Soil Colloids

1. Soil Colloids: Properties, Nature, Types and Significance.
Layer silicates: Genesis and sources of charges
Soil Colloid
 The term colloid is coined from the Greek word “Kolla” meaning
glue.
The fraction of the soil particles having size less than 0.002 mm
in size.
Particles having size less than 0.001 mm possess colloidal
properties and are known as soil colloids.
 These are also called as chemically reactive particles
Colloid is a state of matter consisting of very fine particles but
never approach molecular size.
Examples: Milk, Cheese, Clouds, Fog etc. Dr.AB Jadhav, SSAC,AC, Pune

2. Dr.AB Jadhav, SSAC,AC, Pune

3. Significance of Soil
Colloids
Soil Chemistry
& nutrient
availability
Soil
management
Soil pesticide
interaction and soil
pollution
Nutrient fixation:
Illite, vermiculite, and
montmorillonite-K/
NH4
Interaction with
humuc substances
Soil Physical
properties
Dr.AB Jadhav, SSAC,AC, Pune

4. Types of Soil Colloids
Layer Silicate
Clays
e.g. Kaolinite,
Montmorrilonite
Illite
Iron &
Aluminum
oxide clays or
sesquioxide
clays
e.g. Gibbesite,
Geothite
Allophane
and
associated
amorphous
clays
e.g. Allophane,
Immogiliter
Humus
e.g. Organic colloids
Dr.AB Jadhav, SSAC,AC, Pune

5. Types of Layer Silicate Clays
A) 1:1 Type
One Si-Tetra
One Al-Octa
e.g. Kaolonite
B) 2:1 Type
Two Si-Tetra
One Al-Octa
e.g. Smectite
Non-expanding type
Illite
Expanding Type
Smectite
Vermiculite
C) 2:1:1 Type
Two Si-Tetra
One Al-Octa & again
One Al-octa
e.g. Chlorite
Dr.AB Jadhav, SSAC,AC, Pune

6. Silica Tetrahedron
Aluminium or Magnesium octahedron
Dr.AB Jadhav, SSAC,AC, Pune

7. 1. Size 2. Surface area
3. Surface charges
i) pH dependent charge
ii) Isomorphus substitution
iii) Brocken edges of bonds
4. Adsorption of cations
5) Adsorption of water 6) Cohesion
7) Adhesion 8) Swelling and shrinkage
9) Dispersion and flocculation 10) Brownian movement
11) Non permeability
General Properties of Soil Colloids
Dr.AB Jadhav, SSAC,AC, Pune

8. Sources of charge on colloids
1) pH dependent charge
2) Isomorphus substitution
3) Brocken edges of bonds
1) pH dependent charge
 The total charge soil particles frequently varies with pH
 The positive charge developed at low pH and the excess negetive
charge developed at high pH are collectively called as pH dependent
charge.
 Most soils have negative charge because of the negative charges on
layer silicates and organic matter. But some highly weathered soils
dominated by allophane may have net positive charge.
 The primary source of pH dependent charge is considered to be gain
or loss of H from functional groups on the surfaces of soil colloidsDr.AB Jadhav, SSAC,AC, Pune

9. 2) Isomorphus Substitution
This is the major source in 2:1 and 2:1:1 type layer silicates
Definition:
1. It is the substitution of one ion for another of similar size within a crystal
lattice Si (0.42 Ao) or 0.042 mm in the tetrahedral layer is replaced by ion of
similar size Al3+ (0.51 Ao) or 0.051 mm.
2. In the same manner, part of Al3+ in the octohedral sheet may be replaced by
Mg2+ (0.66 Ao) without disturbing the crystal structure.
3. The resulting – ve charge is considered as permanent charge, since it will not
change with pH.
4. The substituting ion may have a greater, equal or lower charge than the ion
for which it substitute.
5. If a cation of lower valence substitute for one of higher valence like Mg2+ for
Al3+ or Al3+ for Si4+, negative chrge of O2- and OH- ions in the mineral
structure is left unbalanced by yielding a net negative charge on the mineral.
Dr.AB Jadhav, SSAC,AC, Pune