Chemistry of Water in Soil OxidationReduction Redox Potential

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Plant Growth
Engineering
Soils in the Environment Wednesday, Nov. 5
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Soil Attributes and Processes

• Gases and Water in soil
• How Acid is Your Soil?
• An Exploration in the World of the Smallest
  Particles in Soil!
• Why are Certain Soils More
• Fertile than Others?

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Important properties of clay in soils

• Extremely small
• lt 2 µm
• 2. Extremely large surface areas
• gt 800 m2 /g
• 3. Negative charge (in our part of the world)
• 4. Adsorption of water cations

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Tetrahedron and OctohedronSharing the Oxygens
Oxygen
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General Properties of 11 type minerals

• 1 tetrahedral (Si) sheet 1 octahedral (Al) sheet

• Held by sharing O within a layer

• Held by hydrogen between layers makes for a
  very strong bond.

• Fixed structure (does not change size)
• e.g. Kaolinite

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11 Mineral
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Kaolinite in Nature
FYI ONLY
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Uses of Kaolinite

• Numerous fillers and coatings in/on papers, for
  making china and various other earthenware items,

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General Properties of 21 type minerals

• 2 tetrahedral (Si) sheet
• 1 octahedral (Al) sheet

• Held by sharing Oxygen within a layer

• Between layers held by
• Water Smectite
• Mg Vermiculite
• K Illite

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21 Mineral
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Uses for Smectite

• Oil well drilling, wine, animal litter trays and
  oil and grease absorbents, as carriers for
  insecticides, and for decolorizing of oils and
  fats.

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Why does this house have cracks?
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Expanding Minerals
H2O

• Water molecules are in the interlayer positions
  that swell the structure of the mineral. When
  the water is removed the structure shrinks.

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Water
No water
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Construction

• When constructing a home, the builder must know
  if the soil contains clays that will shrink and
  swell.
• The mineralogy may be determined through X-Ray
  Diffraction

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X-Ray Diffraction
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If expandable Clays are present

• The builder must bring in sand fill and raise the
  foundation
• Excavate to a point where the expandable clays
  are not present
• Put in deep pilings
• Move to a more hospitable location

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Buyer Beware!

• If the house is not build properly, cracks may
  develop.
• It may cost thousands of dollars to repair.

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Soil Attributes and Processes

• Gases and Water in soil
• How Acid is Your Soil?
• An Exploration in the World of the Smallest
  Particles in Soil!
• Why are Certain Soils More Fertile than Others?

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Why are certain soils more naturally fertile
than other soils?Its in their chemistry!
Prairie soils (Mollisols) vs Tropical soils
(Oxisols)
(Midwest)
(Hawaii)
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Charges of Clays

• Constant (permanent) charges
• Isomorphous substitution
• 21 minerals
• pH-dependent (variable) charges
• Associated with OH or COOH groups
• 11 minerals-kaolinite
• Fe Al oxides
• humus (organic matter)

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Isomorphous Substitution
Isomorphous Substitution Substitution of a
cation in mineral by another cation with similar
charge and size without changing the mineral
structure
Si Si Si Si Si Si
Al Al Al Al Al Al Al Al
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Octahedral substitution
Si Si Si Si Si Si
Si tetrahedra
Al octahedra
Al3 Mg2 Al3 Al3 Mg2 Al3
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Isomorphous Substitution
Si4 Al3 Si4 Si4 Al3
Si4
Tetrahedral substitution Octahedral
substitution
Al Al Al Al Al Al Al Al
Si Si Si Si Si Si
Al3 Mg2 Al3 Al3 Mg2 Al3
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How does Isomorphic Substitution work?

• If Mg2 substitutes for Al3 in the octahedron,
  the anion (negative charge) must be satisfied.
  Other cations satisfy the anion charge. This is
  the permanent charge, part of the CEC.
• You must keep in mind the sizes (Ionic Radii) of
  the cations being replaced and replacing
  (exchanged).

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Ionic Radii of Elements in Soil
Ca2 - 0.99 nm
K - 0.133 nm
H2O - .295 nm
Mg2 - 0.066 nm
Al3 - 0.051 nm
Si4 - 0.042 nm
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An Example

• If you have Al in the octahedron, then you
  have 6- that must be satisfied by 2 x Al 6.
  Dioctahedron.
•   All Aluminum protons are giving ½ to the bond.
  Strong chemical bond.
• Likewise, if Mg is present it is in the form 3
  x Mg 6. Trioctahedron.
•   All Magnesium protons are giving 1/3 to the
  bond. Weak chemical bond.

Al
Al
Mg
Mg
Mg
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Dioctahedral vs Trioctahedral

• Dioctahedral
• 2 Al3
• Very strong bond
• Fits well in the octahedral

• Trioctahedral
• 3 Mg2
• Relatively weak bond
• Does not fix as well in the octahedral

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What is Cation Exchange Capacity (CEC)?

• What is a soil cation?
• In soil the cations are H, Al3, Ca2, Mg,
  Na, and K.
• What do we mean by exchange?
• These cations can replace each other or
  exchange for each other. BUT the charges must
  be satisfied.
• What do we mean by capacity?
• Every soil has an ability to hold cations.
  Some soils hold more (high CEC). Some soils hold
  less (low CEC). The units are cmol ()/kg of
  soil.

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Factors Determining Cation Preference How do
we know which cation will replace which?

• Concentration
• Charge (1, 2,3)
• Size

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• How do the cations exchange one for the other?

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Cation Exchange

• Types of cations
• Higher for Lower charges

Solid
Solution
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Cation Exchange

• Concentrations of cations
• Higher for Lower concentrations
• Equal molar charge ()
• 2 Al3 or 6 K for 3 Ca2

Solid
Solution
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Measurement of Cation Exchange Capacity
NH4
Determine NH4 concentrations

• By measuring the NH4 concentration, we can
  determine how much cations a soil adsorbs

Solution
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Importance to Plants Scientists

• CEC can give you a relative idea of plant
  available nutrients.
• It is better to know the base saturation than
  CEC.
• Why?

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Why?

• Base saturation or base saturation tells you
  the relative amount of good cations for plants.
• Ca, Mg, K, Na

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Base Saturation

• Base saturation is determined by adding the four
  bases. This is called summing the bases.
• We can divide by the CEC to get the fraction of
  the CEC that is attributed to the bases.
• Most of the time we determine the BS.

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Base Saturation Measurement
NH4
Determine the individual bases displaced into
the solution. The amount of Ca, K, Na, and Mg is
the base saturation of the soil
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Base Saturation Cation Exchange Capacity

• Base Saturation is a calculation.
• You add up the amount of Ca, K, Mg, Na, Al, and H
  in the soil. This is called the Cation Exchange
  Capacity (CEC)
• You add up the amount of Ca, K, Mg, and Na. This
  is called the Sum of Bases.
• You take the sum of bases and divide by the CEC
  multiple by 100 and you get base saturation.

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Determining the bases

• Lab procedure to determine the amount of each
  individual base is very similar to CEC.
• NH4OAC pH 7
• Atomic Adsorption Unit (AA) used to determine the
  amount of the individual base