Clays

Page 1: Introduction to Soil Chemistry and Mineralogy

  • Soil Texture: Refers to the grain size distribution, specifically the relative proportion of sand, silt, and clay.

  • Grain Size Classification:

    • Sand: 0.05 mm – 2 mm

    • Silt: 0.002 – 0.05 mm

    • Clay: < 0.002 mm

    • Analyzed in fractions smaller than 2 mm (Fine earth)

Page 2: Importance of Particle Size

  • Role of Smaller Particles: Smaller particles have a significant role in soil functioning, particularly clay.

  • Soil Texture Exam: Example of a soil with 30% clay, 10% silt, and remaining sand.

  • Soil Texture Triangle: Highlights the importance of clay in determining soil texture.

Page 3: Formation of Clays

  • Clay Formation Process: Clays are produced through weathering as a breakdown and synthesis process.

  • Weathering Types:

    • Physical: Reduces size, increases surface area.

    • Chemical: Alters mineral composition (e.g., influenced by moisture and acidity).

Page 4: Weathering Processes

  • Physical Weathering: Includes freeze/thaw cycles, temperature changes, root expansion, and abrasion.

  • Chemical Weathering: Involves decomposition via water and acids, affecting mineral stability & soil quality.

  • Examples: Some minerals dissolve completely, others may partially dissolve leading to new materials (e.g., clay).

Page 5: Weathering Reactions

  • General Reaction:

    • Mineral + H2O + acid ± O2 → clay mineral + soluble cations/anions.

  • Examples: Feldspar weathers to kaolinite.

  • Environmental Impacts: Relates to soil parent material and runoff water quality.

Page 6: Clay Characteristics

  • Two Processes in Clay Formation: Breakdown (physical weathering) and synthesis (recombination of weathered materials).

  • Properties of Clays:

    • Large surface area (10 m²/g to 800 m²/g).

    • Reactive surfaces with electrostatic charges, allowing ion adsorption.

    • Important for nutrient retention and water-holding capacity.

Page 7: Types of Clays

  • Clay Composition:

    • Crystalline: Defined morphology and layered structure.

    • Amorphous: Lacks a defined structure.

  • Behavior Influence: Clay type affects nutrient retention, malleability, and water absorption.

Page 8: Colloid Types in Soil

  1. Crystalline Silicate Clays: Layered structure, predominantly negative charge.

  2. Amorphous Silicate Clays: High variable surface charge.

  3. Oxides of Fe and Al: Variable charge, found in highly weathered soils.

  4. Organic: Contains high net negative charges, important across soil orders.

Page 9: Crystalline Silicate Clay Structure

  • Layered Structures:

    • 1:1 Type: One tetrahedral sheet, one octahedral sheet.

    • 2:1 Type: Two tetrahedral sheets for one octahedral sheet.

  • Unique Surface Areas: Each type contributes differently to soil characteristics.

Page 10: Charge Types in Colloids

  • Two Charge Types:

    • Permanent Charge: From isomorphous substitution in clay structure.

    • Variable Charge: pH-dependent and occurs at the colloid surface.

  • Influences: Charge affects nutrient adsorption and soil behavior.

Page 11: Charge Differences

  • Highlights the comparison of total charge, permanent vs. variable charge, and interaction with soil solution.

Page 12: Ion Interaction with Colloids

  • Cation Exchange: Clay surfaces adsorb cations and can exchange them with soil solution ions.

  • Interrelation: Positively charged waters lead to different retention behaviors in soils.

Page 13: Clay Types and Weathering

  • Soil Orders Related to Weathering: Different orders like Ultisols and Oxisols are associated with varying weathering intensity.

Page 14: Recap of Clay Formation and Types

  • Key Takeaways:

    • Clays are colloids and products of weathering.

    • Smaller particles (like clays) exert significant effects on soil function.

    • Types of colloids vary in morphology, charge, and behavior.