Question

Which soil has more K fixation capacity

• A. Montmorillonite
• B. Vermiculite
• C. Illite
• D. Kaolinite

Hint:

Fixation happens in 2:1 minerals only.
Vermiculite = High Charge = High Fixation.
$NH_4^+$ and $K^+$ are both "fixed" similarly because they have almost identical ionic radii!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Potassium fixation refers to the "trapping" of $K^+$ ions in the interlayer spaces of certain clay minerals. The question asks which mineral has the highest capacity to fix added Potassium.
Detailed Explanation:

• Mechanism of K-Fixation:
K-fixation occurs in $2:1$ type minerals where the "layer charge" is high and originated in the tetrahedral sheet.

• Comparing Minerals:
Vermiculite: Has a higher layer charge than Illite or Montmorillonite. Because of this high charge, when $K^+$ ions (which have a small hydrated radius) enter the interlayer, the electrostatic attraction is so strong that the layers collapse, permanently trapping the $K^+$. It has the highest capacity to fix "added" Potassium.
Illite: Already contains $K^+$ in its interlayer (it is "born" with K). While it can fix more, its available "capacity" is often less than Vermiculite because many sites are already occupied.
Montmorillonite: Has a lower layer charge. It usually does not fix K unless it is repeatedly dried and wetted.
Kaolinite: A $1:1$ mineral with no interlayer space; it has zero fixation capacity.

• Agronomic Impact:
In soils with high Vermiculite, a large portion of applied K fertilizer might be "fixed" and become unavailable to crops in the short term.

Step 2: Final Answer:
Vermiculite has the highest potential and capacity for Potassium fixation due to its high layer charge density.