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Soil Chemistry

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Basic chemistry. Atomic weight (g mol-1) Mol, Molar, Molarity, Normality, equivalent, mequiv ... Soil Chemistry. What you must know for this chapter. Soil Colloids ... – PowerPoint PPT presentation

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Title: Soil Chemistry


1
Soil Chemistry
  • Dr. Jorge Hernandez
  • 145H Agriculture
  • E-mail suelos_at_siu.edu
  • suelos (soo-eh-los) soils (Spanish)
  • Phone 453 1794
  • Office hours M-W-F 200 300 PM
  • OPEN DOOR POLICY

2
Soil Chemistry
  • What you must know?
  • Basic chemistry
  • Atomic weight (g mol-1)
  • Mol, Molar, Molarity, Normality, equivalent,
    mequiv
  • Functional groups (OH-)
  • Ions and bonding properties
  • pH

3
The Colloidal Fraction
  • Introduction
  • What is a colloid?
  • Why this is important in understanding soils?
  • How can we understand this fraction?
  • What are the fundamental basics of this fraction?

4
Review last class
  • Soil water balance
  • Soil-water problem
  • Soil Chemistry
  • What you must know for this chapter

5
Soil Colloids
  • The most chemically active fraction of soils.
  • They are very small, less than 2 µm in diameter
  • Shape
  • Colloids can be either
  • mineral (clays) or organic (humus)
  • crystalline (definite structure) or amorphous.

6
Colloids
  • Properties imparted to soils
  • Static Vs. dynamic properties
  • Sand and Silt (no colloids)
  • Static and occupy space
  • Clay and Humus (colloids)
  • Dynamic and very active (charges)

7
Colloids
  • Properties imparted to soils by colloids
  • Chemical Vs Physical properties
  • Chemical
  • Sources of ions for plant nutrition
  • Source of electro-negativity (CEC)
  • Buffering capacity
  • Chemical cement agents
  • Physical
  • Large surface area per unit of mass (cm2/g)
  • Plasticity

8
Types of Soil Colloids
  • Crystalline silicate clays
  • Non-crystalline silicate clays
  • Iron and aluminum oxide
  • Organic material (Humus)

9
Soil Colloids Silicate Clays
  • Kaolinite Montmorillonite

10
Crystalline Silicate Clays
  • What is it?
  • Shape x, y and z.
  • Surface area 2 sources
  • External Vs Internal
  • Composition of crystalline structures
  • Silicon-Oxygen and Aluminum- Hydroxide others
  • Silicon Tetrahedral and Aluminum Octahedral
    sheets Do you picture it?
  • Si-O Tetrahedral sheet (tetrafour void spaces)
  • Al-OH Octahedral sheet (0ctaeight void spaces)
  • 4 models to explain crystalline clay morphology

11
Models
  • Space filling
  • Ball and Stick
  • Polyhedral
  • 2D
  • Lets take a peak!
  • M.J. Eick and R.B. Burgholzer from Department of
    Crop and Soil Environmental Sciences, Virginia
    Polytechnic Institute and State University

12
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13
Charges
  • Isomorphous Substitutions
  • Process in which one element substitutes another
    of comparable size in the crystalline structure
  • Al is slightly larger than Si, consequently Si
    may replace Al!
  • IF Al3 and Si2 then what?

14
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15
Charges
  • Permanents
  • Isomorphous substitutions
  • pH dependant (non-permanents)
  • Broken Edges
  • Al-OH OH H- Al- O- H2O
  • (no charge) (- charge)
  • C-OH OH H- -C- O- H20
  • (no charge) (- charge)


16
Structures (2D)
  • Si Tetrahedral Sheet
  • Al Octahedral Sheet
  • Al-Si combined

17
Structures (2D)
  • Si Tetrahedral Sheet
  • Al Octahedral Sheet
  • Al-Si combined

18
Example 11 type clay
  • Sheets
    Layer

  • Interlayer
  • Sheets
    Layer

19
Clay silicate crystals
  • 11 type
    KAOLINITE
  • 21 type
    SMECTITE
    VERMICULITE

  • MONTMORILLONITE



20
Clay silicate crystal
  • 211 type
    CHLORITE

21
Clay silicate crystals
  • 11 type
    KAOLINITE
  • 4
    O and 1 Si
  • 6
    OH and 1 Al

Hydrogen ion STRONG BOND! NO WATER and NO OTHER I
ON!
22
11 type clays
  • Stable and non expanding clay
  • Low total charges
  • Relative low specific surface area
  • pH dependant charges
  • Good physical properties
  • Limiting holding capacity for nutrients
  • Good for roads, buildings, ceramic and bricks.
  • Hexagonal shape

23
Clay silicate crystals
  • 21 type Expanding
  • 4
    O and 1 Si
  • Montmorillonite 6 OH
    and 1 Al
  • 4 O and 1 Si
  • O
    bonding (WEAK)

Hydrated exchangeable cations
Non Hydrated ions

24
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25
21 Clay expanding type
  • Expanding clays
  • Shrinking and swelling constantly
  • Poor physical characteristics
  • Abundant charges and surface
  • Rich in nutrients
  • Good soils for crops if managed properly
  • Not affected much by pH

26
Clay silicate crystals
  • 21 type (Fine Mica) Non expanding

  • Charges 20 Al octa- by Si
    tetrahedral
  • Strong
    bonding forces

Large Net Charge Attract ions K and NH4 Fits p
erfectly in hexahedral holes
27
Hexahedral holes
28
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29
21 Clay non expanding type
  • Limited expanding
  • Good physical properties
  • Medium total charges
  • Lower specific surface area than expanding 21
    clays
  • Good soils for crops
  • Challenging management for K and NH4
  • Why is that?

30
Clay silicate crystal
  • 211 type

  • Fe/Mg instead

  • of Al octahedral

  • Mg dominated sheet
  • CHLORITE

Hydrogen STRONG BOND
31
211 Clay type
  • Non Expanding clays
  • Very limited shrinking and swelling
  • Good physical characteristics
  • Limited charges and surface
  • Good soils for crops if managed properly
  • Not affected much by pH

32
Discovering a new world!
  • Distance between UNITS of crystalline structures
  • 1.41 nm Vermiculite
  • 1.00 nm Micas
  • 0.71 Kaolinite
  • How do we measure it? How do we know it?

33
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34
Non silicate Clays
  • Alone or mixed with silicate clays
  • Organic colloids Humus
  • Large molecules ( and charges)
  • Iron and Aluminum oxides
  • Modified octahedral sheets with substitutions
  • No tetrahedral sheets
  • Gibbsite (Al(OH)3) Oxisol and Ultisol
  • Goethite (FeOOH) yellow brown color
  • Hematite Fe2O3 red color

35
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36
Sources of charges on Colloids
  • Permanent or constant charges
  • Isomorphus substitutions
  • Montmorillonite, Illite, zeolite, and etc.
  • Variable or pH dependant charges
  • Broken edges
  • OH groups
  • Kaolinite, humus, Al/Fe oxides
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