Colloidal Particles in Soil: 8 Properties

This article throws light upon the eight important properties of colloidal particles present in soil. The properties are: 1. Adsorption 2. Brownian Movement 3. Electrical Charge 4. Flocculation 5. Plasticity 6. Cohesion and Adhesion 7. Swelling and Shrinkage 8. Non-Permeability.

Property # 1. Adsorption:

The surface adsorption is very large when large amount of colloidal materials are present in a substance, because of having large surface area contributed by its presence. The adsorption of ions is governed by the type and nature of ion and the type of colloidal particle.

The higher the valence of cations, the more strongly adsorbed it is. Hydrogen (H⁺) ions behave as polyvalent ions and hence it is adsorbed on the surfaces of colloids more strongly than those divalent cations like Ca²⁺.  Among the clay minerals, kaolinitic clay has a greater anion (e.g. phosphate adsorbing capacity than montmorillonitic or illitic clay.

Property # 2. Brownian Movement:

Colloidal particles are found to be in continual motion. The Oscillation is due to the collision of colloidal particles or molecules of the liquid in the dispersion medium. This movement is primarily responsible for the coagulation or flocculation of soil colloidal materials (clay and humus).

Property # 3. Electrical Charge:

Colloidal particles usually have an electrical charge—some positive and some negative when clay colloids suspended in water, it carries a negative electric charge and thereby attracts positively charged ions (cations).

Property # 4. Flocculation:

The colloidal particles are coagulated by adding an oppositely charged ion. This process of formation of floes is known as flocculation. As for example, clay is coagulated by the use of aluminium (Al³⁺). This condition is generally beneficial in relation to agriculture since it is the first step in the formation of stable aggregates or granules. The ability of common cations to flocculate soil colloids is in the order of,

A1 > Ca and H > Mg > K > Na.

In soils of humid and sub-humid regions, the colloidal complexes are dominated by aluminium, hydrogen and calcium and these ions encourage flocculation and consequently they help to form aggregates. Whereas in soils of arid regions, sodium ions have become dominant on the exchange complex and thereby poor physical condition of the soil develops resulting from the dispersion of the soil colloids which is not desirable.

So the phenomenon of flocculation plays an important role in the field of agriculture by improving the physical condition as well as other related condition of the soil which is beneficial for the plant growth.

Property # 5. Plasticity:

Soils containing more than 15 per cent of colloidal clay exhibit plasticity. Plasticity occurs only when soils are moist or wet. Due to this property clay colloids can be molded to any shape. Plasticity phenomenon is extremely important because it encourages a change in soil structure which is most related to the tillage operations.

Property # 6. Cohesion and Adhesion:

Colloidal clay particles exhibit the properties of cohesion and adhesion. Due to cohesion force clay particles are able to form aggregates and also due to adhesive force clay particles envelope sand particles. Both the forces develop in the presence of water and these two forces help in the retention of water in the soil as well as absorption to the plants and micro-organisms.

Property # 7. Swelling and Shrinkage:

Contraction and expansion in volume of soil with the variation in moisture are influenced by the presence of colloidal clay particles in soil. Swelling phenomenon occurs when colloidal clay particles are allowed to be placed in contact with moisture due to imbibition of water.

Whereas shrinkage occurs in dry condition of the soils containing sufficient amount of colloidal particles and causes decrease in volume with cracks in variable nature. Shrinkage and swelling are more pronounced in montmorillonite clays (expanding 2: 1 type) than that of kaolinite (non-expanding 1: 1 type) clays and these phenomena plays a significant role in the formation of soil structure.

Property # 8. Non-Permeability:

Colloids are unable to pass through a semi permeable membrane and this membrane allows passing water and other dissolved substances but retains colloidal materials.