Soil Colloidal Chemistry Compiled and Edited by Dr. Syed Ismail, Marthwada Agril. University Parbhani,MS, India 1
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 5
Types of Soil Colloids Crystalline silicate clays Non-crystalline silicate clays Iron and aluminum oxide Organic material (Humus) 6
Soil Colloids: Silicate Clays Kaolinite Montmorillonite 7
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) 8
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- + H 2 O (no charge) (- charge) C-OH + OH ==H- == -C- O- + H 2 0 (no charge) (- charge) 11
Structures (2D) Si Tetrahedral Sheet Al Octahedral Sheet Al-Si combined 12
Example 1:1 type clay Sheets Layer Sheets Interlayer Layer 13
Clay silicate crystals 1:1 type KAOLINITE 2:1 type SMECTITE VERMICULITE MONTMORILLONITE 14
Clay silicate crystal 2:1:1 type CHLORITE 15
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! 16
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 17
Clay silicate crystals 2:1 type Montmorillonite Expanding 4 O and 1 Si 6 OH and 1 Al 4 O and 1 Si O bonding (WEAK) Hydrated exchangeable cations Non Hydrated ions 18
Smectite (includes montmorillonite) 19
kaolinite illite montmorillonite humus (fulvic acid) 20
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 21
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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 23
Clay silicate crystals 2:1 type (Fine Mica) tetrahedral Non expanding Charges: 20% Al octaby Si Strong bonding forces Large Net Charge Attract ions K + and NH 4 + 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 NH 4 + 27
Clay silicate crystal 2:1:1 type Fe/Mg instead of Al octahedral CHLORITE Mg dominated sheet 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 30
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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 Fe 2 O 3 red color 32
Clay Minerals Comparison Kaolinite Illite Vermiculite Smectite Tetrahedral 0 20% Al 3+ 10% Al 3+ 2.5% Al 3+ Octahedral 0 0 15% Mg 2+ 15% Mg 2+ Tetrahedral --- 20% Al 3+ 10% Al 3+ 2.5% Al 3+ 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. 33
Thanks 34