Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
Assertion (A) : Soil clay has maximum influence on soil behavior.
Reason (R) : Clay has greater surface area per unit mass and is most physicochemical active.
In light of the above statements, 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:

To test if the Reason (R) explains the Assertion (A), read them together using the connector "because":
"Soil clay dominates soil behavior BECAUSE it has an extremely high surface area and is highly physicochemical active."
This reads as a logical, scientifically correct explanation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Soil texture consists of three major mineral fractions: sand, silt, and clay.
Among these, the clay fraction ($< 0.002\text{ mm}$ in diameter) dominates the soil's chemical, physical, and mechanical behavior due to its colloidal size and mineral structure.

Step 3: Detailed Explanation:
Let us analyze the assertion and reason:
Assertion (A): "Soil clay has maximum influence on soil behavior."
This statement is correct.
Clay particles control key soil properties, including water retention, swelling-shrinkage behavior, plasticity, cohesion, and nutrient adsorption.
Even a small percentage of clay (e.g., $15 - 20\%$) can dominate these properties, whereas sand and silt act as relatively inert frameworks.
Reason (R): "Clay has greater surface area per unit mass and is most physicochemical active."
This statement is correct.
Because clay particles are extremely small ($< 0.002\text{ mm}$), they have an exceptionally high specific surface area.
For example, sand has a specific surface area of about $0.1\text{ m}^2/\text{g}$, while montmorillonite clay can exceed $800\text{ m}^2/\text{g}$.
This massive surface area, combined with the permanent negative charges resulting from isomorphous substitution, makes clay highly active in adsorption, hydration, and cation exchange.
This high physicochemical activity directly explains why clay has such a dominant influence on overall soil behavior.
Thus, (R) is the correct explanation of (A).

Step 4: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A).
This corresponds to Option (1).