Summary to Date

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Summary to Date

• Field Scale
• 3-Phase System
• Soil Profile
• Soil Horizons
• Factors of Formation
• Taxonomy
• Nomenclature

• Lab (Meso) Scale
• Mass Volume
• Soil Separates
• Specific Surface
• Soil Texture
• Soil Structure
• Measurement

Micro Scale
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PHYSICAL CHEMICAL PROPERTIES Inorganic and
Organic Colloidal Fractions
Crystalline Inorganic Mineral Colloids The
principal structural units are silica
tetrahedrons and the aluminum octahedrons which
combine to form sheets similar to a sheet of
paper. These sheets of silica tetrahedra and
alumina octahedra then condense to form linked
layers. The number of linked layers constitute
the specific mineralogy of the various layer
silicates.
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Layer Silicates
11 Kaolinite Halloysite 21 Micas Vermiculi
te Smectite 22 (211) Brucite Chlorite
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Si Al Si Si Al Si
Si Al Si Al
Unit Cell 7 Ao
Unit Cell 10-14 Ao
Si Al Si O Si Al Si
Unit Cell 14 Ao
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Non-Crystalline Amorphous Minerals
Oxides, Hydroxides, Hydroxyoxides of Fe and
Aluminum Aluminum Oxides Gibbsite, Boehmite Iron
Oxides Hematite, Goethite Allophane (linked but
not layered Si-Tetrahedra Imogolite Zeolites
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Organic Non-Mineral Colloids
Amorphous high molecular weight organic polymers
humic acid, fulvic acid Typical functional groups
OH Hydroxyls COOH Carboxyls
C-C-N-C-COOH Amine Linkage
Benzoic Acid C7 H6 O2
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Colloid Properties Characteristics Charge
Development
Mineral Fraction pH Independent (Permanent) pH
Dependent (Variable) Organic Fraction pH
Dependent (Variable)
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Mineral Fraction
Layer Silicates pH Independent Due to isomorphic
substitution The substitution of similar sized
trivalent cations (Al, Fe) for tetravalent
Si in the tetrahedral coordination and of
similar sized divalent cations (Ca, Mg, Fe)
in the octahedral coordination at the time of
crystal formation. Always a net negative
charge. pH Dependent Due to the ionization of
free radicals associated with broken bonds and
edges. May be a net negative or positive charge.
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Mineral Oxides pH Dependent Due to the
ionization or protonation of free radicals. May
be a net negative or positive charge. Organic
Fraction pH Dependent Due to the ionization of
functional groups or protonation of amine
linkages. May be a net negative or positive
charge.
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Ionization of free radicals or functional
groups Alkaline pH
O
O-
OH
HOH
alkaline pH
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Ionization of free radicals or functional
groups Acid pH
H
..
..
C-C-N-C-COOH H Amine Linkage
C-C-N-C-COOH
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Summary pH Independent Charge Always
Negative (-) pH Dependent Charge Negative at
high pH (alkaline conditions) Positive at low
pH (acidic conditions) Which source is the most
important in soils?
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Expansibility
Non-Expanding Kaolinite (Kandites) 11
Unit Cell 7 Ao
O O O O O O O O H H H H H
H H H O O O O O O O O
H - Bonding
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Non-Expanding Micas (Muscovite, Biotite) 21
Unit Cell 10 Ao
K K K K
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Non-Expanding Chlorite (Brucite) 22 or 211
Unit Cell 14 Ao
Brucite (Mg(OH)2) Interlayer
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Non-Expanding but Collapsible Illite (Less K)
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Unit Cell 14 Ao
H K HOH K
2K
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Non-Expanding but Collapsible Vermiculite (Much
Less K) 21
Unit Cell 14 Ao
HOH H HOH K
3K
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Freely Expanding Smectites (Monmorillonite)
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Unit Cell 20 Ao
Ca2 Na HOH K
Which clay is most suitable for making pottery
and why?
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Interactions Among Properties
Jury and Horton Table 1.1 p. 16 (specific
surface s, si, c) Table 1.2 p. 17 (specific
surface, CEC, and charge density for various
clay types) Hillel Table 4.1 p. 58 (typical
properties of selected clay
minerals) NOTE You do not need to memorize
specific numbers but you should be able to
discuss the various properties and interactions
in a relative context.