Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Cations adsorbed on soil colloids determine aggregate formation.
Reason (R): Cations form electropositive links between electronegative soil particles.
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:

Flocculation is the primary step in aggregation. Polyvalent cations (like $\text{Ca}^{2+}$) have a thin hydration shell and high charge, which allows them to effectively bridge clay particles together.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Soil aggregation is the process whereby primary soil particles (sand, silt, and clay) are bound together into stable units. This process is heavily influenced by the chemical and electrical properties of soil colloids and exchangeable cations.

Step 2: Detailed Explanation:
- Assertion (A) is correct: The nature of the cations adsorbed on soil colloids determines whether soil particles flocculate (aggregate) or disperse. Polyvalent cations like $\text{Ca}^{2+}$, $\text{Mg}^{2+}$, and $\text{Al}^{3+}$ promote aggregation, while monovalent hydrated cations like $\text{Na}^+$ cause dispersion.
- Reason (R) is correct: Soil clay minerals and organic matter carry net negative surface charges (electronegativity). Multivalent cations, carrying positive charges, act as electrostatic bridges ("electropositive links") that bind adjacent negatively charged soil particles together into microaggregates.
- Relationship: Because polyvalent cations form these electropositive bridges (R), they physically pull the particles together to initiate aggregation (A). Therefore, (R) is the correct explanation of (A).

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