LAST LECTURE
Soil acidity (pH)
Soil aeration
Role of water in soil aeration
Redox reactions
Role of soil temperature
THIS LECTURE
Soil COLLOIDS
Clay structure
Electropositive and electronegative charge in soil
SOIL COLLOIDS
CLAY MICELLES ARE VERY ATTRACTIVE
Clay crystalline
structure is often
plate-like.
• Particles < 1 - 2 μm behave as soil colloids
• Total surface area ranges from 10-800 m2·g-1 !!!
Has both internal
and external
surfaces.
• Internal and external surfaces have electronegative (-) or
electropositive (+) charges Æ (-) charge dominant
Negative charges
are shown as
dashes around the
soil particle.
• Each micelle adsorbs thousands of hydrated Al3+, Ca2+, H+, K+, Mg2+
and Na+ ions (enclosed within several H2O molecules)
A micelle is a submicroscopic aggregation of molecules in the clay colloidal aggregate
There is a
difference in
concentration of
cations with
distance from the
soil colloid.
• Cation exchange occurs when ions break away into the soil solution
and are replaced by other ions
• Ionic double layer: negatively charged micelle surrounded by a
swarm of cations
CRYSTALLINE SILICATE CLAYS
SOIL COLLOIDS
1. Crystalline Silicate clays
•
Dominant colloid in most soils (not andisols,
oxisols or organic soils)
3. Iron and Aluminium Oxides
•
Crystals layered as in a book (lattice structure)
4. Humus
•
2-4 sheets of tightly-bonded O, Si and Al atoms
in each layer
•
Eg. kaolinite, montmorillonite
2. Non-crystalline Silicate clays
1
EXTERNAL SURFACE AREA
Size range of clay particles
influences soil properties & fertility.
Surface area of clays is very large.
Surface area of clays dependent on
whether the clay mineral has
expanding lattice or not
Noncrystalline Silicate Clays
•
Not organized into crystalline sheets
•
Both + and – charges; can adsorb anions
such as phosphate
•
High water-holding capacity
•
Malleable when wet, but not sticky
•
Often form in volcanic soils (especially in Andisols)
•
e.g. allophane and imogolite
IRON AND ALUMINIUM OXIDES
•
Fe or Al replaced Si
•
Found in highly weathered soils of warm, humid regions
(e.g. oxisols)
•
Fe (goethite, hematite) and Al (gibbsite) atoms connected
to O atoms or OH- groups
•
Some form crystalline sheets (eg. gibbsite & goethite),
but often amorphous
•
Low plasticity and stickiness
HUMUS
Kaolinite
•
Present in nearly all soils, especially the A horizon
•
Not mineral or crystalline
•
Consist of very very complicated chains of C atoms,
bonded to H, O & N
•
Very high water adsorption capacity
•
Not plastic or sticky
•
Highly negatively charged
Mica
(kandite)
Montmorillonite
(smectite)
Humic
Acid
2
BASIC UNITS OF CLAY MINERALS:
Molecules and Sheets
BASIC UNITS OF CLAY MINERALS
Central small ‘cation’ surrounded by Oxygen (O) or hydroxyl groups (OH-)
Silica - tetrahedron
1 Si, 4 O
Aluminium - octahedron
1 Al, 6 O
Figure 2–11 Summary of
aluminosilicate clay
structures. (A) Building
blocks: Oxygen, OH, or
H2O—each 0.3 nm
diameter—coordinate
around smaller atoms of
Si and Al, forming the two
basic building blocks: the
Si–O tetrahedron and the
Al–O, OH octahedron.
These units are
represented in three ways:
as polyhedra, as stickand-ball drawings showing
positions of atom centers
and bonds, or as space-fill
(sphere-packing) drawings
indicating volumes filled
by oxygen electron shells.
(Parentheses—(Al), (Mg,
Fe)—indicate possible
isomorphous
substitutions.) (B) Sheet
structures: These are
formed by Si–O
tetrahedra, each sharing
three of their oxygens, or
by octahedra sharing all
six of their OH or O.
Sheets combine to form
layers.
BASIC UNITS OF CLAY MINERALS
Combined sheets
PHYLLOSILICATES: Sheets/Leaves
Kaolinite
Illite
Montmorillonite
Tetrahedron:
• Two planes of O, with
Si in between
• Basic building block is
silicon atom,
connected to 4 O
atoms
Most clay minerals are based
on combined sheets
1:1
Octahedron:
2:1
2:1
Either in 1:1 or 2:1
configuration
• Two planes of O, with
Al or Mg inbetween
• Basic building block is
Al (or Mg), connected
to six hydroxyl groups
or O atoms
There are many
layers in each micelle
IONIC RADII AND CLAY SHEETS
CLAY SHEETS AND REMAINING CHARGE
Trioctahedral
Sheet
Dioctahedral
Sheet
3 Mg2+ atoms
Charge = 0
2 Al3+ atoms
Charge = 0
Isomorphous
substitution
1 Al3+ atom,
1 Mg2+ atom
Charge = -1
3
ISOMORPHOUS SUBSTITUTION
1:1 SILICATE CLAYS
Each layer contains 1 tetrahedral and 1 octahedral sheet
Eg. Kaolinite, halloysite, nacrite and dickite
• Each Mg2+ ion that
substitutes for Al3+ causes a
negative charge in a
dioctahedral sheet
•Sheets are held together because the apical oxygen
in each tetrahedron also forms the bottom corner of
one or more octahedrons in the adjoining sheet
• Each Al3+ ion that substitutes
for Si4+ causes a negative
charge in a tetrahedral sheet
1:1 SILICATE CLAYS
2:1 SILICATE CLAYS
Each layer contains one octahedral sheet sandwiched
between 2 tetrahedral sheets
Hydroxyl plane is exposed: removal/addition of hydrogen ions
can produce + or - charges (hydroxylated surface also binds with
anions)
Hydroxyls of octahedral sheet alongside Oxygens of the
tetrahedral sheet:
strong hydrogen bonding results Æ no swelling in kaolinites!
