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Soil Colloidal
Chemistry
1
Compiled and Edited by
Dr. Syed Ismail,
Marthwada Agril. University
Parbhani,MS, India
The Colloidal Fraction
Introduction
What is a colloid?
Why this is important in understanding
soils?
How can we understand this fraction?
What are the fundamental basics of this
fraction?
2
Soil Colloids
The most chemically active fraction of soils.
They are very small, less than 2 µm in
diameter
Shape
Colloids can be either
– mineral (clays) or organic (humus)
– crystalline (definite structure) or amorphous.
3
Colloids
Properties imparted to soils
– Static Vs. dynamic properties
– Sand and Silt (no colloids)
Static and occupy space
– Clay and Humus (colloids)
Dynamic and very active (charges)
4
Colloids
Properties imparted to soils by colloids
– Chemical Vs Physical properties
Chemical
– Sources of ions for plant nutrition
– Source of electro-negativity (CEC)
– Buffering capacity
– Chemical cement agents
Physical
– Large surface area per unit of mass (cm2/g)
– Plasticity
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Types of Soil Colloids
Crystalline silicate clays
Non-crystalline silicate clays
Iron and aluminum oxide
Organic material (Humus)
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Soil Colloids: Silicate Clays
Kaolinite Montmorillonite
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Crystalline Silicate Clays
What is it?
Shape: x, y and z.
Surface area: 2 sources ***
– External Vs Internal
Composition of crystalline structures:
– Silicon-Oxygen and Aluminum- Hydroxide … others
– Silicon Tetrahedral and Aluminum Octahedral sheets
– Si-O Tetrahedral sheet (tetra=four void spaces)
– Al-OH Octahedral sheet (0cta=eight void spaces)
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Charges
Isomorphous Substitutions
Process in which one element substitutes
another of comparable size in the crystalline
structure
Al is slightly larger than Si, consequently Si
may replace Al!
IF Al+3 and Si+2 then what?
9
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Charges
Permanents
– Isomorphous substitutions
pH dependant (non-permanents)
– Broken Edges
– Al-OH + OH ==H- == Al- O- + H2O
(no charge) (- charge)
– C-OH + OH ==H- == -C- O- + H20
(no charge) (- charge)
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Structures (2D)
Si Tetrahedral Sheet
Al Octahedral Sheet
Al-Si combined
12
Example 1:1 type clay
Sheets Layer
Interlayer
Sheets Layer
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Clay silicate crystals
1:1 type KAOLINITE
2:1 type SMECTITE
VERMICULITE
MONTMORILLONITE
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Clay silicate crystal
2:1:1 type CHLORITE
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Clay silicate crystals
1:1 type KAOLINITE
4 O and 1 Si
6 OH and 1 Al
Hydrogen ion
STRONG BOND!
NO WATER and
NO OTHER ION!
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1:1 type clays
Stable and non expanding clay
Low total charges
Relative low specific surface area
pH dependant charges
Good physical properties
Limiting holding capacity for nutrients
Good for roads, buildings, ceramic and bricks.
Hexagonal shape
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Clay silicate crystals
2:1 type Expanding
4 O and 1 Si
Montmorillonite 6 OH and 1 Al
4 O and 1 Si
O bonding (WEAK)
Hydrated
exchangeable
cations
Non Hydrated ions
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Smectite (includes montmorillonite)
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kaolinite illite
mont-
morillonite
humus
(fulvic acid)
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Sheets and Layers
Al sheet
Al sheet
Al sheet
Al sheet
Si sheet
kaolinite
Si sheet
Si sheet
Si sheet
illite smectite and
vermiculite
1:1 non-expanding 2:1 non-expanding 2:1 expanding
+ +
+
+
+ +
+
+
+
+
+ +
+ +
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2:1 Clay expanding type
Expanding clays
Shrinking and swelling constantly
Poor physical characteristics
Abundant charges and surface
Rich in nutrients
Good soils for crops if managed properly
Not affected much by pH
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Clay silicate crystals
2:1 type (Fine Mica) Non expanding
Charges: 20% Al octa-
by Si
tetrahedral
Strong bonding forces
Large Net Charge
Attract ions K+ and NH4+
Fits perfectly in hexahedral
holes 24
Hexahedral holes
25
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2:1 Clay non expanding type
Limited expanding
Good physical properties
Medium total charges
Lower specific surface area than expanding 2:1 clays
Good soils for crops
Challenging management for K+ and NH4+
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Clay silicate crystal
2:1:1 type
Fe/Mg instead
of Al octahedral
Mg dominated sheet
CHLORITE Hydrogen
STRONG
BOND 28
2:1:1 Clay type
Non Expanding clays
Very limited shrinking and swelling
Good physical characteristics
Limited charges and surface
Good soils for crops if managed properly
Not affected much by pH
29
Distance between UNITS of crystalline
structures
1.41 nm Vermiculite
1.00 nm Micas
0.71 Kaolinite
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Non silicate Clays
Alone or mixed with silicate clays
Organic colloids – Humus
– Large molecules (+ and – charges)
Iron and Aluminum oxides
– Modified octahedral sheets with substitutions
– No tetrahedral sheets
Gibbsite (Al(OH)3) Oxisol and Ultisol
Goethite (FeOOH) yellow brown color
Hematite Fe2O3 red color
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Clay Minerals Comparison
Kaolinite Illite Vermiculite Smectite
Tetrahedral 0 20% Al3+ 10% Al3+ 2.5% Al3+
Octahedral 0 0 15% Mg2+ 15% Mg2+
Tetrahedral --- 20% Al3+ 10% Al3+ 2.5% Al3+
CEC me/100g 3-15 (edges) 30 150 80 - 150
Shrink-swell Low None Mod to High High
Interlayer H-bonds Fixed K+ Exch. cations Exch. cations
Origin Recrystallization
under intense
acid weathering
Early
alteration of
micas
Intermediate
alteration of
micas
Recrystallization
under moderate
neutral to
alkaline weath.
Thanks
34