- organic and inorganic matter with small particle size- clays and humus.
- Cannot be seen with microscope but have large surface area per unit mass.
- Colloidal surfaces carry positive or negative charges, mostly negative change
- this influences most chemical, physical and biological properties of soils
- exchange of ions between soil particles and plant roots is vital to life
- cation and anion exchanges take place on surface of colloids
- Same weight of clay has 1000 times more surface area than sand particle-
each colloid has large external surface per unit mass.
- These internal surfaces occur between platelike crystal units that make up
each particle and commonly greatly exceed the external surface area
4 TYPES OF SOIL COLLOIDS:
- Layer (crystalline ) silicate clays:
- dominant inorganic colloids in most all soils
- each particle made up of series of layers like pages of book
- layer comprised of planes of closed packed oxygen atoms held together by
silicon, aluminum, magnesium, and or iron, eg. Kaolinite
- exact chemical composition and internal arrangement of atoms in each
layer accounts for particles surface change and abikity to hold and exchange
ions, as well as physical properties such as stickiness and plasticity.
- Allophane and other amorphous material:
-non crystalline – no geometrical form
- allophane found in soils developed in volcanic ash.
- Iron and Aluminum oxide clays
- clays which dominate in highly weathered soils of tropics and temperate