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Soil Colloids
•Particles less than 1 or 2 m behave as soil colloids
•Total surface area ranges from 10-800 m2·g-1 !!!
•Internal and external surfaces have electronegative or
electropositive charges (electronegative charge dominant)
•Each micelle adsorbs thousands of hydrated Al3+, Ca2+, H+,
K+, Mg2+ and Na+ ions (enclosed within several H2O molecules)
•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
I.
Crystalline Silicate Clays
•
Dominant colloid in most soils (not andisols,
oxisols or organic soils)
•
Crystals layered as in a book
•
2-4 sheets of tightly-bonded O, Si and Al atoms
in each layer
•
Eg. kaolinite, montmorillonite
II. 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)
•
Eg. allophane and imogolite
III. Iron and aluminium oxides
•
Found in highly weathered soils
of warm, humid regions (eg. oxisols)
•
Consist of Fe and Al atoms connected to
oxygen atoms or hydroxyl groups
•
Some form crystalline sheets (eg. gibbsite and
geothite), but often amorphous
•
Low plasticity and stickiness
IV. Humus
•
Present in nearly all soils, especially A horizon
•
Not mineral or crystalline
•
Consist of chains of C atoms, bonded to H, O & N
•
Very high water adsorption capacity
•
Not plastic or sticky
•
Negatively charged
Kaolinite
Mica
(kandite)
Montmorillonite
(smectite)
Humic
Acid
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.
©2002 Prentice Hall, Inc. Pearson Education
Upper Saddle River, New Jersey 07458
Soils: An Introduction, 5th Edition
andDonald
Munns,
2002)
by Michael J.(Singer
Singer and
N. Munns
Phyllosilicates
Tetrahedron:
•Two planes of O,
with Si in between
•Basic building block
is silicon atom,
connected to 4 O
atoms
Octahedron:
•Two planes of O,
with Al or Mg in
between
•Basic building block
is Al (or Mg),
connected to six
hydroxyl groups or
O atoms
There are many
layers in each micelle
Trioctahedral
Sheet
Dioctahedral
Sheet
Isomorphous
substitution
3 Mg2+ atoms
Charge = 0
2 Al3+ atoms
Charge = 0
1 Al3+ atom,
1 Mg2+ atom
Charge = -1
Isomorphous
substitution
•Each Mg2+ ion that
substitutes for Al3+
causes a negative charge
in a dioctahedral sheet
•Each Al3+ ion that
substitutes for Si4+
causes a
negative charge in a
tetrahedral sheet
1:1 Silicate Clay
Each layer contains one tetrahedral and one
octahedral sheet
Eg. Kaolinite, halloysite, nacrite and dickite
•Sheets are held together because the apical oxygen
in each tetrahedron also forms the bottom corner of
one or more octahedra in the adjoining sheet
•Hydroxyl plane is exposed: removal or addition of
hydrogen ions can produce positive or negative
charges (hydroxylated surface also binds with anions)
•Hydroxyls of octahedral sheet are alongside
Oxygens of the tetrahedral sheet: hydrogen bonding
results, with no swelling in kaolinites!
•Kaolinite useful for roadbeds, building foundations
and ceramics (hardens irreversibly)
2:1 Silicate Clay
Each layer contains one octahedral sheet sandwiched
between two tetrahedral sheets
O on both ends
No attraction without cations
Expanding 2:1 Silicate Clays
Smectite group: Interlayer expansion may occur as
H2O fills spaces between layers in dry clay
•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
Figure 2–11 Continued. (C) Layer structures: The two basic types, 1:1 and 2:1, are
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.
©2002 Prentice Hall, Inc. Pearson Education
Upper Saddle River, New Jersey 07458
Soils: An Introduction, 5th Edition
andDonald
Munns,
2002)
by Michael J.(Singer
Singer and
N. Munns
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
•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.
