Question

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

• 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:

Zero Point Charge shifts reflect soil chemical changes; organic matter and metal oxides are both influential factors.

Answer 1

The Correct Option is A

Zero Point Charge (ZPC) increases with accumulation of sesquioxides like Fe and Al due to intense weathering. Organic matter can also influence ZPC through surface interactions, justifying both statements.

Answer 2

Variable-charge surface reasoning: Iron and aluminium oxides are variable-charge minerals whose surface charge depends on the protonation state of surface hydroxyl groups, and they carry a comparatively high point of zero charge, typically in the pH 8 to 9 range.
As weathering intensifies, primary silicate minerals break down and sesquioxides of Fe and Al accumulate relative to permanent-charge clay minerals, so the soil's overall zero point of charge shifts upward toward that high oxide value, supporting the assertion.
Organic matter also carries functional groups such as carboxyl, phenolic, and hydroxyl groups that interact with and coat these oxide surfaces, and in oxide-rich, weathered soils this surface interaction is described as reinforcing the same upward shift in the zero point of charge, aligning with the reason given.
With both effects tied to the oxide-surface-charge mechanism, both statements are true and connected.