Soil colloidal chemistry

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Soil colloidal chemistry

  1. 1. Soil Colloidal Chemistry Compiled and Edited by Dr. Syed Ismail, Marthwada Agril. University Parbhani,MS, India 1
  2. 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? 2
  3. 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. 3
  4. 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) 4
  5. 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 5
  6. 6. Types of Soil Colloids Crystalline silicate clays Non-crystalline silicate clays Iron and aluminum oxide Organic material (Humus) 6
  7. 7. Soil Colloids: Silicate Clays Kaolinite Montmorillonite 7
  8. 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) 8
  9. 9. ChargesIsomorphous SubstitutionsProcess in which one element substitutes another of comparable size in the crystalline structureAl is slightly larger than Si, consequently Si may replace Al!IF Al+3 and Si+2 then what? 9
  10. 10. 10
  11. 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) 11
  12. 12. Structures (2D) Si Tetrahedral Sheet Al Octahedral SheetAl-Si combined 12
  13. 13. Example 1:1 type clay Sheets Layer Interlayer Sheets Layer 13
  14. 14. Clay silicate crystals 1:1 type KAOLINITE 2:1 type SMECTITE VERMICULITE MONTMORILLONITE 14
  15. 15. Clay silicate crystal 2:1:1 type CHLORITE 15
  16. 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! 16
  17. 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 17
  18. 18. Clay silicate crystals 2:1 type Expanding 4 O and 1 SiMontmorillonite 6 OH and 1 Al 4 O and 1 Si O bonding (WEAK) Hydrated exchangeable cations Non Hydrated ions 18
  19. 19. Smectite (includes montmorillonite) 19
  20. 20. kaolinite illite humusmont- (fulvic acid)morillonite 20
  21. 21. Sheets and Layers1:1 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
  22. 22. 22
  23. 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 23
  24. 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 24
  25. 25. Hexahedral holes 25
  26. 26. 26
  27. 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+ 27
  28. 28. Clay silicate crystal 2:1:1 type Fe/Mg instead of Al octahedral Mg dominated sheetCHLORITE Hydrogen STRONG BOND 28
  29. 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 29
  30. 30.  Distance between UNITS of crystalline structures 1.41 nm Vermiculite 1.00 nm Micas 0.71 Kaolinite 30
  31. 31. 31
  32. 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 32
  33. 33. Clay Minerals Comparison Kaolinite Illite Vermiculite SmectiteTetrahedral 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 - 150Shrink-swell Low None Mod to High HighInterlayer H-bonds Fixed K+ Exch. cations Exch. cations Early Intermediate Recrystallization Recrystallization under moderateOrigin under intense alteration of alteration of neutral to acid weathering micas micas alkaline weath. 33
  34. 34. Thanks 34

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