Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Fixation and release of \(\text{K}^+\) in soil is influenced by absence or presence of ammonium ion (\(\text{NH}_4^+\)).
Reason (R): The physics of \(\text{NH}_4^+\) is closely related to that of \(\text{K}^+\) because both ions have similar ionic radii and low hydration energy.
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:

Because of their similar ionic size (\(\text{K}^+ = 1.38 \text{ \AA}\) and \(\text{NH}_4^+ = 1.43 \text{ \AA}\)), these two ions compete for the same interlayer fixation sites in 2:1 clay minerals.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Interlayer ion fixation occurs in 2:1 expanding clay minerals (such as illite and vermiculite) where certain cations fit into ditrigonal cavities within the clay sheets.

Step 2: Detailed Explanation:
- Comparison of Cations: Potassium (\(\text{K}^+\)) and ammonium (\(\text{NH}_4^+\)) have very similar unhydrated ionic radii (\(1.38 \text{ \AA}\) and \(1.43 \text{ \AA}\), respectively) and low hydration energies.
- Fixation Mechanism: Because of their size and low hydration, both ions can lose their hydration shells and fit into the interlayer hexagonal cavities of 2:1 clay minerals, causing the clay sheets to collapse and trap the ions.
- Competition: Due to these identical physical properties, \(\text{NH}_4^+\) directly competes with \(\text{K}^+\) for interlayer binding sites. The presence of \(\text{NH}_4^+\) can block \(\text{K}^+\) fixation or displace previously fixed \(\text{K}^+\), validating the Assertion.
- Because the Reason explains this competitive behavior using their physical characteristics, it is the correct explanation.

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A).