General properties of soil colloids:

1. Size of the colloids:

• Extremely in small size, microscopic (cannot be seen using an ordinary light microscope but can be seen only with an electron microscope.
• 1 µm to 1 nm (10-6 m to 10-9 m) or <2 µm in diameter.

(1mm=1000 micrometer=1000000 nanometer)

2. Surface area:

• Because of their small size, all soil colloids have a larger external surface area per unit mass.
• The external surface area of 1g of colloidal clay is 1000 times that of 1g of coarse sand.
• The total surface area of soil colloids ranges from 10m2 / g for clays with only external surfaces to more than 800m2/ g for clays with extensive internal surfaces.
• The colloid surface area in the upper 15cm of a hectare of a clay soil could be as high as 700,000 km2 g-1

3. Surface charge

• Both external and internal surfaces of soil colloids carry negative and / or positive charges. Most of the organic and inorganic soil colloids carry a negative charge.
• When an electric current is passed through a suspension of soil colloidal particles they migrate to anode, the positive electrode indicating that they carry a negative charge.
• The magnitude of the charge is known as zeta potential.
• The presence and intensity of the particle charge influence the attraction and repulsion of the particles towards each other, there by influencing both physical and chemical properties.

4. Adsorption of cations:

• As soil colloids possess negative charge they attract and attach the ions of positive charge on the colloidal surfaces.
• They attract cations like H+, Al3+, Ca2+ and Mg2+. This gives rise to anionic double layer.
• The Isomorphous substitution in the colloidal particle makes the external and internal layers of clay minerals negatively charged and these surfaces act as huge anions, which form the inner layer of the double layer.
• The outer layer is made up of a swarm of loosely held (adsorbed) cations attracted to the negatively charged surfaces.

5. Adsorption of water:

• A large number of water molecules are associated with soil colloidal particles.
• Some water molecules are attracted to the adsorbed cations and the cation is said to be in hydrated state.
• Others water molecules are held in the internal surfaces of the colloidal clay particles.

6. Cohesion: (Attractive force between similar molecules or materials).

• Cohesion indicates the tendency of clay particles to stick together.
• This tendency is due to the attraction of clay particles for water molecule sheld between them.
• When colloidal substances are wetted, water first adheres to individual clay particles and then brings about cohesion between two or more adjacent colloidal particles.

7. Adhesion (Attraction between different molecules or materials).

• Adhesion refers to the attraction of colloidal materials to the surface of any other body or substance with which it comes in contact.

8. Swelling and shrinkage:

• Some soil clay colloids belonging to smectite group like Montmorillonites well when wet and shrink when dry.
• After a prolonged dry spell, soils high in smectite clay (e.g. Black soil-Vertisols) often show crises-cross wide and deep cracks.
• The shrinkage first allow rain to penetrate rapidly. Later, because of swelling, the cracks will close and become impervious.
• But soils dominated by kaolinite, chlorite, or fine grained micas do not swell or shrink. Vermiculite is intermediate in its swelling and shrinking characteristics.

9. Dispersion and flocculation:

• As long as the colloidal particles remain negatively charged, they repel each other and the suspension remains stable.
• If on any account they loose their charge, or if the magnitude of the charge is reduced, the particles coalesce, form flock or loose aggregates, and settle down.
• This phenomenon of coalescence and formation of flocks is known as flocculation.
• The reverse process of the breaking up of flocks into individual particles is known as de-flocculation or dispersion.

10. Brownian movement:

• When a suspension of colloidal particles is examined under a microscope the particles seem to oscillate. The oscillation is due to the collision of colloidal particles or molecules with those of the liquid in which they are suspended.
• Soil colloidal particles with those of water in which they are suspended are always in a constant state of motion.
• The smaller the particle, the more rapid is its movement.

11. Non permeability:

• Colloids, as opposed to crystalloids, are unable to pass through a semi-permeable membrane.
• Even though the colloidal particles are extremely small, they are bigger than molecules of crystalloid dissolved in water.
• The membrane allows the passage of water and of the dissolved substance through its pores, but retains the colloidal particles.