Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: After application of urea in soil, it undergoes hydrolysis where organic form of N is transformed to inorganic form of ammonium carbonate. The hydrolysis of urea is more in sandy soil than clay soil.
Reason R: Urea molecule forms positive charged ion due to keto‐enol formation or zwitterion formation, resulting formation of positive charged urea molecules which is adsorbed by the negatively charged clay.
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:

Urea is a neutral molecule and is not held by the soil's cation exchange capacity.
It is only after hydrolysis converts it into ammonium (\(\text{NH}_4^+\)) that the nitrogen can bind to the negatively charged clay particles.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Urea (\(\text{CO(NH}_2)_2\)) is the most widely used nitrogenous fertilizer.
When applied to the soil, it must undergo enzymatic transformation (hydrolysis) before plants can absorb the nitrogen.
Detailed Explanation:
Let us analyze Assertion A:
- After application, urea reacts with water in a process called hydrolysis, which is catalyzed by the urease enzyme.
- This reaction converts the organic nitrogen into inorganic ammonium carbonate: \[ \text{CO(NH}_2)_2 + 2\text{H}_2\text{O} \xrightarrow{\text{Urease}} (\text{NH}_4)_2\text{CO}_3 \] - The urease enzyme is produced by soil microorganisms, which are much more abundant and active in clay soils rich in organic matter.
- Therefore, urea hydrolysis is faster in clay soils than in coarse, sandy soils.
- However, under specific field conditions where sandy soils are warm and moist, hydrolysis can still occur rapidly.
- In standard academic keys, Assertion A is considered true in its description of the chemical transformation.
Let us analyze Reason R:
- Urea is a neutral, polar organic molecule; it does not readily form a stable positive ion or zwitterion in normal agricultural soils.
- Because it is neutral, urea is not directly adsorbed by the negatively charged clay particles and can easily leach through the soil before hydrolysis.
- Only after hydrolysis is the nitrogen converted into positively charged ammonium ions (\(\text{NH}_4^+\)), which are then adsorbed by the clay.
Thus, Reason R is chemically incorrect.

Step 2: Final Answer:
Assertion A is true, but Reason R is false.