Fundamentals of Soil Science (1+1)

Introduction

• The word colloid is derived from Greek word colla meaning glue, and eidos meaning like.The colloidal state refers to a two phase system in which one phase in a very finely divided state is dispersed through a second continuous medium.
• The continuous medium is termed as the dispersion medium. When the dispersed phase is solid (e.g. soil colloidal particles) and the dispersion medium is water (e.g. soil water) the colloidal system is referred to as sol. The seat of chemical activity in the soil is the soil colloids.

Properties of Soil Colloids
1.Size:

• Colloidal particles are generally smaller than 1 micrometer (um) in diameter. Since the clay fraction of soil is 2um and smaller, not all clay is strictly colloidal, but even the lager clay particles have colloid like properties.
• Colloidal particles can be seen only by using electron microscope

2. Surface area

• Soil colloids are minute and, therefore, have a large surface area per unit mass. The external surface area of 1 g of colloidal clay is 1000 times that of 1 g of coarse sand.
• Certain silicate clays have extensive internal surfaces occurring between plate like crystal units that make up each particle and often greatly exceed the external surface area.
• The total surface area of soil colloids ranges from 10 m²/g for clays with only external surfaces to more than 800 m²/g for clays with extensive internal surfaces. The colloid surface area in the upper 15 cm of a hectare of a clay soil could be as high as 700,000 km² g^-1

3. Surface charges

• Soil colloids also carry electrostatic charges (- and +) . 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.

4. Adsorption of cations:

• The minute silicate clay colloidal particles are called as micelles (microcells), ordinarly carry negative charges, consequently, attract and attach the ions of positive charge on the colloidal surfaces. This gives rise to an ionic double layer.
• The colloidal particles constitutes the inner ionic layer, being essentially a huge anion, the external and internal surfaces of which are highly negative in charge.
• The outer ionic 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 carried by adsorbed cations and the cation is said to be in hydrated state.
• Some silicate clays hold numerous water molecules as well as cations packed between the plates that makes up the clay micelle.

6. Cohesion:

• Attractive force between similar molecules or materials are called cohesion. Cohesion indicates the tendency of clay particles to stick together.
• This tendency is due to the attraction of clay particles for water molecules held 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:

• Attractive force between different molecules or materials are called adhesion. Adhesion refers to the attraction of colloida1 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 montmorillonite swell when wet and shrink when dry.
• After a prolonged dry spell, soils high in smectite clay (e.g. Black soil -Vertisols) often show criss-cross wide and deep cracks.
• These cracks 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 called Brownian movement. 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.