Soil Colloids: Seat of Soil Chemical and

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Chapter 8.
Soil Colloids Seat of Soil Chemical and
Physical Activity
What ya lookin at?
Found me some soil colloids!
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Q What is a soil colloid?
A Extra extra small, very tiny, very
very tiny, particles of mineral (clay) and
organic matter (humus) dispersed in
the soil matrix!
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General colloid properties.

• Enormous reactive surface areas.
• Natures great electrostatic chemical reactor!
• Influence most all soil ecosystem functions.
• Site of cation absorption (loosely held) that are
  exchangeable.
• Also bind other things, like pesticides and
  viruses.

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Cation Exchange Capacity (CEC)
CEC the sum of exchangeable cations that
a soil can absorb.
CEC is expressed as the number of moles of
positive charge adsorbed per unit mass.
Units
Cmol () kg-1 or
Meq per 100 g soil
(Its the same number!)
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Charge characteristics of selected colloids.
colloid type Ave. CEC pH dependent
organic amorphous 200 90
smectite 21 100 5
vermicullite 21 150 5
Fine micas 21 30 20
chlorite 211 30 20
kaolinite 11 8 95
Gibbsite (Al) Non-silicate 4 100
Geothite (Fe) Non-silicate 4 100
Allophane Si Non-cystalline 30 90
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Yep, theres colloids in this soil, pretty
important property of soils. Any questions?