Soil Colloids

Definition and Basics

• The Colloidal State:A two-phase system where one material in a finely divided state (dispersed phase) is distributed throughout another material (dispersion medium).
• Examples:
  • Solid in Liquid:Clay particles dispersed in water.
  • Liquid in Gas:Fog or clouds in the atmosphere.
• Soil Colloids:Soil particles less than 001 mm in size possess colloidal properties and are known as soil colloids. (These are a subset of the clay fraction, which includes particles less than 0.002 mm).

Types of Soil Colloids: There are two primary types of colloids in soil, which together form the “colloidal complex” responsible for most of the soil’s chemical activity.

i) Inorganic (Mineral) Colloids

• Composition:These are primarily composed of silicate clay minerals, made up of silica, alumina, iron, and combined water. These four constituents make up about 90% of colloidal clay.
• Charge:They possess a permanent negative charge (anions), which allows them to attract and hold onto positively charged plant nutrient ions (cations)
• Importance:They form the major portion of the colloidal complex in most soils and are a major reservoir for essential plant nutrients like Potassium (K⁺), Calcium (Ca⁺⁺), and Magnesium (Mg⁺⁺).
• Global Distribution:
  • Tropical/Sub-tropical regions:The colloidal complex consists almost entirely of inorganic colloids, often dominated by Iron and Aluminium Hydroxide clays.
  • Temperate regions:Silicate clays are the characteristic inorganic colloids.

ii) Organic (Humus) Colloids

• Composition:These are composed of humus, which is the stable end product of the decomposition of plant and animal residues. Their primary elements are Carbon, Hydrogen, Oxygen, and Nitrogen, unlike the Si and Al in clay colloids.
• Charge:They also carry a negative charge, but it is variable and dependent on soil pH.
• Importance:Humus is extremely important for soil fertility, water retention, and aggregation.
• Global Distribution:Soils in temperate regions usually contain a higher proportion of organic colloids compared to tropical soils.

3. Structure of Inorganic Clay Colloids

The structure is based on the arrangement of two fundamental building blocks: Silica tetrahedron (a layer of Silicon and Oxygen) and Alumina octahedron (a layer of Aluminium and Oxygen). These sheets combine to form different types of clay minerals.

a) 1:1 Type (Two-Layer Type)

• Structure:One sheet of silica tetrahedrons bonded to one sheet of alumina octahedrons.
• Properties:Non-expanding type. Has a relatively low capacity to hold nutrients and water.
• Example:Kaolinite.

b) 2:1 Type (Three-Layer Type)

• Structure:One sheet of alumina octahedrons is sandwiched between two sheets of silica tetrahedrons.
• Properties:This group includes both expanding and non-expanding clays.
  • Expanding Type:Example – Montmorillonite. Can swell when wet and shrink when dry.
  • Non-Expanding Type:Example – Illite (Hydrous Mica). Potassium ions firmly hold the layers together, preventing expansion.

c) 2:1:1 Type (or 2:2 Type)

• Structure:Similar to the 2:1 type, but with an additional octahedral sheet of Magnesium (Mg) or Aluminium (Al) hydroxide.
• Properties:Non-expanding.
• Example:Chlorites.