Soil Colloids and Silicate Clay Minerals
- 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).
- Colloidal Solutions vs True Solutions
Property | True Solution | Colloidal Solution |
Particle Size | <1 nanometer | 1 nm to 1000 nm (1 μm) |
Visibility | Invisible under microscope | Visible under ultramicroscope |
Filtration | Passes through membranes | Does not pass parchment membrane |
Settling | Does not settle | Remains suspended |
- 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).
- 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)
- Acidic vs Alkaline Clays
Region Type | Predominant Cations | Nature of Clay |
Humid | H⁺, Al³⁺ | Acid Clay (Al-H clay) |
Arid | Ca²⁺, Mg²⁺ | Calcium Clay (Neutral) |
Arid + Sodium | Na⁺ | Sodium-Calcium Clay (Alkaline) |
In humid soils, H⁺ & Al³⁺ dominate → acidic
In arid soils, Ca²⁺ & Mg²⁺ dominate → neutral
If Na⁺ accumulates → alkaline reaction
- 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
- 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
- Silica-Sesquioxide Ratio
- Molar ratio = Silica / Sesquioxides in clay fraction
- Original rock: ~6.0
- In soils: 1.25 – 3.6
🔹 Ratios in Soils:
Soil Type | Ratio Range |
Black Soils | High |
Ashy Grey & Black | Medium |
Red/Brown Soils | 2.0–2.5 |
Laterite Soils | Low |
Higher ratio = High CEC, moisture retention, fertility
Lower ratio = Leached, acidic, low fertility
- Common Colloidal Minerals in Soil
Mineral | Formula | Notes |
Gibbsite | Al₂O₃·3H₂O or Al(OH)₃ | Common Al oxide |
Goethite | FeO(OH) or Fe₂O₃·H₂O | Common Fe oxide |
Allophane | Al₂O₃·2SiO₂·H₂O | Amorphous, volcanic ash soils |
- Silicate Clay Minerals
🔹 Phyllosilicates (“Phyllon” = leaf-like)
- Most important group of silicate clays
- Structure: Sheets/layers of mineral cations
Clay Structural Units
Type of Sheet | Dominant Element | Structure |
Tetrahedral Sheet | Silicon (Si⁴⁺) | 4-sided (SiO₄) |
Octahedral Sheet | Al³⁺ or Mg²⁺ | 6-sided (Al(OH)₆ / Mg(OH)₆) |
- 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
