Soil Colloid : the Important constitute of Soil


Soil is a natural made body that constitute organic matter, essential elements, nutrients, etc. Sometimes Soil is also called three dimensional body because they have length, breadth and height also. Basically soils are formed by Physical or Chemical weathering of Regolith (parent materials) and composed of tiny particles. even Soil colloid also comes under basic component of Soil.

Soil Colloid –

Soil colloid refers to the colloidal complex of Soil that consists smallest Particles of clay and Humus and Plays an important Role in ion exchange and fertility.

Colloid = A Colloid is a mixture in Which one insoluble Particles are dispersed into it, is Suspended throughout the mixture not Precipitated into the Solution or Mixture.

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And the Property that is Shown by this type of Solution (colloid) are known as Colloidal Property.

For example-

  • Soil in Liquid = Dispersion of Clay in Water.
  • Liquid in Gas = Fog, Clouds in Atmosphere.

General Properties of Soil Colloids

There are several Properties that describing below-

  1. Size
  2. Surface Area
  3. Surface Charge
  4. Absorption of Cation
  5. Absorption of Water
  6. Cohesion
  7. Adhesion
  8. Swelling and Shrinkage Properties
  9. Dispersion
  10. Brownian movement
  11. Non Permiability

(1). SIZE

microscopic view of Particles of Soil colloid
microscopic view of Particles of Soil colloid

Colloids are Basically Two types (Organic or Inorganic Colloids), thus the organic or Inorganic Colloids are extremely Small size that is Smaller than 2 micro metre in Diameter.

The Particles can not be seen by using Simple microscope but can be Seen only With an Electron microscope.

(2). Surface Area –

Because of their Small Size, all Soil Colloids have a Larger External Surface Area Per unit Mass (1 gram).

For example – The External Surface Area of One gram of Colloidal clay is thousand time that of 1g of Coarse Sand.

  • The Colloidal Surface Area in Upper 15 cm of a hectare of Clay Soil could be as High as 7,00,000 km2/g
  • Total Surface Area of Soil Colloids – Range:

External Surface = 10 m2/g

Internal Surface = more than 800 m2/g

(3). Surface Charge –

soil colloid particles contain negative charge on their surface
soil colloid particles contain negative charge on their surface

Both Internal and External Surface of Soil Colloids carry negative and/or Positive Charge.

  • Most Organic and Inorganic Soil colloids Carry a negative Charge.
  • In Electrolytic Solution when an electric current is Passed through the Suspension of Soil colloid Particles, they migrate to Anode shown as Figure-
  • Magnitude of Charge is known as ZETA POTENTIAL.
  • The Source of negative Charge on Clay Surface comes from-
  1. Ionizable Hydrogen ion,
  2. Isomorphus Substitution.

a). Ionizable Hydrogen ion = These are the Hydrogen from –OH  ion on clay Surface. As shown as figure –

  • (H) depends upon Soil Ph. Hense these negative charges are Ph dependent charges.
  • More ionization occurs in Alkaline (Basic) solution.

b). Isomorphus Substitution = The term Isomorphus  is derived from two other words i.e., ISO and MORPHUS [ ISO means Same and MORPHUS means Structure] And Substitution means Replacement.

  • This is the Substitution of Cation (higher Valence) by Anion (lower valence) but It is necessary that both have Similar Size.
  • In Clay crystal; Si+4 is in Tetrahedral Site and Al+3 is at Octahedral Sites.

Al+3 has Valancy = 3

Si+4 has Valancy = 4

So, Si+4 is replaced by Al+3 

  • It means Positive Charge decrease and negative Charge of O2 remains unchanged that’s why negative Charge be Increased.


(4). Adsorption of Cation –

cation absorbed on surface of Soil colloid
cation absorbed on surface of Soil colloid

AS the Soil colloid carry negative charge they attract and attach positive charged ion (cation) on colloidal surface such as H+, Al3+, Ca2+, Mg2+.

  • This gives rise to an ionic double Layer.

(5). Adsorption of water –

A large number of water molecules are associated with soil colloidal Particles.

  • Some H2O molecules attracted to absorbed cation is said to hydrated state.
  • Other H2O molecules are held in Internal Surface surface of colloidal clay Particles.
  • These water molecules plays a critical role in Physical and chemical Properties of Soil.

(6). Cohesion –

Cohesion refers to the attractive force between the similar molecules or materials. Cohesion indicates the tendency of lay particles to Stick together. (This Tendency is due to attraction of clay Particles for water molecules held between them).

