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

This question gives an Assertion about how organic sulphur becomes available to crops and a Reason about acidification, then asks whether the Reason actually explains the Assertion.

1. Both (A) and (R) are true and (R) is the correct explanation of (A): This would only hold if the acid-producing part of the reaction were the actual cause of sulphate becoming available. It is not, availability comes from the conversion of organic-S into sulphate itself, not from the acid released alongside it, so this option is wrong.
2. Both (A) and (R) are true but (R) is NOT the correct explanation of (A): Organic-S really does mineralize to sulphate under aerobic conditions through bacteria like Thiobacillus, so (A) is true. Oxidation of sulphur to sulphate really does release \(H^+\) and acidify the soil, so (R) is true too. But (R) explains a side effect, falling pH, not the mechanism that makes sulphur available to roots, so (R) is not the explanation of (A). This fits the situation exactly.
3. (A) is true but (R) is false: (R) is a genuine, well documented reaction, sulphur oxidation releasing \(H^+\), so calling it false is wrong.
4. (A) is false but (R) is true: (A) correctly describes a known pathway of S nutrition in aerobic soils, so (A) is not false.

The correct choice is that both statements are true, but the Reason describes the acidifying side effect of sulphur oxidation, not the reason sulphate becomes available to the plant.

Let's summarize:

• Organic-S mineralizes to sulphate under aerobic conditions through S-oxidizing bacteria such as Thiobacillus, making (A) true.
• That same oxidation releases \(H^+\) and acidifies the soil, making (R) true as a separate fact.
• (R) explains a pH change, not the plant-availability mechanism in (A), so it is not the correct explanation.

So the answer is option 2, both (A) and (R) are true, but (R) does not explain (A).