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Soil Chemical Properties

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Here is a schematic diagram of a negatively charged clay particle surrounded by cations. The soil liquid (soil solution) ... Titration Curve Weak Acid. pH. – PowerPoint PPT presentation

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Title: Soil Chemical Properties


1
Soil Chemical Properties
  • Section B
  • Soil Fertility and Plant Nutrition

2
Soil Texture
  • The proportions of sand, silt, and clay particles
    in soilsSand 2 to 0.05 mm effective
    diameterSilt 0.05 to 0.002 mmClay lt0.002 mm
  • The most reactive fraction is ___________.

clay
3
Soil Colloids
  • Soil particles lt0.001 mm in diameter
  • Are the most reactive of soil particles because
    of _________________ and _________________________
    .
  • Types of soil colloids

surface area
electrical charge
Inorganic clay minerals, oxide
minerals Organic soil organic matter
4
Organic Colloids
  • Mostly soil humus, the chemically resistant
    organic matter in soils, that results from
    organic matter decomposition.
  • Characteristics variably charged (usually -),
    high cation exchange capacity (CEC)

5
Humus
Carbon Hydrogen Oxygen Nitrogen
6
Building Blocks of layer silicates
  • Tetrahedral (Si4 bonded to four O-2)
  • Octahedral (Al3 bonded to six OH-)
  • The long chains or layers of tetrahedra and
    octahedra are bonded together to form layer
    silicates.

7
Mineral Colloids
  • Layer silicate clays
  • 11 clays (Kaolinite)
  • 21 clays (Micas, Illite, Vermiculite,
    Montmorillonite)
  • 211 clays (chlorite)

8
11 Clay Mineral
Layer
9
21 Clay Mineral
Interlayer
10
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11
Layer Silicate Clays
  • PropertiesSurface AreaChargeExpansion

12
Layer Silicate Clays
  • Have a charge because of
  • Isomorphous substitutionSubstitution of cations
    of equal or lesser charge within tetrahedrons or
    octahedrons. This can create a negative charge
    deficit on the clay particle.
  • pH dependent chargeH may attach to or detach
    from (depending on pH) O atoms located on the
    clay edges. Creates a negative or positive
    charge deficit.

13
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14
Layer Silicate Clays
  • Have a charge because of
  • Isomorphous substitutionSubstitution of cations
    of equal or lesser charge within tetrahedrons or
    octahedrons. This can create a negative charge
    deficit on the clay particle.
  • pH dependent chargeH may attach to or detach
    from (depending on pH) O atoms located on the
    clay edges. Creates a negative or positive
    charge deficit.

15
Hematite Fe2O3
H
H
H
16
H
On kaolinite, most pH- dependent charge occurs on
exposed octahedral Surfaces.
Kaolinite
17
- H
Increasing pH

charge
pzc
-
pH
18
Na
H2
H
-
H2
H2
H
H
-
H
H2
H
H
Na
H
-
-
Na
H2
H
-
Increasing pH

charge
pzc
-
?q
pH
19
Soil Colloids
  • Other soil minerals may occur as colloidal
    particles
  • Fe, Al oxides - can have a pH dependent charge
  • Poorly crystalline clays such as allophane - also
    have pH dependent charge

20
Sources of Charge on Common Soil Clays
  • 21 clays (smectites, vermiculite, etc.)
  • Most charge is due to isomorphous substitution
    (always negative)
  • Little pH-dependent charge
  • 11 clays (kaolinite)
  • Little isomorphous substitution
  • Most charge is due to pH-dependent charge
    (positive or negative)

21
Cation Exchange
  • Definition The exchange of cations adsorbed
    (attached) onto colloid surfaces with cations in
    solution.
  • Exchangeable cations are those attached to
    colloid surfaces.
  • Cations in solution and on colloid surfaces tend
    toward a state of _______________.
  • Exchangeable cations can be manipulated. e.g.