  • When colloidal Substances are wetted, water firstly adheres (attach) to Individual Clay Particles and then brings about cohesion between two or more adjacent colloidal Particles.

(7).  Adhesion –

Adhesion refers to the attractive force between the different molecules or materials.

  • Adhesion means to attraction of colloidal materials to the surface of any other body which it comes in contact.

(8). Swelling and Shrinkage –

some soil clay swell when wet and shrink when Dry. For example – Smectite group of Soil = montmorillonite or Vertisols, when dry they shoe deep crack.

  • These cracks first allow Rain to penetrate rapidly. Later, because of Swelling, the Crack will close.
  • But Soils such as Kaolinite, Chlorite, micas etc. do not have Swelling and Shrinking Properties.

(9). Dispersion and Flocculation –

Approx all colloidal Particles have negatively charged ion, so they repel each other and suspension remains stable.

  • If On any account, they loose their charge or magnitude will be reduced, they form flock and settle down. This process of Formation of flocks is known as Flocculation.
  • While the reverse process of breaking down of flocks is known as de-flocculation or also known as dispersion.

(10). Brownian movement –

Brownian movement of Soil colloid particles
movement of Soil colloid particles

when a suspension of colloidal particles is examined under microscope, the Particles are seem to be Oscillate (movement).

  • The Brownian movement is named on the Scientist name Robert Brown.
  • The Oscillation is due to-
  1. Collision of colloidal particles with liquid in which they are suspended.
  2. Constant state of motion.
  • Smaller the particle, the more rapid is its movement.

(11). Non-Permiability –

The colloidal particles are unable to pass through semi permeable membrane.

  • The semi permeable membrane is a biological membrane that allow the Passage of water and dissolved Substances.
  • The colloidal particles are extremely small but they are bigger than molecules of crystalloid dissolved in water.

Types of Soil colloids-

Soil colloids are basically classified into four types, i.e.,

  1. Layer Silicate clay or Silicate clay
  2. Sesquioxide clay or Iron and Aluminium oxide clay
  3. Allophane and associated amorphous clay
  4. Humus

If we categorized them into Organic or Inorganic colloids then Humus (4) comes under Organic colloids but rest (1,2,3) are comes under Inorganic colloids.

(1). Layer Silicate clay –

Layer Silicate clay are also known as Silicate clay and Phyllosilicates. The term Phyllosilicates is derived from the word Phyllon which means Leaf because of their leaf like or Plate like structure.

  • Phyllosilicates or Layer Silicate clay are made up of horizontal sheets-
  1. Dominated by Si.
  2. Dominated by Al + Mg / Al or Mg.

(2). Sesquioxide clay –

A Sesquioxide ia an Oxide compound that contains three atoms of Oxygen with two atoms of another elements.

  • There are many metal Sesquioxide are Present in Nature but mainly Sesquioxide of Al and Fe are present into the Soil.
  1. Sesquioxide of Al = Aluminium oxide [Al2O3]
  2. Sesquioxide of Fe = Iron oxide [Fe2o3]
  • Sesquioxide mainly contains the metal of +3 oxidation State. For example = Al or Fe.

(3). Allophane and associated amorphous clay-

  • Allophane = Allophane is an amorphous mineral of various color consisting of a hydrous aluminium silicate.
  • These Silicate clays are the mixture of silica and aluminium.
  • Allophanes are Amorphus in Nature.
  • Allophanes are derived by Volcanic ash materials and constitute amajor component of Volcanic Soil.
  • Example of Alophane = Imogolite (Si Al4O10.5H2O)
  • These clays have high Anion or Cation exchange capacity.

(4). Humus –

  • Color = Dark brown to Black
  • Humus are Amorphus in Nature.
  • They are insoluble in water but mostly soluble in alkali solution.
  • Humus are the product made after decomposition of Plants and Animals.
  • The Humus is referred to as Organic colloid and consists of various chains of linked carbon atom.
  • Humus is not crystalline in Nature
  • They are composed basically of C, H, O2 rather than of Si, Al, Fe.




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It is Arpan Priy Das from west champaran, Bihar. Arpan started his blog about agricultural information named as “ECOHILLAGRI.COM ” He started his blog to help many agricultural or other students as well as farmers. so, it is necessary for them to learn about agriculture that is useful for students as well as farmers and that all concepts are present in this blog.

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