Non-Expanding 2:1 Silicate Minerals
Mica Group (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
Chlorites are also non-expansive:
Mg-dominated hydroxide sheet fits between adjacent
2:1 layers (2:1:1). H-bonded to O atoms between sheets
Fe or Mg occupy most octahedral sites
Iron and 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
Variable Charge (pH-dependent)
• Hydrous oxides whether crystalline or
amorphous get their charge from surface
protonation and deprotonation
• >AlO- + H+  >AlOH + H+  AlOH2+
Negative
Neutral
Positive
pH decreasing 
• 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
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)
Box 2-3 Fixed and Variable Charge
©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
Less effective
cation exchange
More effective
cation exchange
Cation exchange capacity
is highest in soils with:
•High humus content
•High swelling capacity
•High pH
Humus
•A non-crystalline, organic substance
•Very large, organic molecules
•50% C, 40% O, 5% H, 3% N and sometimes S
•Structure highly variable
•Very large negative charge due to three types
of -OH groups (H+ ions gained or lost)
(i)
carboxyl group COOH
(ii) phenolic hydroxyl group (due to
partial decomposition of lignin by
microorganisms)
(iii) alcoholic hydroxyl group
State of organic
residues one year
after incorporation
into a soil
Humic Substances
• Microbes break down complex components
• Simpler compounds created; CO2 is released
• Synthesize new biomolecules, using C not respired,
as well as N, S & O
• Lignin not completely broken down: complex
residual molecules often retain lignin characteristics
• Microbes polymerize new, simpler molecules with
one another and with residual molecules
• This creates long, complex chains, resistant to
further decomposition
• Chains interact with amino compounds
• Polymerization process is stimulated by colloidal
clays
After one year:
•
1/5 to 1/3 of carbon remains in soil
(i)
live biomass (5%)
(ii)
humic fraction (20%)
(iii)
nonhumic fraction (5%)
Humic substances include:
(i)
(ii)
(iii)
Fulvic acids: lowest molecular weight and
lightest colour (most susceptible to microbes)
Humic acid (intermediate)
Humin: highest molecular weight, darkest,
least soluble and most resistant to microbes
Humus:
Amorphous and
colloidal mixture
of complex organic
substances no
longer identifiable
as tissues
Note: non-humic
substances are biomolecules
produced by microbes
Soil Acidification
1. Carbonic acid
Carbon dioxide gas from soil air dissolves in water
Root respiration and soil decomposition provide extra CO2
CO2 + H2O  H2CO3  HCO3- + H+
2. Acids from Biological Metabolism
Microbes break down organic matter, producing organic
acids such as citric acid, carboxylic acids and phenolic acids
RCH2OH… + O2 + H2O  RCOOH  RCOO- + H+
3. Accumulation of Organic Matter
(i) Loss of cations by leaching due to soluble humic
complexes combining with non-acid nutrient cations (eg. Ca2+)
(ii) Organic matter is a source of H+ ions
4. Oxidation of Nitrogen (Nitrification)
Nitrogen enters soils as NH4+
Converted to nitric acid
NH4+ + 2O2  H2O + H+ + H+ + NO35. Oxidation of Sulphur
6. Acids in Precipitation
H2SO4  SO42- + 2H+
HNO3  NO3- + H+
7. Plant Uptake of Cations
Plants exude H+ ions or take up anions (eg. SO42-) to
balance off cation uptake
Aluminium Toxicity
H+ ions adsorbed onto clay surfaces may attack the mineral
structure and release Al3+ ions in the process
Aluminium is highly toxic to most plants
Al promotes hydrolysis of H2O (see Fig. 9.12)
Al combines with OH-, leaving H+ ions in the soil solution
Tolerant plants secrete organic acids into the soil around
the root. Organic acids such as (eg. malate or citrate) are
able to chelate the Al that is in the soil solution near the root
tip. Al that is bound to organic acids cannot enter
the plant root.
•Acids are neutralized in soils with available bases
•Canadian Shield severely affected in central and eastern
Canada
H+ + HSO3sun
SO2
O2
2N20 + O2
SO3
H 2O
H2SO4
4NO
2O2

2H+ + SO42-
4NO2
2H2O
2HNO3 + 2HNO2
H+ + NO3-
Acidity of Rainfall in New Hampshire
Susceptibility to Acidification
•
Weathering of non-acid cations from minerals
An example is the weathering of calcium from silicates
Ca-silicate + 2H+
H4SiO4 + Ca2+
•
Soil maintains its alkalinity if the release of cations from
weathering minerals exceeds leaching losses
•
Acid soils therefore form:
(i)
(ii)
(iii)
in a high rainfall environment
where parent materials are low in Ca, Mg, K
and Na
where there is a high degree of biological
activity, resulting in H2CO3 formation
Effect of soil pH
on cation exchange
capacity
Increase in CEC with pH
due to:
(i) Binding and release of
H+ ions on pHdependent charge
sites
(ii) Hydrolysis reactions
involving Al
Percent “base” saturation
= cmol of exchangeable Ca2+ + Mg2+ + K+ + Na+
cmolc of CEC
= 100 – percent acid saturation
Note: Percent acid saturation, though less often cited, is determined by
cmolc Al3+ & H+ ions divided by cmolc of CEC. This is actually more
meaningful, because Ca, Mg, K and Na ions are not true bases!
