Chapter 4A
SOIL COLLOIDS
Chemical nature of soil constituents
Three phases:
Solid - skeletal framework of soils
- mixture of inorganic and organic material
Liquid - soil solution, carries and moves dissolved
nutrients
Gas
- soil air
Percentage composition by volume:
O2
N2
CO2
Soil air
20.0
78.6
0.50
Atmospheric air
21.0
78.03
0.03
Chemical Nature of Soil Constituents
Detailed structure of a soil showing solid, water and air
phases
SOIL COLLOIDS
Seat of chemical activity or reactions in
soils
Very small particles whose sizes range from
0.2 to 1 micrometer (m)
1m = 1/1000 mm
= 0.001mm
= 0.0001 cm
SOIL COLLOIDS
Organic and inorganic components of soil
having very small particle size and a
corresponding large surface area per
unit soil mass [<.001 mm or 1 micron
(µ)].
Does not include all the clay components
(<.002 mm or 2 micron).
CLASSIFICATION
1. Organic colloids – humus
2. Inorganic colloids
a. Silicate clays
b. Oxide clays
c. Amorphous clays
Inorganic Colloids
1. Clay – refers to soil particles that have a
diameter of <2 micrometer (µm) (1 µm=106 meter)
2. It is a secondary mineral formed or
synthesized from primary minerals or their
weathering products.
3. It is made up of layers of plates or flakes
Inorganic Colloids
a. Silicate clays
montmorillionite
illite
vermiculite
chlorite
kaolinite
b. Oxide clays (Non-silicate clays):
Fe oxide and hydrous oxide – Hematite, Goethite
Al oxide and hydrous oxide – Gibbsite, Boehmite
c. Amorphous clays
Allophanes and Imogolite
Silicate Clays (Layer silicates)
Basic structural units:
1. Silica tetrahedron
2. Alumina octahedron
Silicate Clays (Layer silicates)
Silica sheet
Octahedral sheet
Silicate Clays (Layer silicates)
Diagrammatic representations:
tetrahedral sheet:
octahedral sheet:
Silicate Clays (Layer silicates)
Classification of silicate clays (based on number of
tetrahedral to octahedral sheet):
1:1 Type:
Unit layer
Silicate Clays (Layer silicates)
2:1 Type:
Unit
layer
Variety of clay minerals based on the arrangements
of the tetrahedral and octahedral sheets.
Three structural groupings:
a. 1:1 layer phyllosilicate – a tetrahedral sheet
attached to one side of an octahedral sheet.
b. 2:1 layer phyllosilicate – symmetrical arrangement
of two tetrahedral sheets about a central
octahedral sheet.
c. 2:1:1 or 2:2 layer phyllosilicate – presence of an
octahedral sheet between *adjacent 2:1 layers.
Chemical Nature of Soil Constituents
Silicate clays
Important Silicate Clays Minerals
1. Kaolinite
2. Smectite (Montmorillonite)
3. Vermiculite
4. Micas
5. Illite
*6. Chlorite
a) 1:1 type - kaolinite
• two sheets are held together by oxygen
atoms that are mutually shared by Si and
Al
• unit layers are held together tightly by Hbonding
restricts expansion
limits the reactive area to external
surface
Specific surface area: 10-20 m2/g
Kaolinite
• Little isomorphous substitution
• Low colloidal property
CEC : 3 – 15 cmolc/kg
• Occur as hexagonal crystals
Transmission electron
micrograph of kaolinite
mineral
Kaolinite
-1:1 type of structure)
-Low CEC (3-15
meq/100g)
-Low specific surface
(m2/g)= 5-20
-Low water holding
capacity
-Low shrink-swell
*potential
• Soils high in kaolinitie do not swell nor
shrink on wetting and drying
• Soils dominated by kaolinite are:
 good bases for roadbeds and building
foundations
 commonly used in making bricks
 easy to cultivate for agriculture
b) 2:1 expanding type: Montmorillonite
• Most common member of smectite group
• unit layers are loosely held together by weak
oxygen-to-oxygen and cation-to-oxygen linkages
• When saturated with water, basal spacing
between layers can approach 20ºA (2.0 nm)
Under dry conditions it may be reduced to less
than 10ºA (1.0 nm)
Montmorillonite
- 2:1 type of structure)
- High CEC (60-100 meq/100g
- High Specific surface (m2/g)= 700-800
- High water holding capacity
- High shrink-swell potential
b) 2:1 expanding type: Vermiculite
• Layer structure resembles that of mica from which
it is derived
• Aluminum dominated dioctahedral:
(Ca,Mg)y(Si8-xAlx)(Al,Mg,Fe)6O20(OH)4.nH2O
• Mg dominated trioctahedral:
(Ca,Mg)y(Si8-xAlx)(Mg,Fe)6O20(OH)4.nH2O
• capable only of limited expansion ; swells less than
montmorillonite because of higher layer charge
Al3+ substituting for Si4+ in the tetrahedral
sheet
Mg2+ and Fe2+ as the octahedral cations
-
-
-
Vermiculite
-2:1 type of structure)
-High CEC (100-150 meq/100g)
-Limited expansion compared with
smectite.
-Larger crystal than smectite.
c) 2:1 non-expanding: illite (fine-grained mica)
• chemical composition is similar to muscovite, but
contains more SiO2 and less K
• contains interlayer K, the unit layers are bonded
more strongly than montmorillonite and vermiculite
hence no expansion
• CEC: 20-40 cmolc/kg
• Specific surface: 40-180 m2/g
- K
K
-
Unit
layer
K
Mica (Illite)
- 2:1 type of structure)
- Low CEC (15-40 meq/100g)
- Limited expansion compared with montmorillonite
- Variable d-spacing
14 Å expanded
10 Å collapsed
- Larger crystal than montmorillonite
- Specific surface
(m2 /g)= 100-120
Chemical Nature of Soil Constituents
Silicate clays
d) 2:1:1 or 2:2 non-expanding type chlorite
• 2:1 layers alternate with a hydroxide sheet
commonly dominated by magnesium
• no water adsorption between the unit layers
hence its non-expansive nature
• CEC: 10-40 cmolc/kg
Chemical Nature of Soil Constituents
Silicate clays
2:1:1 type - chlorite
unit layer
Mineral colloids other than silicates
1. Hydrous oxide clays of iron and aluminum

