Soil Colloids and Silicate Clay Minerals

1. Introduction to Soil Colloids

• Colloidal particles are typically <1 micron (µm) in diameter.
• While clay particles are <2 µm, not all are colloidal, but many display colloidal behavior.
• Colloids:
  • Do not dissolve in water like crystalloids.
  • Exist in ‘sol’ (suspended) or ‘gel’ (solid) states.
  • Undergo Brownian movement due to constant repulsion (similar electric charges).

2. Colloidal Solutions vs True Solutions

3. Clay Minerals: Definition & Classification

🔹 Minerals; Naturally occurring, inorganic with defined composition.

🔹 Types:

• Primary Minerals: Original, anhydrous (e.g., feldspar, mica).
• Secondary Minerals: Formed via weathering, hydrous (e.g., kaolinite, montmorillonite, illite).

4. Composition of Clay

• Most common clays = Silicate Clays (temperate regions).
• Sesquioxide Clays (Al and Fe hydroxides): Occur in tropical regions.

Crystalline Nature:

• Clay = Crystalline, plate-like structure (not amorphous)
• Humus = Amorphous

5. Micelles & Clay Charge

• Micelles = Fine colloidal clay particles with net negative charge
• Ionic Double Layer: Surrounding positive ions form:
  • Helmholtz Double Layer
  • Stern Layer (hydrated cations)
  • Gunny Layer (ions + water between clay plates)

6. Acidic vs Alkaline Clays

In humid soils, H⁺ & Al³⁺ dominate → acidic
In arid soils, Ca²⁺ & Mg²⁺ dominate → neutral
If Na⁺ accumulates → alkaline reaction

7. Adsorption Power of Cations (Flocculation Order)

Cation adsorption strength (ability to flocculate soil colloids):

👉 Al³⁺ > H⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺

• Al³⁺ = Strongest flocculator
• Na⁺ = Weakest; causes dispersion of colloids
• Ba²⁺ also helps in flocculating soil colloids

8. Sesquioxide Clays

• Formed in high rainfall areas due to leaching of silica
• Residual clay becomes rich in Fe and Al oxides (Fe₂O₃, Al₂O₃)
• Known as sesquioxide clays

🔸 Properties:

• Low plasticity & cohesion
• Low base exchange capacity (CEC)
• Low fertility
• Fixes phosphorus as Fe/Al phosphate

9. Silica-Sesquioxide Ratio

• Molar ratio = Silica / Sesquioxides in clay fraction
• Original rock: ~6.0
• In soils: 1.25 – 3.6

🔹 Ratios in Soils:

Higher ratio = High CEC, moisture retention, fertility
Lower ratio = Leached, acidic, low fertility

10. Common Colloidal Minerals in Soil

11. Silicate Clay Minerals

🔹 Phyllosilicates (“Phyllon” = leaf-like)

• Most important group of silicate clays
• Structure: Sheets/layers of mineral cations

Clay Structural Units

• Di-octahedral: 2 Al³⁺ (common)
• Tri-octahedral: 3 Mg²⁺

🔗 These sheets stack to form clay minerals like kaolinite, montmorillonite, illite, etc.

Quick Revision Points for Competitive Exams

• Clay is crystalline, not amorphous
• Micelles carry negative charge → attract cations
• Silicate clays dominate in temperate regions
• Sesquioxide clays dominate in tropics
• Al³⁺ has the highest flocculation power
• Na⁺ causes dispersion
• Gibbsite (Al), Goethite (Fe), Allophane (volcanic ash) = common soil minerals
• Tetrahedral (Si⁴⁺), Octahedral (Al³⁺/Mg²⁺) = clay layers
• Silica:Sesquioxide ratio indicates fertility & weathering degree