exer 5

• Soil colloids


  Organic or inorganic matter with particles less than 0.001 mm in size and a correspondingly large surface area per unit of mass

  
• Soil colloids


  • The most active constituent of the soil
  • Determine the physical and chemical properties of the soil

  
• Important properties of soil colloids


  • Large specific surface area (area per unit amount of colloid)
  • Extremely small particle size
  • Presence of electric charges, (positive and negative)

  
• Exchangeable ions


  Weakly held, in contact with soil solution, ready for quick replacement, available for plants
• Nonexchangeable ions


  Adsorbed by strong bonds or held in inaccessible places (e.g., the K+ between layers of illite)
• Classification of soil colloids


  • Inorganic (clay) colloids
  • Organic colloids

  
• Inorganic colloids


  • Made up of Si, Al, O
  • Inorganic and crystalline/ amorphous
  • Relatively stable
  • Have definite and well-known structure

  
• Organic colloids


  • Made up of C, H, O
  • Complex amorphous organic colloid
  • More dynamic
  • Complex structure
• Inorganic soil colloids


  • Crystalline silicates
  • Non-silicates or Fe/Al oxides
  • Amorphous silicates
  • Non-silicates or Fe/Al oxides

  
• Crystalline silicate clay minerals


  • Kaolinite
  • Montmorillonite
  • Vermiculite
  • Illite
  • Chlorite

  
• Isomorphous substitution


  Replacing one structural cation for another of similar size
• Kaolinite


  • 1:1 type - one Tetrahedral (Th) sheet linked to an Octahedral (Oh) sheet
  • Adjacent layers are tightly bound together by H-bond
  • Non-expanding mineral, unable to absorb water into the interlayer position
  • Failure of soils to swell or shrink much on wetting and drying
  • Basal spacing fixed at 0.72 nm (7 Å)
  • Lacks significant isomorphous substitution - small capacity to adsorb exchangeable cations

  
• Montmorillonite


  • High (-) charges from isomorphous substitution (Mg2+ replaces Al3+ in Oh layer; Al3+ replaces Si4+ in Th layer)
  • High CEC (80-100 meq/100g)
  • Specific surface (m2/g)= 700-800
  • Adjacent layers are loosely bound by weak O-O linkage & cation-O linkage
  • High water holding capacity (ultramicropores)
  • High shrink-swell potential
• Vermiculite


  • High CEC (100-150 meq/100g)
  • Limited expansion compared with smectite (interlayer constituents)
  • Variable d-spacing (14 Å expanded, 10 Å collapsed)
  • Larger crystal than smectite
  • 2:1 type of structure
• Hydrous Mica (Illite)


  • 2:1 type minerals containing interlayer K+ resulting to no expansion on wetting
  • Replacement of Si4+ with Al3+, each substitution leaves 1 (-) charge
  • K+ ions are a bridge between layers and hold them tightly together
  • The K+ content of hydrous mica is less than that of micas
• Chlorite


  • Non-expanding, contains two octahedral sheets, thus a 2:1:1 layer mineral
  • Chlorites differ from other 2:1 layer minerals in that they contain a stable, positively charged Oh sheet rather than adsorbed cations in the interlayer space
  • By virtue of its positive charge, the interlayer sheet neutralize the negative charge of the 2:1 sheets

  
• Mineral colloids other than silicates

  • Hydrous oxide clays of iron and aluminum
  • Allophane and other amorphous minerals
• Hydrous oxide clays of iron and aluminum


  • They are in the 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
• Amorphous clay minerals


  • High specific areas, such as allophane, imogolite, and other non-crystalline soil colloids
  • Have high variable charge, high water retention capacity (thixotropic), high water permeability and low bulk density (<0.9 Mg/m3)
  • Allophane - chemical combination of Aluminum and silicon (Al2O3.2SiO2.H2O)
  • CEC varies from 20-150 meq/100g - large amount of negative charges are pH dependent
  • Large surface area
  • High phosphate fixing capacity
  • High water holding capacity

  
• Summary of clay mineral properties

  • Kaolinite
  • Montmorillonite
  • Vermiculite
  • Hydrous Mica
  • Chlorite
  • Allophane

  
• Gentian violet


  Net positive charge
• Eosin red


  Net negative charge