Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: Sulphuric acid can be used to amend water quality and can be applied directly to soil or in the irrigation water
Reason R: Sulphuric acid rapidly neutralizes the Na constituents of water or reacts with lime in the soil to produce soluble Ca
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

Sulfuric acid acts as an indirect source of calcium.
It works by reacting with the insoluble calcium carbonate (lime) already present in the soil to produce soluble calcium ions.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Sodic soils and high-sodium irrigation water present significant challenges in agriculture by degrading soil physical structure and reducing permeability.
Chemical amendments are used to neutralize sodium or replace exchangeable sodium on clay particles with calcium ions.

Step 2: Detailed Explanation:
Let us analyze both the Assertion and the Reason:
- Assertion A: Concentrated sulfuric acid (\( \text{H}_2\text{SO}_4 \)) is a powerful chemical amendment.
It can be applied directly to the soil using specialized equipment or metered into irrigation water to improve both water quality and soil structure.
Therefore, Assertion A is correct.
- Reason R: When sulfuric acid is applied to calcareous soils (soils containing calcium carbonate or lime), it reacts rapidly with the lime to produce calcium sulfate (gypsum):
\[ \text{H}_2\text{SO}_4 + \text{CaCO}_3 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2 \]
The resulting calcium sulfate is highly soluble, releasing calcium (\( \text{Ca}^{2+} \)) ions into the soil solution.
These soluble calcium ions then displace exchangeable sodium (\( \text{Na}^+ \)) ions from the clay exchange complex:
\[ \text{Clay-Na} + \text{Ca}^{2+} \rightarrow \text{Clay-Ca} + 2\text{Na}^+ \]
The displaced sodium is then leached down below the root zone, improving soil structure and water infiltration.
Therefore, Reason R is correct and provides the direct chemical explanation for Assertion A.

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