This document discusses potassium fixation in different clay minerals. It explains that potassium fixation was first reported in 1887 and involves potassium penetrating between clay layers and becoming tightly held. The degree of potassium fixation varies between clay types, with vermiculite showing the highest fixation due to its high charge density and large interlayer space, followed by illite, montmorillonite, and kaolinite. Factors like charge density, interlayer space size, solution concentration, and presence of other cations can influence how much potassium is fixed within clay minerals. The practical implication is that fixed potassium contributes to long-term potassium availability in soils.

1. POTASSIUM (K) FIXATION IN DIFFERENT CLAY
MINERALS

2. CONTENTS
• Potassium
• Forms of potassium
• Fixation of potassium
• Relation to clay minerals
• Factor affecting potassium fixation
• Practical implications of fixed potassium
• References

3. POTASSIUM (K)
• K Constitutes About 2.5% Of Earth Crusts, The Seventh Abundant Element And Fourth Abundant
Mineral. It Is Found In Large Concentration 1-5% In Plants.
• Roles Of K+ In Plants
• Osmotic Regulation,
• Translocation Of Assimilates,
• Metabolic Processes,
• Improving Quality Of Fruits, Flowers And Vegetables,
• Imparting Resistance To Disease, Pests And Environmental Stress.

4. Forms of potassium

5. FIXATION OF POTASSIUM
• Fixation Of Potassium Was First Reported By Kellner In 1887 And Dyer In 1894
• The K Which Penetrates The Intermicellar And Gets Fit Snugly Into The Holes And such are held
tightly as they come in contact.
• Becomes Relatively Nonexchangeable Called Fixed (Rich 1968, Bolt Et Al 1976, Van Der Marel
1959)

6. LATTICE HOLE THEORY
•According to PAGE AND BAVER,1940, the exposed surface and surfaces
between sheets of 2:1 layer mineral consist of oxygen ions, arranged
hexagonally.
•The opening within the hexagon is equal to the diameter of an oxygen ion (2.8Å).
•Any cation(K) with similar size can get fitted

7. EXPANDIBLE LAYER SILICATE

8. Order of K fixation in clay minerals
Vermiculite > Illite > Montmorillonite >
Kaolinite

10. •VERMICULITE:
•High Charge Density
•More exchange sites
•ILLITE:
•Interlayer Space Is Same As K+ Ion
•Non Expanding
•MONTMORILLONITE:
•It Has Large Interlayer Space
•FILLED WITH Ca & Mg
•KAOLINITE:
•Low Charge Density
•Non Expanding
•No Interlayer Space

11. FACTORS AFFECTING POTASSIUM FIXATION
• CHARGE DENSITY OF CLAY:
Vermiculites fix more potassium compared to smectites due to
internal charge density.
• EXTENT OF WEDGE ZONE:
Deeper the wedge zone higher the amount of potassium fixed.

12. • SOLUTION CONCENTRATION
The solution K+ concentration affects the equilibrium between
exchangeable and fixed K+ in soil.
• NATURE AND CONCENTRATION OF OTHER CATIONS
The presence of cations like NH4+ (2.86Å) and Rb+(2.96Å) compete
with the K+ ion for interlayer space.

13. PRACTICAL IMPLICATION OF FIXED K+
• TISDALAND NELSON (1975) Said That Potassium Fixation Is A Process Of Conserving
Potassium In Nature.
• Non Exchangeable K+ Contributes Substantially To K+ Availability In Soils.
• HAYLOCKS (1956) Introduced Terms Of “STEP-K” And “CONSTANT RATE K” As A
Measure Of Potassium Reserve In Soils.

14. REFERENCES
• FUNDAMENTALS OF SOIL SCIENCE – ISSS (A.SUBBA RAO, M.S.BRAR)
• INTRODUCTORY SOIL SCIENCE – D.K DAS