Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Saline soils are dominated by neutral soluble salts consisting of chlorides and sulphates of sodium, calcium and magnesium.
Reason (R): Sodic soils contain sodium salts capable of causing alkaline hydrolysis, mainly \(\text{Na}_2\text{CO}_3\).
Choose the correct answer from the options given below:

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

- Saline soils are dominated by neutral salts (chlorides and sulphates), keeping pH \(< 8.5\).
- Sodic soils are dominated by alkaline-hydrolyzing salts (carbonates and bicarbonates), raising pH \(\ge 8.5\).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Saline and sodic soils are two distinct categories of salt-affected soils defined by their chemical compositions and behaviors in water.

Step 2: Detailed Explanation:
- Assertion analysis: Saline soils contain an excess of neutral soluble salts, primarily chlorides (\(\text{Cl}^-\)) and sulphates (\(\text{SO}_4^{2-}\)) of sodium (\(\text{Na}^+\)), calcium (\(\text{Ca}^{2+}\)), and magnesium (\(\text{Mg}^{2+}\)). These neutral salts keep the soil pH below \(8.5\). This statement is true.
- Reason analysis: Sodic soils are dominated by exchangeable sodium and soluble sodium salts capable of alkaline hydrolysis, such as sodium carbonate (\(\text{Na}_2\text{CO}_3\)) and sodium bicarbonate (\(\text{NaHCO}_3\)): \[ \text{Na}_2\text{CO}_3 + 2\text{H}_2\text{O} \rightleftharpoons 2\text{Na}^+ + 2\text{OH}^- + \text{H}_2\text{CO}_3 \] This hydrolysis produces hydroxyl ions (\(\text{OH}^-\)), raising the soil pH above \(8.5\). This statement is true.
- Relationship: Although both statements are correct scientific facts describing different categories of salt-affected soils, the Reason does not explain why saline soils are dominated by neutral soluble salts. Thus, (R) is not the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are true, but (R) is not the correct explanation of (A).