Fundamentals of Soil Science (1+1)

Classification of Soil Colloids

Classification of Soil Colloids
Soil colloids can be broadly classified in two types, depending on the nature of the linkages present and the types of compound formed.

These are:
i) Inorganic colloids
ii) Organic colloids.
Both inorganic and organic colloids are intimately mixed with other soil solids. Thus, the bulk of the soil solids are essentially inert and the majority of the soil's physical and chemical character is a result of the colloids present.
Inorganic soil colloids:

• Layer aluminosilicates which consist of thin layers of repeated structural units. These are the dominant clay minerals in temperate regions.
• Amorphous aluminosilicates that form from volcanic ash
• Al and Fe oxides which may be crystalline or amorphous. These are common in subtropical and tropical regions.

Organic colloids:

• Include highly decomposed organic matter generally called humus.
• Organic colloids are more reactive chemically and generally have a greater influence on soil properties per unit weight than the inorganic colloids.
• The negative charges of humus are associated with partially dissociated enolic (-OH), carboxyl (-COOH), and phenolic groups; these groups in turn are associated with central units of varying size and complexity.
• The complex humus colloid is composed of C,H and O rather than Al, Si and O like the silicate clays.
• Humus is amorphous and its chemical and physical characteristics are not well defined. The organic colloidal particles vary in size, but they may be at least as small as the silicate clay particles.
• They are not stable as clay and thus more dynamic, being formed and destroyed more rapidly than clay.

Layer Aluminosilicates:

• The most important silicate clays are known as phyllosilicate (Gr. Phullon, leaf) because of their leaf or plate like structure.
• Two types of structural units are basic in the layer lattice structure of most clay minerals that are tetrahedral and octahedral unit.

Silica tetrahedron:

• The tetrahedral unit is SiO^{4 –} in which silicon ion is equidistant from the four oxygen anions (Fig.1). It is called silica tetrahedron because of its four sided configuration.
• Many tetrahedra are linked together horizontally by shared oxygen anions gives a tetrahedral sheet.

Tetrahedron Structure Octahedron Structure

Alumina octahedron:

• Aluminium and/or magnesium ions are the key cations surrounded by six oxygen atoms or hydroxyl group giving an eight sided building block termed octahedron ( Fig.1)
• Many octahedra are linked horizontally to form an octahedral sheet. An aluminum-dominated sheet is known as a di-octahedral sheet, whereas one dominated by magnesium is called a tri-octahedral sheet.
• The distinction is due to the fact that two aluminum ions in a di-octahedral sheet satisfy the same negative charge from surrounding oxygen and hydroxyls as three magnesium ions in a tri-octahedral sheet.
• The tetrahedral and octahedral sheets are the fundamental structural units of silicate clays. These different sheets are bonded together to form crystalline units composed of alternating sheets of Si tetrahedra and Al (or Mg) octahedra.
• The Si tetrahedral sheet is chemically bonded to the one or two adjacent Al (or Mg) octahedral sheet(s) via shared oxygen atoms.
• The specific nature and combination of sheets, called layers, vary from one type of clay to another and control the physical and chemical properties of each clay.