Question

Given below are two statements, one labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Organic-S is made available to plants under aerobic upland conditions by mineralization into sulphates by S-oxidizing bacteria such as Thiobacillus.
Reason (R): Mineralization of organic-S results in production of \(H^+\) ions leading to the acidification of soil.
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:

Ask separately: is A true, is R true, and does R explain WHY A happens.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The question checks two facts about sulphur (S) in soil at the same time, how organic-S becomes available to plants, and what happens to soil pH while this process runs.

Step 2: Key Formula or Approach:
For an Assertion-Reason question, decide first if the Assertion (A) is true on its own, then decide if the Reason (R) is true on its own, and only then ask whether R actually explains why A happens.

Step 3: Detailed Explanation:
Checking Assertion (A): In upland, well aerated (aerobic) soils, most of the sulphur locked inside organic matter cannot be used by plants directly. Soil microbes, including S-oxidizing bacteria such as \(Thiobacillus\), break down this organic-S and turn it into sulphate (\(SO_4^{2-}\)), the form roots actually take up. This mineralization into sulphate under aerobic conditions is a well established process, so Assertion (A) is true.
Checking Reason (R): When reduced or organic forms of sulphur get oxidized to sulphate by these bacteria, the reaction also releases hydrogen ions (\(H^+\)) into the soil solution. This is the same reason elemental sulphur is deliberately added to alkaline soils, to bring the pH down. So the claim that mineralization of organic-S produces \(H^+\) ions and acidifies the soil is also true, and Reason (R) is true.
Checking the link between them: Assertion (A) is about why sulphur becomes available to plants, it gets converted to sulphate. Reason (R) describes a side effect of that same oxidation reaction, the release of acid. R does not explain why the sulphate becomes plant-available, it only tells us what else happens to soil pH during the process. So R is a true statement, but it is not the correct explanation of A.

Step 4: Final Answer:
Both (A) and (R) are individually true, but (R) is not the reason behind (A), so option 2 is correct.