Chemical Properties of Soil and Clay Fraction

Reminders

  • Reminder at 3:1.

Introduction to Chemical Properties

  • Introduction to chemical properties of soil.

Chemical Properties

  • Chemical properties affect:
    • The speciation of nutrients and availability of nutrients to plants.
    • The fate of chemicals (pollutants) in the soil.
    • Soil physical properties and microbial activity and survival.

Clay Fraction of Soil

  • Clay is the smallest of the three textural classes, with a size of less than 0.0020.002 mm.
  • Colloid: Mineral or organic particle with a diameter less than 0.0010.001 mm (within the clay size range); it is the smallest of the clays.
  • Chemical reactions take place on colloid surfaces.
  • Colloids are within the clay size range.
  • The chemistry of a soil body/soil mass is largely a function of the properties of the clay fraction.

Importance of Clay Properties

  • It is important to know the properties of clays.
  • Specifically, it is important to know what clay minerals are present and what their contributions are to the soil properties.

The Clay Fraction

  • Clay = minerals with an effective diameter of < 0.0020.002 mm.
  • Three types of clay minerals:
    • Iron and Aluminum Oxides
    • Non-crystalline Silicate Clays
    • Crystalline Silicate Clays

Iron and Aluminum Oxides

  • Iron and Aluminum Oxides (advanced stages of weathering).
  • Common in humid tropical environments (highly weathered – like Ultisols and Oxisols) and have a bright red color.

Non-crystalline Silicate Clays

  • Usually found in soils that developed from Volcanic Ash (Andisols).

Crystalline Silicate Clays

  • Also called "Layer Silicates".
  • Layers that make up the clay particles are made of 2 Basic Structural Units:
    • Tetrahedron/Tetrahedral Unit
    • Octahedron/Octahedral Unit

Layer Silicates - Structural Units

  • Si Tetrahedron
  • Al Octahedron
  • Tetrahedral Unit: Si4+ surrounded by oxygen (O2-)
  • Octahedral Unit: Aluminum (Al3+) surrounded by hydroxide (OH-)

Silicate Tetrahedral Sheets

  • Combination of Structural Units
  • Tetrahedral Sheet: Arrangement of linked tetrahedra.
  • Octahedral Sheet: Arrangement of linked octahedra.
  • Clay Layer/Lamellae: Stacked sheets forming a clay layer.

Structure of Layer Silicates

  • Clay Particles: Multiple layers stacked together.

Structure of Layer Silicates

  • 1:1 layer silicates: One tetrahedral sheet and one octahedral sheet (e.g., Kaolinite).
  • 2:1 layer silicates: Two tetrahedral sheets and one octahedral sheet (e.g., Montmorillonite, Vermiculite, Illite).

Analogy for Layer Silicates

  • Oreo with two cookies and cream: 2:1 mineral.
  • Oreo with one cookie and cream: 1:1 clay mineral.
  • What holds them together and what pushes them apart allows us to categorize them.

Question: Why are clay particles very reactive?

  • Answer: They have a very high surface area, and the surfaces are usually charged.

Sources and Types of Surface Charges

  • Permanent charge (fixed charge) of clays results from isomorphous substitution.
    • Source of charge for 2:1 clays.
  • Variable Charge (pH-dependent)
    • Develops on exposed functional groups on surfaces of organic matter and clay edges.
    • Significant source of charge of organic matter, 1:1 clay (kaolinite), non-crystalline silicate clays, and in Fe and Al Oxides.

Fixed Charge: Isomorphous Substitution

  • In a Tetrahedron, Al3+ replaces Si4+, leaving the clay particle with extra negative charge.
  • In an Octahedron, Mg2+ replaces Al3+, leaving the clay particle with extra negative charge.

Isomorphic Substitution

  • A1+3
  • Unpaired negative charge
  • Recall
  • Tetrahedron is negatively charged.
  • Note that this happens in a lot of tetrahedrons.
  • Leads to a tremendous amount of negative charge in the soil.

Surface Charge of Different Clays

  • Montmorillonite (Smectite): -80 to -150 cmol/kg
  • Vermiculite: -100 to -200 cmol/kg
  • Illite: -10 to -40 cmol/kg
  • Chlorite: -10 to -40 cmol/kg

Kaolinite

  • 1:1 Clay Mineral
  • Tetrahedral sheet (cookie).
  • Octahedral sheet (cream).
  • Interlayer Bond: Strong hydrogen bonds.
  • Relatively large - ranging from 0.1 to 5 microns.
  • No Isomorphous Substitution; no permanent charge.

Illite (Mica) 2:1 Non-Expanding

  • Tetrahedral sheet.
  • Octahedral sheet.
  • Tetrahedral sheet.
  • Source of charge: Isomorphous substitution that occurs only in the tetrahedral sheet.
    • Fixed K+ interlayer cation that tends to bond layers together.
    • Important! Water cannot easily enter the interlayer to pry layers apart, so illite does not swell much.

Vermiculite 2:1 Expanding

  • Tetrahedral sheet.
  • Octahedral sheet.
  • Tetrahedral sheet.
  • Source of charge: Isomorphous substitution that occurs only in the tetrahedral sheet.
    • Very strong net negative charge.
    • Cations in the interlayer are not limited to K+.
    • Allows some expansion, but ions tend to act primarily as a bridge to hold units together.
    • Similar to illite, but interlayer contains a Cation other than K+.

Practice: Illite, Vermiculite, or Both?

  • 2:1? Both
  • Fixed exclusively by K+? Illite
  • Expansion when hydrated? Vermiculite
  • Isomorphic substitution occurs in the tetrahedron? Both
    • Permanent

Montmorillonite (Smectite)

  • Isomorphic substitution occurs in Octahedron.
  • Tetrahedral sheet
  • Octahedral sheet
  • Tetrahedral sheet
  • Charge felt on inter-later is lower than in Vermiculites
  • Why does it expand?

Key Differences

  • Isomorphic substitution occurs in both tetrahedral and octahedral sheets for smectite.
  • Both vermiculite and smectite expand, but vermiculite expands less than smectite.

Chlorite 2:1 Non-Expanding

  • Tetrahedral sheet.
  • Octahedral sheet.
  • Mg Oxide layer.
  • Does not expand!!
  • Where does isomorphic substitution occur?

Summary Table

  • Kaolinite: 1:1, no permanent charge.
  • Smectite: 2:1, has permanent charge.
  • Illite: 2:1, has permanent charge.

Properties Table

  • All 2:1 minerals have permanent charge.