equilibrium
22
Cation Exchange Capacity
  • CEC is The mass of exchangeable cations that a
    given soil can retain per unit weight. Units are
    cmol()/kg soil or meq/100g.
  • Soils have CEC because of
  • Soils have many more exchangeable cations than
    cations in solution (buffering capacity)

23
Definitions
  • Atomic weight is weight in grams of 6 x 1023
    atoms of a substance. One mole of substance is 6
    x 1023 atoms, molecules etc. Thus, atom weight
    is grams/per mole.
  • Equivalent weight is the mass of substance that
    will react or displace 1 gram of H, which is 6 x
    1023 charges (- or ).
  • Thus equivalent weight is atomic weight divided
    by valence.

24
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25
CEC
  • Is the quantity of negative charges per kg of
    soil
  • Expressed in units of cmol()/kg (i.e meq/100g)
  • 1 mole of () is 6.023 x 1023 ()
  • 1 cmol of () is 0.01 mol ()
  • 1 mol of Na is 23 g and contains 1 mol ()
  • 1 cmol of Na is 0.23 g and contains 1 cmol ()
  • 1 mol of Ca2 is 40 g and contains 2 mol ()
  • 1 cmol of Ca2 is 0.40 g and contains 2 cmol ()

26
Cation Exchange
Cation
Clay particle
27
High CEC
Low CEC
2
2
28
Strength of Adsorption
  • Cations attraction to clays is a function of
    charge and size.
  • Strength of attractionAl3 gt Ca2 gt Mg2 gt K
    NH4 gt Na

29
Clays and CEC
  • Kaolinite 2-5 cmol()/kg
  • Illite (fine mica) 15-40 cmol()/kg
  • Vermiculite 100-180 cmol()/kg
  • Montmorillonite 80-120 cmol()/kg
  • Humus 100-550 cmol()/kg

30
Clays and CEC
  • What will be the CEC of a clay loam soil with 30
    kaolinite clay?
  • 5 cmol()/kg clay x 30 kg clay/100 kg soil ____
    cmol()/kg soil
  • What will be the CEC of a clay loam soil with 30
    montmorollonite clay?
  • 90 cmol()/kg clay x 30 kg clay/100 kg soil
    ____ cmol()/kg soil

1.5
27.0
31
Brady and Weil, Figure 8.11
32
Measuring CEC
  • CEC is commonly measured in laboratories
    by1. Saturating soil cation exchange sites with
    a cation (e.g. NH4)2. Extracting the soil with
    another cation to remove the NH43. Measure NH4
    extracted

33
Exchangeable Cations
  • The exchangeable cations have very important
    influences on soil properties
  • Ca2 is the dominant exchangeable cation in most
    soils.
  • Soils become acidic when they contain significant
    amounts of exchangeable _______ .
  • Soils have poor structure when they contain
    significant amounts of exchangeable _____ .

Al3
Na
34
Weathering and Soil Minerals
  • Soil mineralogy depends on
  • Parent material
  • Weathering
  • Soils that are not highly weathered will tend to
    contain smectite and illite (mica) colloids in
    the clay fraction.
  • Soils that are highly weathered will tend to
    contain kaolinite and oxide colloids in the clay
    fraction
  • How does this affect soil CEC?

35
Anion Exchange Capacity
  • Some soil colloids can have a positive charge,
    leading to an anion exchange capacity.
  • This is due only to a pH dependent charge, not
    isomorphous substitution.
  • Most important where oxide (Al, Fe) minerals are
    abundant in soils

36
Buffering Capacity
  • Definition
  • The soil solids control or buffer the
    composition of the soil solution.
  • Caused by dissolution of minerals,
    adsorption/desorption of exchangeable cations.

