Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).

Assertion (A) : Intense weathering of soil shifts its Zero Point Charge towards higher pH owing to greater accumulation of iron and aluminum oxides.
Reason (R) : The application of organic matter in soil also pushes the soil Zero Point Charge towards higher pH.

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:

Fe/Al oxides shift soil ZPC towards higher pH, whereas organic matter and silicate clays shift the ZPC towards lower pH.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Zero Point of Charge (ZPC) is the pH value at which the net variable charge on soil particle surfaces is zero. Highly weathered soils and organic matter have different effects on this property.

Step 2: Detailed Explanation:
Assertion (A) is correct because intense weathering removes silica and accumulates stable iron and aluminum hydrous oxides (such as goethite and gibbsite).
These oxides have high ZPC values (typically between pH 7 and 9). As they accumulate, they shift the soil's overall ZPC toward a higher pH.
Reason (R) is incorrect because soil organic matter contains highly acidic functional groups (like carboxyl and phenolic groups) with low ZPC values (typically around pH 2 to 3).
Applying organic matter adds negative variable charges, which shifts the soil's ZPC toward a lower pH, rather than a higher pH.

Step 3: Final Answer:
Thus, Assertion (A) is correct, but Reason (R) is incorrect.