Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): High Na-concentration in irrigation water may reduce oxygen supply to the root system.
Reason (R): High sodium content in irrigation water causes flocculation of soil particles.

• 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:

Remember that sodium (\(\text{Na}^+\)) is a dispersing agent, whereas calcium (\(\text{Ca}^{2+}\)) and organic matter act as flocculating agents in soil. Dispersion ruins soil physical structure, leading to poor root aeration.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Irrigation water containing elevated concentrations of sodium (\(\text{Na}^+\)) relative to divalent cations (\(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\)) affects soil physical properties.
High exchangeable sodium on soil particles leads to soil dispersion (deflocculation) rather than flocculation.

Step 2: Detailed Explanation:
1. Sodium is a highly hydrated monovalent ion with a weak charge density. When it dominates the exchange complex, it causes soil colloid swelling and dispersion of soil aggregates (deflocculation).
2. Dispersed clay particles clog macro-pores in the soil. This structural deterioration creates a dense, compacted layer that impairs water infiltration and limits soil aeration.
3. This block in soil aeration decreases oxygen diffusion rate (ODR), resulting in poor oxygen supply to the root system, which validates the Assertion.
4. Since sodium causes dispersion (deflocculation) and not flocculation, the Reason is scientifically incorrect.

Step 3: Final Answer:
Assertion (A) is correct and Reason (R) is false.