The resistance of the soil solution to a change
in composition.
37
Titration CurveWeak Acid

Alkaline
pH
Buffering
Acid
Base added
38
Buffering in Solutions
  • Acetic Acid in waterHC2H3O2 ? H C2H3O2-Keq
    10-5
  • Add a baseNaOH H ? Na H2O

39
Buffering
40
Buffering Capacity
Poorly buffered
10 gallon fuel tank
What about fertilization?
Highly buffered
  • Poorly buffered soils
  • Store limited amounts of available nutrients
  • Should be fertilized more often
  • Should be fertilized with lesser amounts

30 gallon fuel tank
41
Buffering Capacity
  • The amount of buffering capacity is
  • Proportional to minerals present (e.g. soils high
    in K-feldspars will be highly buffered with
    respect to K).
  • Proportional to amount of exchangeable cations
    (e.g. soils high in exchangeable Ca will be
    highly buffered with respect to Ca)
  • Typically, highly-weathered soils are less
    well-buffered with respect to nutrients than are
    lightly-weathered soils (more CEC, more primary
    minerals)

42
Buffering Capacity
Affects how frequently some soil amendments,
fertilizers need to be added, and how much.
Highly Buffered
Amt. Of exch. Or mineral nutrient

?y
Poorly Buffered

?y


?x2
?x1
Solution Concentration
43
Potassium Buffering Capacity
Exchangeable K mmol/kg
From Barber, 1984 p.37
K in soil solution mmol/L
44
Oxidation-Reduction (Redox)
  • Involves exchange of electrons between chemical
    species.
  • In soils, redox reactions often are catalyzed by
    ____________________.
  • Oxidation is _______________________.
  • Reduction is _______________________.
  • Oxidation and reduction always occur together.

microorganisms
a loss of electrons
a gain of electrons
45
Redox Reaction
  • 2FeO 2H2O ? 2FeOOH 2H 2e-(oxidation )
  • ½ O2 2H 2e- ? H2O
  • (reduction)
  • _________________________________
  • 2FeO 1/2O2 H2O ? 2FeOOH
  • (oxidation-reduction redox)

Represents Fe oxidation in an aerobic soil
environment
46
Redox Reactions (1)
  • Typical redox reaction in an aerobic soilCH2O
    ½ O2 ? CO2 H2O
  • Represents the decomposition of organic matter in
    soils. C in CH2O is oxidized in the reaction, O
    in O2 is reduced in the reaction. The O2 is
    called the electron acceptor.

47
Redox Reactions (2)
  • If a soil becomes anaerobic because of
    waterlogging, O2 is not present, so another
    electron acceptor is needed3 CH2O 2 NO3- ? 3
    CO2 N2 2 H2O 2H
  • Represents the decomposition of organic matter in
    an anaerobic soil. C in CH2O is oxidized in the
    reaction, N in NO3- is reduced in the reaction.
    The NO3- is called the electron acceptor.

48
Redox
  • Organisms gain energy by oxidizing compounds (e-
    donors). They have to dispose of the electrons
    using other compounds (e- acceptors).
  • Common e- donors in soils
  • Organic matter, NH4 , S, Fe2
  • Common e- acceptors in soils
  • O2, NO3-, Fe3, SO42-, Mn4

49
Oxidation State
  • The oxidation state is the difference between the
    charge of an atom in its current state and the
    charge of the neutral atom. Is equal to the
    number of electrons gained or lost.
  • In redox reactions, electron gain and loss must
    be balanced.

50
Redox
  • Redox reactions have very important effects on
    many nutrients in soilsOxidized Reduced
  • NO3- NH4, N2 Fe3 Fe2
  • Mn3 Mn2
  • SO42- H2S

51
Soil Redox Potential
  • Aerobic soils have sufficient supplies of O2,
    which is the primary e- acceptor. Inorganic N,
    Mn, Fe, and S tend to be present in their
    oxidized forms.
  • Anaerobic soils have little or no O2. An
    anaerobic condition may be caused by
    _________________. In this case, N, Mn, Fe, and
    S may be used as e- acceptors.
  • N and S availability to plants decrease when
    reduced, availability of Fe and Mn increase when
    reduced.

flooded soil
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