Soil Chemistry

1
Soil Chemistry

• Dr. Jorge Hernandez
• 145H Agriculture
• E-mail suelos_at_siu.edu
• suelos (soo-eh-los) soils (Spanish)
• Phone [phone]
• 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
(No Transcript)
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
(No Transcript)
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
ION!
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
(No Transcript)
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
perfectly in hexahedral holes
27
Hexahedral holes
28
(No Transcript)
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
(No Transcript)
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
(No Transcript)
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