advance stage of weathering usually found in
humid tropics

Gibbsite – Al(OH)3 or [Al2O3.XH2O]

Goethite – Fe(OH)3 or[Fe2O3.XH2O]

Limonite - Fe2O3.XH2O
2. Allophane and other amorphous minerals
Allophanes
• Non-crystalline; amorphous to x-ray
diffraction analysis; exhibit featureless xray diffraction pattern
• Abundant in soils derived from volcanic ash
deposits
• Have high phosphate fixation capacity
• Most soils containing allophane have black A
horizons, extremely high in OM
Imogolite
• First found in weathered volcanic ash or
pumice beds in Japan called imogo
• Chemical composition is assumed to be:
SiO2.Al2O3.2.5H2O
• Crystal structure is better defined than
allophane
• Electron microscopy shows the presence of
hair-like or spaghetti-like crystal forms
Soil Organic Matter (SOM)
 Sum total of all carbon-containing
substances in the soil
 consists of living organisms, dead
plant and animal residues, and
other organic materials in various
phases of decomposition
Humus
•
a dark, complex mixture of organic
substances modified from original
organic tissue
•
not biologically active and is the pool
responsible for many of the soil
chemical and physical properties
associated with SOM and soil
quality.
Fractionation of SOM:
Soil organic matter
Humic matter
Non-Humic matter
+ alkali
Insoluble
Humin
Soluble
+ acid
Not precipitated
Fulvic acid
Precipitated
Humic acid
+ alcohol
Soluble
Hymatomelanic acid
Insoluble
Gray Humic acid
Non-humic substances
include carbohydrates, lipids, amino acids,
proteins
Humic fraction include amorphous, colloidal,
polydispersed substances with yellow to
brown to black color and relatively high
molecular weights
Important characteristics of soil colloids
related to chemical reactivity:
1. Fineness and large specific surface area
To illustrate:
1 cm
1 face = 1 x 1 = 1 cm2
since there are 6 faces
total surface area = 6 cm2
If we split the cube:
0.5 cm
1 face = 0.5 x 0.5 = 0.25 cm2
area of 1 cube = 0.25 x 6 = 1.5 cm2
total surface area = 8 x 1.5 = 12 cm2
2. Presence of electric charges
Sources of negative charges
a. Exposed edges of clay mineral structure
b. Isomorphous substitution
1. Exposed crystal edge
a. Unsatisfied valences at the broken edges of the
silica
and alumina sheets
b. Exposed oxygen and hydroxyl groups attached to
silicon and aluminum atoms
Exposed crystal edges
(1)
at high pH
Al—OH + OHat low pH
Al—OH + H+
Al—O- + H2O
(2)
At high pH
Fe—OH + OHAt low pH
*Fe—OH + H+
Fe—O- + H2O
Al—OH2+
Fe—OH2+
b) Ionization of exposed –OH groups

At high pH, the hydrogen of these exposed
hydroxyls, dissociates and the surface of the
clay is left with the negative charge of the
oxygen ions.
Alkaline medium: -Al-OH + OH- = -Al-O- + H2O

this type of negative charge is called variable or
pH-dependent charge
Isomorphous substitution is primarily responsible in
the development of permanent charges in crystal
lattice.
1. pH dependent charges
negative (or positive) charges of clay particles
resulting from exposed oxygen (O-) or hydroxyl
(OH-) ions which act as negatively charged sites;
affected by changes in pH or by exchange
reaction
2. Permanent charges
not affected by changes in pH or by ion-exchange
reactions and form through isomorphous
substitution
Specific surface area of some clay
minerals and CEC
---------------------------------------------------------------------- - - - - - - - - - CLAY MINERALS SPECIFIC SURFACE
CEC
AREA (m2/g)
meq/100g
-----------------------------------------------Kaolinite
7-30
3-15
Montmorillonite
600-800
80-100
Vermiculite
600-800
100-150
Illite
65- 100
15-40
Allophane
100-800
20-150
_______________________________________________
REVIEW QUESTIONS
REVIEW QUESTIONS
7. Of the following minerals, the one that expands on wetting is
a. muscovite
c. montmorillionite
b. illite
d. kaolinite
8. Which is the seat of chemical activities in the soil?
a. Sand
c. colloid
b. Silt
d. clay
THANK YOU
&
GOOD LUCK!!!