Analysis: Gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)) is about 100 times more soluble than calcium carbonate. When applied to the surface, it dissolves and leaches into the subsoil, making it an effective amendment for subsoil acidity, validating the Assertion.
- Reason Analysis: Upon dissolution, gypsum releases \(\text{Ca}^{2+}\) and \(\text{SO}_4^{2-}\) ions: \[ \text{CaSO}_4 \rightleftharpoons \text{Ca}^{2+} + \text{SO}_4^{2-} \] - \(\text{Ca}^{2+}\) ions compete with and displace exchangeable \(\text{Al}^{3+}\) from the clay complex, allowing it to leach.
- \(\text{SO}_4^{2-}\) ions react with toxic \(\text{Al}^{3+}\) to form non-toxic ligand complexes like \(\text{AlSO}_4^+\) and neutral precipitates. This chemical pathway reduces exchangeable and solution aluminum levels, making the Reason correct.
- Since the Reason explains the mechanism of subsoil acid management by gypsum, it is the correct explanation.

Step 3: Final Answer:
Both (A) and (R) are correct, and (R) is the correct explanation of (A).