Soil Colloids Flashcards
Most chemically active portion of the soil
Soil Colloids
Soil particles [____] or [_____] in diameter
<1μ, 0.001 mm
Types of Soil Colloids
- Inorganic
(______)
(______)
(______) - Organic
(______)
Silicate clays
Oxides of Fe and Al
Allophanes
Humus
A silicate clay crystal
Micelle
Types of Silicate clays
1:1
2:1 non expanding type
2:1 expanding type
2:1:1 or 2:2
One tetrahedral sheet attached to one side of an octahedral sheet
- Represented by the mineral kaolinite
- Other 1:1 include halloysite, nacrite, dickite
1:1 Silicate clay
If a trivalent is the central cation, two-thirds of the available cation sites are filled
Dioctahedral
- Symmetrical arrangement of two tetrahedral sheets and a central octahedral sheet
- Represented by montmorillonite or smectite group of clay; also by vermiculite
- Isomorphous substitution is common (Al3+ to Si4+, Mg2+ to Al3+)
- Expand or contract as water molecules are added or removed
2:1 Expanding Silicate Clay
- Same composition/structure as the
other 2:1 type but doesn’t expand
due to the presence of K+ ions
“bridging” the silicate clays - Represented by muscovite (mica)
2:1 Non expanding silicate clay
- Structure: two silica tetrahedral sheet and two Mg-dominated octahedral sheet
- Represented by chlorites
2:1:1 or 2:2 Silicate clay
Commonly dominant in the highly weathered soils of tropics and subtropics
Hydrous oxide of Fe and Al
A colloidal matter that is either amorphous or the crystalline structure is not sufficiently ordered to be detected by x-rays
Allophanes and other amorphous minerals
Composed of C, H and O, that is non crystalline
Humus
Prevalent in soils developed from volcanic ash (Andisols)
Allophanes and other amorphous minerals
Sources of negative charges
Dissociation of exposed OH groups at the broken edges of the mineral
Dissociation of functional groups in organic colloids
The interchange between a cation in solution and another cation on the surface of any surface-active material such as clay or organic matter
Cation Exchange
- The sum of the exchangeable cations that a soil can adsorb
- Expressed in terms of moles of positive charge adsorbed per unit mass (cmol+ kg-1 soil)
- Used to be milliequivalent per 100g soil (m.e. 100g-1 soil)
Cation exchange capacity
The quantity of a substance that exactly reacts with, or is equal to the combining value of, an arbitrarily fixed quantity of
another substance in a particular reaction
Equivalent weight
Proportion of CEC that is made up of exchangeable bases
Base saturation
Characteristics of cation exchange
Reversible
Loss=gain
Instantaneous or rapid
