Question

Given below are two statements, one is labeled as Assertion (A) and the other one is labeled as Reason (R).
Assertion (A) : Humic acids are more resistant than fulvic acid.
Reason (R) : Humic acids have low molecular weight and simple structure than fulvic acids.
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:

To remember the solubility differences: Humic acid is soluble in alkali but insoluble in acid; Fulvic acid is soluble in both alkali and acid; and Humin is insoluble in both.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Soil organic matter is composed of humic substances, which are further divided into three main fractions based on their solubility in acids and bases: humic acid, fulvic acid, and humin.

Step 3: Detailed Explanation:
Let us evaluate both statements individually:
- Assertion (A): Humic acids are highly polymerous, dark-colored organic compounds.
Because they contain dense, highly condensed aromatic rings, they are highly stable and resistant to microbial decomposition in the soil.
In contrast, fulvic acids are more aliphatic and contain more accessible oxygen-containing functional groups, making them easier for soil microbes to break down.
Therefore, Assertion (A) is correct.
- Reason (R): This statement asserts that humic acids have lower molecular weight and a simpler structure than fulvic acids.
This is biochemically incorrect.
In reality, humic acids have much higher molecular weights (ranging from $10,000\text{ to }100,000\text{ Daltons}$) and much more complex, condensed aromatic structures compared to fulvic acids.
Fulvic acids have lower molecular weights (typically $1,000\text{ to }10,000\text{ Daltons}$) and simpler, less condensed chemical structures.
Because the statement claims humic acids have lower molecular weight and simpler structures, Reason (R) is incorrect.
Thus, (A) is true, but (R) is incorrect.

Step 4: Final Answer:
The correct option is (C) as (A) is true but (R) is false.