Question

Assertion (A): Ammonium-based fertilisers are the major contributors to soil acidification.
Reason (R): Especially the non-leachable nitrogen ions which have been taken up by plants contribute to the soil acidity

• 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:

Soil acidification is common with ammonium-based fertilizers due to nitrification and plant uptake processes.

Answer 1

The Correct Option is A

Ammonium-based fertilizers acidify soil due to nitrification releasing hydrogen ions. The uptake of non-leachable ions by plants reduces basic cations, increasing acidity—thus, (R) correctly explains (A).

Answer 2

Reaction/mechanism approach:
Nitrification of applied ammonium fertiliser can be written as \( \text{NH}_4^+ + 2\text{O}_2 \rightarrow \text{NO}_3^- + \text{H}_2\text{O} + 2\text{H}^+ \), directly releasing acidifying \( \text{H}^+ \) ions into the soil. Separately, when plant roots absorb \( \text{NH}_4^+ \), a cation that is largely retained on exchange sites rather than leached, they release an equivalent \( \text{H}^+ \) ion back into the rhizosphere to maintain internal charge balance, a process distinct from nitrification but adding further acidity. Because both pathways, microbial oxidation and root-level cation exchange, funnel back to \( \text{H}^+ \) release tied to non-leachable ammonium uptake, Reason (R) correctly explains why ammonium fertilisers acidify soil.