1. Cation Exchange Capacity
Soil Colloids and
Cation Exchange Capacity

2. Soil Colloids Particles less than 1 or 2 m behave as soil colloids
Total surface area ranges from 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

4. 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

7. 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

8. 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

9. 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

11. (Singer and Munns, 2002)

12. Phyllosilicates Tetrahedron: Two planes of O, with Si in between
Basic building block
is silicon atom,
connected to 4 O
atoms
Octahedron:
with Al or Mg in
between
is Al (or Mg),
connected to six
hydroxyl groups or
O atoms
There are many
layers in each micelle

13. Trioctahedral
Sheet
Dioctahedral
Sheet
Isomorphous
substitution
1 Al3+ atom,
1 Mg2+ atom
Charge = -1
3 Mg2+ atoms
Charge = 0
2 Al3+ atoms
Charge = 0

14. 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

16. 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

17. 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)

18. Each layer contains one octahedral sheet sandwiched
2:1 Silicate Clay
Each layer contains one octahedral sheet sandwiched
between two tetrahedral sheets
O on both ends
No attraction without cations

19. 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:cation
linkages between layers
leads to plasticity, stickiness,
swelling and a very high
specific surface area

20. (Singer and Munns, 2002)

21. 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.

22. 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

23. Chlorites are also non-expansive:
Mg-dominated trioctahedral hydroxide sheet fits between
2:1 layers (2:1:1). H-bonded to O atoms between sheets
Fe or Mg occupy most octahedral sites

25. 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

27. 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

28. 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)

30. Less effective
cation exchange
More effective
cation exchange

31. Cation exchange capacity
is highest in soils with:
High humus content
High swelling capacity
High pH

32. 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

33. State of organic
residues one year
after incorporation
into a soil

34. 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

37. 1/5 to 1/3 of carbon remains in soil (i) live biomass (5%)
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) Fulvic acids: lowest molecular weight and
lightest colour (most susceptible to microbes)
(ii) Humic acid (intermediate)
(iii) Humin: highest molecular weight, darkest,
least soluble and most resistant to microbes

38. Humus: Amorphous and colloidal mixture of complex organic
substances no
longer identifiable
as tissues
Note: non-humic
substances are biomolecules
produced by microbes

39. Soil Acidity:
Causes and Impacts

43. Soil Acidification Carbonic acid
Carbon dioxide gas from soil air dissolves in water
Root respiration and soil decomposition provide extra CO2
CO2 + H2O  H2CO3  HCO3- + H+
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+
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

44. Oxidation of Nitrogen (Nitrification)
Nitrogen enters soils as NH4+
Converted to nitric acid
NH4+ + 2O2  H2O + H+ + H+ + NO3-
Oxidation of Sulphur
Acids in Precipitation
H2SO4  SO H+
HNO3  NO3- + H+
Plant Uptake of Cations
Plants exude H+ ions or take up anions (eg. SO42-) to
balance off cation uptake

45. 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.

47. Acids are neutralized in soils with available bases
Canadian Shield severely affected in central and eastern
Canada

48.  H+ + HSO3- SO2 SO3 H2SO4 2H+ + SO42- 4NO 4NO2 2N20 + O2
sun
H2O 
SO2
SO3
H2SO4
2H+ + SO42- O2
2O2
4NO
4NO2
2H2O
2N20 + O2
2HNO3 + 2HNO2
H+ + NO3-

49. Acidity of Precipitation
in the United States

50. Acidity of Rainfall in New Hampshire

52. 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) in a high rainfall environment
(ii) where parent materials are low in Ca, Mg, K
and Na
(iii) where there is a high degree of biological
activity, resulting in H2CO3 formation

53. Effect of soil pH on cation exchange capacity Increase in CEC with pH
due to:
Binding and release of
H+ ions on pH-
dependent charge
sites
(ii) Hydrolysis reactions
involving Al

54. 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!

55. Soils with high clay or organic content tend to
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:
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.

57. 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
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).
Carbonate dissolution and precipitation (Eq. 9.18)

58. Liming Liming materials react with CO2 and H2O,
   
Soil Colloid + CaCO3               Soil Colloid-Ca++ + H2O + CO2
Liming
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

62. Effect of soil pH
on nutrient content
and soil
microorganisms

63. pH Determination 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)
Potentiometric method
Difference between H+ ion activity in soil suspension and
glass electrode gives pH

64. Soil sampling sites at Tambito, Cauca, Colombia
Lower Montane Cloud Forest
(LMCF)

67. Soil Organic Matter

70. N of Taber, Alberta on Highway 36
Organic matter
decomposition rates
are higher
ET > P

71. SW of MacGrath Wind Farm, Alberta

72. 8 metres of organic material Accumulation of organic
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

73. Composition
of Green Plant
Materials

74. Decomposition Rates of Organic Materials Rapid
Very slow
Sugars and Starches
Proteins
Hemicellulose
Cellulose
Fats, Waxes and Oils
Lignins and phenolic compounds
Review: Oxidation products are
CO2, H2O and energy (478 kJ/mol C)

76. 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

77. Effect of C/N ratio on Decomposition Rate

78. Soil Nutrient Concentrations vs. Successional
Stage (Tambito, Cauca, Colombia)