Kaolinite useful for roadbeds, building foundations and ceramics
(hardens irreversibly)
SMECTITE GROUP
(EXPANDING/SWELLING 2:1 SILICATE CLAYS)
Interlayer expansion occurs as H2O fills spaces between layers in
dry clay
Figure 2–11 Continued.
(C) COLLOIDS
Layer structures: The two basic types, 1:1 and 2:1, are
SOIL
shown. Each is represented (left to right) as polyhedral, stick-and-ball, and space-fill
drawings, each depicting a side view of two unit layers and the interlayer space
between them.
•Montmorillonite is a very common smectite
•Smectites have a large amount of negative charge
due to isomorphous substitution
•Mg2+ often replaces Al3+ in the
octahedral sheet
•Al3+ sometimes replaces Si4+ in the
tetrahedral sheet
•Weak O:O or O:cation
linkages between layers
leads to plasticity, stickiness,
swelling and a very high
specific surface area
©2002 Prentice Hall, Inc. Pearson Education
Upper Saddle River, New Jersey 07458
Soils: An Introduction, 5th Edition
by Michael J. Singer and Donald N. Munns
4
VERMICULITE GROUP
(2:1 EXPANDING SILICATE CLAY)
• Very high negative charge, due to frequent substitution
of of Si4+ ions with Al3+ in the tetrahedral sheets
MICA GROUP
NON-EXPANDING 2:1 SILICATE MINERALS
Æ fine-grained micas, illite and glauconite
• Al3+ substituded for 20% of Si4+ in tetrahedral sheets
• K+ fits tightly into hexagonal holes between tetrahedral
oxygen groups: virtually eliminates swelling
• Cation exchange capacity is higher in vermiculites
than in any other clay
• Swelling occurs, but less than in smectites due to strongly
adsorbed H2O molecules, Al-hydroxy ions and cations,
which act more as bridges than wedges.
CHLORITES
NON-EXPANDING 2:1 SILICATE MINERALS
FORMULAS FOR CLAY MINERALS
Mg-dominated OH sheet fits between adjacent 2:1 layers (2:1:1).
H = bonded to O atoms between sheets
Fe or Mg occupy most octahedral sites
NONSILICATE COLLOIDS
IRON & ALUMINIUM OXIDES
IRON & ALUMINIUM OXIDES
• Modified octahedral sheets with either Fe2+ or Al3+ in the
cation positions
• No tetrahedral sheets and no Silicon
• Lack of isomorphous substitution (little negative charge)
• Small charge (+ or -) due to removal or addition of hydrogen
ions from surface hydroxyl groups
• Non-expansive and relatively little stickiness, plasticity and
cation absorption
5
NONSILICATE COLLOIDS
HUMUS
¾
¾
¾
¾
A non-crystalline, organic substance
Very large, VERY COMPLICATED organic molecules
~ 50% C, 40% O, 5% H, 3% N and sometimes S
Structure highly variable
• Very large NEGATIVE charge due to 3 types of OH
groups (H+ ions gained or lost)
(i) carboxyl group COOH
(ii) phenolic hydroxyl group
Æ partial decomposition of lignin by micro-organisms
(iii) alcoholic hydroxyl group
HUMUS HYDROXYL GROUPS
CHARGES OF COLLOIDS
Constant/permanent charges
ÆThrough isomorphous substitution
ÆMostly in 2:1 silicate clays
Positive
Negative
pH-dependent/Variable charges
ÆIn 1:1 silicate clays (kaolinite), humus, allophane and Fe/Al oxides
ÆCharge related to OH- group
Negative and positive charges vital to the behaviour and fertility of soils
VARIABLE CHARGE (pH-DEPENDENT)
• Hydrous oxides (crystalline or amorphous) get their charge
from surface protonation and deprotonation
• >AlO- + H+
Negative
AlOH + H+
AlOH2+
Neutral
pH decreasing Æ
Positive
• Layer aluminosilicates have a small amount of variable
charge because of OH at the edges
• All the negative charge on humus is variable
• Hydrous oxides are positively charged in some very acid
soils and help retain anions
pH higher Æ more OH- (e.g. removal of AL(OH)2 means high CEC
Negative charge:
•Dissociation of H+ ions,
lack of Al & Si at edge
to associate with O atom
Less Negative to Positive Charge:
•As pH increases, more H+ ions bond to
O atoms at the clay surface
•Protonation at very low pH (H+ ions attach
to surface OH groups)
6
Box 2-3 Fixed and Variable
Charge
SOIL COLLOIDS
See also Fig 8.19 of book
CLAY MINERALS DITRIBUTION
Clay generis/weathering
Alteration and recrystallisation
Clay minerals have a
geographic distribution
(e.g. climate, and parent
material related)
See figure 8.18 in book
Clay minerals also have a
distribution within a soil
profile
More weathered in A and
B horizons
©2002 Prentice Hall, Inc. Pearson Education
Upper Saddle River, New Jersey 07458
Soils: An Introduction, 5th Edition
by Michael J. Singer and Donald N. Munns
READING FOR THURSDAY
CHAPTER 8:
SOIL COLLOIDS: CEC
pp. 331-357
7