Buffering
• Soils with high clay or organic content tend to
have the highest buffering capacity
• Why? Importance of exchangeable and residual
acidity
Examples of Buffering:
(i) Aluminium hydrolysis (in very acid soils)
Al(OH)2+ + H2O  Al(OH)3 + H+
Adding more H+ ions will drive the reaction to the left.
(ii) Protonation and deprotonation of organic matter
H+ ions dissociate when a base is added, preventing
pH from rising as much as expected.
CEC increases as the H+ ions are removed, increasing
the negative charges
(iii) pH-dependent charge sites in clays
Again, adding a base dissociates H+ ions from hydroxyl
groups and oxygen atoms
(iv) Cation exchange
As H+ ions are added, most end up attracted to negative
charge sites so that pH changes less than expected. If a
base is added, they are replaced by H+ ions or Al ions
from exchange sites. (most effective when pH>6).
(v) Carbonate dissolution and precipitation (See eq. 9.20)
H+
Liming
Soil Colloid + CaCO3
CO2
Soil Colloid-Ca++ + H2O +
H+
•Liming materials react with CO2 and H2O,
to produce bicarbonate (HCO3-)
Example:
CO3- + 2H+  CO2 + H2O
CaMg(CO3)2 + 2H2O + 2CO2  Ca2+ + 2HCO3- + Mg2+ + 2HCO3-
•Bicarbonate is reactive with exchangeable and
residual soil acidity
•Ca2+ and Mg2+ replace H+ and Al3+ on clay colloids
Effect of soil pH
on nutrient content
and soil
microorganisms
pH Determination
(i) Color dyes
Certain organic compounds change colour in
response to pH
Drops of dye solution can be placed on white
spot plate (in contact with soil)
(ii) Potentiometric method
Difference between H+ ion activity in soil suspension and
glass electrode gives pH
Soil sampling sites at Tambito, Cauca, Colombia
Lower Montane Cloud Forest
(LMCF)
Nutient Uptake
Rate
Soil Type
*
SoilNTexture
Soil
Soil P (Bray) in
Panamá*
Soil P (Bray) in
Borneo**
SoilK
Organic
*
Soil
Matter Content
Bulk Density
Soil Ca
*
SoilBAcidity
Soil
Cation
Soil
Al Exchange
Capacity
Base Saturation
Root Biomass
Soil Wetness
Lowland
Rainforest
stressed
to low
Upperdue
Montane
transpiration
rates
Cloud Forest
Oxisols and Ultisols
Sandy0.49-0.56%
clay loams;
5cm:
more clay
and
less
25cm:
0.095
-0.405%
sand than
5cm:
<0.5 TMCF
ppm
31-69%
clayppm
25cm:
<0.5
rates and Ultisols
Oxisols
Organic
at surface
5cm:
1.25-1.85%
then sandy
loams or
25cm:
0.40-0.49%
sandy31
clay
loams
5cm:
ppm
5-22% 4clay
25cm:
ppm
14-26% silt
0-15cm:
17-49% sand
1.18-1.56
ppm
17-24% silt
0-15cm:
57-78% sand
0.84-2.70
ppm
0-15cm: 0.80-20.93
ppm
5cm: Varies up to
Highest
0-15cm: 1.74 - 3.41%
5cm:
0.95 meq/100g
25cm:
5cm:meq/100g
0.74 gcm-3 100.39
5cm:
25cm:
29
meq/100g
0.73
gcm-3
25cm:
12 meq/100g
5cm: 15 meq/100g
Higher due to greater
25cm:
18 meq/100g
clay
fraction
and
pH:
5.4-6.5
reduced wetness
5cm: 0.1
54 meq/100g
5cm:
meq/100g
25cm:
meq/100g
25cm: 0.2
37 meq/100g
Higher root biomass
5cm:
67 meq/100g
in
Amazon
(but also
25cm:
42
meq/100g
greater above-ground
High, except dry
biomass)
season
Nutient Uptake
Rate
possibly stressed
due
Lower
Montane
to low transpiration
Cloud
Forest
Higher than TMCF
5 cm:
70..8% meq/100g
0.86-0.90
0-20cm: 9.8-19.5%
25cm:
5cm: 0.26 meq/100g
gcm-3
0.15-0.33
5cm:
10-25cm:
-3
5
- 8 gcm
meq/100g
0.49
25cm:
1-2.9 meq/100g
5cm: 68 meq/100g
5cm: 9.1 ppm
25cm:
25
cm: 26
0.6meq/100g
ppm
pH: 3.7-5.1
65 meq/100g
5cm: 2.5
meq/100g
25cm: 1.8
meq/100g
26 meq/100g
5cm: 4.26% of soil
5cm:weight
12 meq/100g
dry
25cm: 0.49%
4 meq/100g
25cm:
of soil
Wetter
due to cloud
dry
weight
-1
total:
9.45 t·ha
interception
yearIntermediate:
round and cooler
possibly stressed due
to low transpiration
Oxisols and Ultisols
Organic
at surface,
-
then sandy loams
-
-
Lowest
Lowest ?
