Question

The different pools of potassium in soils are
(A). Soil solution K
(B). Fixed K
(C). Exchangeable K
(D). Lattice K
Choose the correct answer from the options given below:

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

Hint:

Remember the dynamic equilibrium of soil K:
\[ \text{Lattice K} \rightleftharpoons \text{Fixed K} \rightleftharpoons \text{Exchangeable K} \rightleftharpoons \text{Soil Solution K} \]

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Soil potassium exists in four distinct chemical pools that are in dynamic equilibrium with one another. These pools range from immediately plant-available to highly insoluble forms.

Step 2: Detailed Explanation:
The four major pools of soil potassium are:
- Soil solution K (A): Immediately available to plants; consists of $\text{K}^+$ ions dissolved in the soil water (typically $<1\%$ of total soil K).
- Exchangeable K (C): Weakly held on the negatively charged exchange sites of clay and organic matter; readily displaces into the soil solution (typically $1\%$--$2\%$ of total soil K).
- Fixed (Non-exchangeable) K (B): Trapped within the interlayer spaces of 2:1 clay minerals; released slowly as solution potassium is depleted (typically $1\%$--$10\%$ of total soil K).
- Lattice (Mineral) K (D): Structural component of primary silicate minerals like feldspars and micas; released very slowly through chemical weathering (typically $90\%$--$98\%$ of total soil K).
Since all four are recognized pools of potassium in the soil, they are all correct.

Step 3: Final Answer:
The correct option is (C).