The document discusses soil colloids, which are the chemically active fraction of soils that are less than 2 μm in diameter. It describes the different types of colloids including mineral clays that can be crystalline or amorphous in structure, and organic colloids like humus. It explains the properties colloids impart to soils through their large surface area and electrostatic charges. The main clay minerals are described in detail, including their crystal structures, layer types, charge characteristics, and properties. Kaolinite is a 1:1 layered clay that is non-expanding, while smectites like montmorillonite are 2:1 layered expanding clays with more surface area and reactivity.

1. Soil Colloidal
Chemistry
Compiled and Edited by
Dr. Syed Ismail,
Marthwada Agril. University
Parbhani,MS, India
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2. 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?
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3. 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.
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4. 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)
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5. 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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6. Types of Soil Colloids
 Crystalline silicate clays
 Non-crystalline silicate clays
 Iron and aluminum oxide
 Organic material (Humus)
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7. Soil Colloids: Silicate Clays
Kaolinite Montmorillonite
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8. 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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9. 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?
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10. 10

11. 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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12. Structures (2D)
 Si Tetrahedral Sheet
 Al Octahedral Sheet
Al-Si combined
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13. Example 1:1 type clay
 Sheets Layer
Interlayer
 Sheets Layer
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14. Clay silicate crystals
 1:1 type KAOLINITE
 2:1 type SMECTITE
VERMICULITE
MONTMORILLONITE
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15. Clay silicate crystal
 2:1:1 type CHLORITE
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16. 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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17. 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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18. 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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19. Smectite (includes montmorillonite)
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20. kaolinite illite
humus
mont-
(fulvic acid)
morillonite
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21. Sheets and Layers
1:1 non-expanding 2:1 non-expanding 2:1 expanding
Al sheet
Si sheet + ++++ + +
Al sheet
Si sheet
Al sheet
Si sheet
Al sheet ++ + ++ + +
Si sheet
kaolinite illite smectite and
vermiculite
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22. 22

23. 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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24. 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
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25. Hexahedral holes
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26. 26

27. 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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28. Clay silicate crystal
 2:1:1 type
Fe/Mg instead
of Al octahedral
Mg dominated sheet
CHLORITE Hydrogen
STRONG
BOND 28

29. 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
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30.  Distance between UNITS of crystalline
structures
 1.41 nm Vermiculite
 1.00 nm Micas
 0.71 Kaolinite
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31. 31

32. 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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33. 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
Early Intermediate Recrystallization
Recrystallization
under moderate
Origin under intense alteration of alteration of neutral to
acid weathering micas micas alkaline weath.
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34. Thanks
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