Question

K fixing power of clay minerals follow the order
(A). Illite
(B). Montmorillonite
(C). Kaolinite
(D). Vermiculite
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D)
• B. (D), (A), (B), (C)
• C. (B), (A), (D), (C)
• D. (C), (B), (D), (A)

Hint:

Remember that non-expanding 1:1 clays like kaolinite do not fix potassium. Fixation is a characteristic feature of 2:1 clays, with vermiculite having the highest capacity due to its high layer charge.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Potassium ($\text{K}^+$) fixation occurs when potassium ions are trapped within the interlayer spaces of specific expanding and semi-expanding 2:1 clay minerals, reducing their immediate availability to plants.

Step 2: Detailed Explanation:
The potassium-fixing capacity of clay minerals depends on their interlayer charge density and structural expandability:
- Vermiculite (D): Has a high interlayer negative charge. When $\text{K}^+$ ions enter the interlayer spaces, they dehydrate, and the sheets collapse, trapping the ions securely. It has the highest $\text{K}$-fixing capacity.
- Illite (A): Is a non-expanding 2:1 mineral that already contains interlayer $\text{K}^+$. However, weathered edge sites can readily fix additional $\text{K}^+$ ions, giving it high fixing power.
- Montmorillonite (B): Has a lower interlayer charge density. Although it is a 2:1 mineral, it fixes less potassium than vermiculite and illite unless it undergoes severe wetting and drying cycles.
- Kaolinite (C): Is a 1:1 non-expanding mineral with no interlayer space or internal charge. It has virtually zero $\text{K}$-fixing capacity.
Thus, the correct decreasing order of potassium-fixing power is Vermiculite $>$ Illite $>$ Montmorillonite $>$ Kaolinite.

Step 3: Final Answer:
The correct sequence is (D), (A), (B), (C).