Viktor Horn
Chapter 8 Summary
General Properties
 Soil Colloids are extremely small in size (< 2Um)
 Surface area is the ratio of total surface area per unit mass
 Soil colloids also carry electrostatic charges that are balanced by
absorbed cations and anions
Four Types of Soil Colloids
 Crystalline silicate clays
o Consist of thin layers of repeated structural units
o Dominant in clay minerals in temperate regions
 Noncrystalline silicate clays
o Tightly bonded silicon, aluminum and oxygen atoms
o and Imogolite => form from volcanic ash
o Lacks of well defined crystal structure
 Iron and aluminum oxide
o Important in highly weathered soils of warm humid
regions
o Mainly iron or aluminum atoms coordinated with Oxygen
atoms
o Gibbsite (Al oxide) and goethite (Fe oxide) form
crystalline sheets
 Organic (humus)
o Important in nearly all soils
o They are not minerals or crystalline in nature
o Consist of long convoluted chains and rings of Carbon
bonded to Hydrogen, Oxygen and Nitrogen
o High capacity to absorb water
Types of layer silicates

1:1 layer silicates => kaolinite
o Hexagonal crystals
o No shrink swell
o Useful for making bricks and ceramics
o Low specific surface area

Geography 3080
2:1 layer silicates => expanding
o Plates to flakes crystals
o Moderate shrink swell
o Undesirable for most construction activities
o High specific surface area
 2:1 layer silicates => none expanding
o Flakes to variable crystals
o Low shrink swell
o Intermediate specific surface area
Two processes of silicate clay development
o Physical and chemical alternation of primary minerals
o Decomposition of primary minerals
Two major sources of charges on soil colloids
o Constant charge => negative and positive charges
o Variable or pH dependent => negative and positive
charges
Absorption of cations and anions
 Temperate region soils
o Anions are absorbed in smaller quantities than cations
 Tropics => acid, rich in 1:1 clay
o Anions are absorbed much more
Cation exchange reactions
 Anion exchange
o Hydrated anion replaces another hydrated anion =>
positively charged site
 Exchangeable (cations and anions)
o Ions held in outer sphere were they can be replaced by
exchange reactions
 Exchange complex (cations or anions)
o Group of colloids in soil, organic, inorganic => holding
exchanging cation or anions