Acid
Lowest due to sandy
texture ?
High
-
Low
Wettest, due to cloud
interception yearLowest:
possibly
round;
little
stressed due to low
transpiration rates
Soil Type
Soil N*
Soil P (Bray) in
Panamá*
Soil P (Bray) in
Borneo**
Soil K*
Soil Ca*
Soil B
Soil Al
Root Biomass
Nutient Uptake
Rate
Lowland
Rainforest
Lower Montane
Cloud Forest
Upper Montane
Cloud Forest
Oxisols and Ultisols
5cm: 0.49-0.56%
25cm: 0.095 -0.405%
5cm: <0.5 ppm
25cm: <0.5 ppm
Oxisols and Ultisols
5cm: 1.25-1.85%
25cm: 0.40-0.49%
5cm: 31 ppm
25cm: 4 ppm
Oxisols and Ultisols
-
0-15cm:
1.18-1.56 ppm
0-15cm:
0.84-2.70 ppm
0-15cm: 0.80-20.93
ppm
5cm:
0.95 meq/100g
25cm:
0.39 meq/100g
5cm:
29 meq/100g
25cm:
12 meq/100g
Higher due to greater
clay fraction and
reduced wetness
5cm: 0.1 meq/100g
25cm: 0.2 meq/100g
Higher root biomass
in Amazon (but also
greater above-ground
biomass)
5 cm:
0.86-0.90 meq/100g
25cm:
0.15-0.33 meq/100g
5cm:
5 - 8 meq/100g
25cm:
1-2.9 meq/100g
5cm: 9.1 ppm
25 cm: 0.6 ppm
-
5cm: 2.5 meq/100g
25cm: 1.8 meq/100g
5cm: 4.26% of soil
dry weight
25cm: 0.49% of soil
dry weight
total: 9.45 t·ha-1
Intermediate:
possibly stressed due
to low transpiration
rates
-
Higher than TMCF
-
Lowest ?
Lowest due to sandy
texture ?
-
Lowest: possibly
stressed due to low
transpiration rates
N of Taber, Alberta on Highway 36
•Organic matter
decomposition rates
are higher
•ET > P
SW of MacGrath Wind Farm, Alberta
Mer Bleue Bog
(SE of Ottawa)
8 metres of organic material
Accumulation of organic
material has exceeded decay
rates (anaerobic) for 12,000
years
P >> ET
Photo: P Lafleur, Trent University
Composition
of Green Plant
Materials
Decomposition Rates of Organic Materials
Rapid
Sugars and Starches
Proteins
Hemicellulose
Cellulose
Fats, Waxes and Oils
Lignins and phenolic compounds
Very slow
Review: Oxidation products are
CO2, H2O and energy (478 kJ/mol C)
Decomposition in Anaerobic Soils
•Very slow
•Release of methane gas, alcohols, organic acids
water and some carbon dioxide
•Provides little energy for organisms involved, so
byproducts contain more energy
Rice paddies
and natural
wetlands
release
methane
Concentrations
on the rise
Effect of C/N ratio on Decomposition Rate
Soil Nutrient Concentrations vs. Successional
Stage (Tambito, Cauca, Colombia)
1.2
15000
40
NITROGEN
POTASSIUM
PHOSPHORUS
30
0.8
10000
1st/late 2nd.
Early 2nd.
Deforested
m.eq.
p.p.m.
p.p.m.
20
0.4
5000
10
0.0
0
0
0
40
80
0
Depth (cm)
8
40
Depth (cm)
0
60
20
40
60
Depth (cm)
5
12
CALCIUM
m.eq.
20
BORON
ALUMINIUM
4
m.eq.
p.p.m.
8
3
4
2
4
1
0
0
0
20
Depth (cm)
40
60
0
0
20
40
Depth (cm)
60
0
10
20
30
Depth (cm)
40
50