Muhammad Nadeem Ashraf

1. Cation Exchange
Capacity
By
Mohsin Zafar
Lecturer

2. Basic Structural Units
Clay minerals are made of two distinct structural
units.
hydroxyl or
oxygen oxygen
aluminium or
silicon magnesium
0.26 nm
0.29 nm
Silicon tetrahedron Aluminium Octahedron

3. Different Clay Minerals
Different combinations of tetrahedral and octahedral
sheets form different clay minerals:
1:1 Clay Mineral (e.g., kaolinite, halloysite):
3

4. Different Clay Minerals
Different combinations of tetrahedral and octahedral
sheets form different clay minerals:
2:1 Clay Mineral (e.g., montmorillonite, illite)
4

5. Absorption: interception of radiant energy or sound waves
Adsorption: adhesion in an extremely thin layer of molecules to the surfaces of
solid bodies or liquids with which they are in contact.
Buffering capacity (BC): represents the ability of the soil to re-supply an ion to
the soil solution.

6. pH independent charge (permanent)
Isomorphic substitution: substitution of one element for another in ionic
crystals without changing the structure of the crystal
a. Substitution of Al+++ for Si++++ in tetrahedral
b. Mg++, Fe++, Fe+++ for Al+++ in octahedral
Leaves a net negative charge (permanent)
pH dependent charge: positive charge developed at low pH and excess
negative charge formed at high pH
Gain or loss of H+ from functional groups on the surface of soil solids.
a. Hydroxy (-OH)
b. Carboxyl (-COOH)
c. Phenolic (-C6H4OH)

7. • Cation exchange- the interchange between a cation in
solution and another cation on a soil surface
• Cation exchange capacity (CEC)- the total sum of
exchangeable cations that a soil can adsorb.

8. Importance of CEC
• Chemical behavior in soils
• Fertility
• Liming rates
– Buffering capacity
• Pesticides
• Contaminants
• Non-acid cation (Base) Saturation

9. Ion exchange
• Sources of charge:
– In 2:1 clays, charge created mostly by
isomorphous substitution.
• Not very pH dependent
– Hydroxyls (OH-) and other functional groups
on the surfaces of colloidal particles that
cause positive or negative charges based on
releasing or accepting H+ ions.
• pH dependent
• Common source of charge on humus, Fe and Al
oxides, 1:1 type clays, and non-crystalline silicates

10. Ion exchange
• Positive and negative
– Anion exchange (negative ions)
– Cation exchange (positive ions)
– Units of : cmolc/kg (centimoles of charge
per kg)

11. The Colloidal Fraction: Seat of Soil Chemical and
Physical Activity
Some of the many types:
• Layer silicate clays
• Iron and Aluminum
oxide clays
• Organic soil colloids:
humus
Colloids are small particles in
soil that act like banks:
managing the exchange of
nutrient currency in the soil
Different soils, like checking accounts, have different
capacities to hold nutrient currency: cations and anions

13. OF GREAT IMPORTANCE:
The influence of clay type on CEC

14. Typical
CEC
Values
Figure 8.13 Ranges in the cation exchange
capacities (at pH 7) that are typical of a
variety of soils and soil materials. The high
CEC of humus shows why this colloid plays
such a prominent role in most soils, and
especially those high in kaolinite and Fe, Al
oxides, clays that have low CECs.

15. Principles of Ionic
Exchange
Reversible Reactions
Charge Balance
Ratio Law
Mass Action
Ion Selectivity
Complementary Cations

16. Reversible Reactions
Can go forwards or backwards
Example:
K+
H+
micelle
micelle + 2K+ + 2H+
K+
H+

17. Balanced by Charge
Charge for Charge…..
NOT ion for ion
Ca++ K+
micelle
micelle + Ca++
+ 2K+
K+

18. The Ratio of Ions on Exchange
Site is Equal to the Ratio of Ions
in the Soil Solution
6 H : 3 Na 4 H : 2 Na 2H : 1Na
before After on colloid After in soln.
H+ H+ Na+
H+
Na+
H+
micelle + Na+ and 2H+
micelle
+ 3Na+
H+
H+ H+ H+
H+
H+

19. Mass Action
H+
micelle micelle + H2O + CO2
+ CaCO3
H+ Ca++
CO2 is a gas and escapes
from the soil easily….
This drives the reaction to
the right.

20. Ion Selectivity
Al+3 > Ca+2 > Mg+2 > K+ = NH4+ > Na+
Held tightly ---------------------------------- Held loosely
Based on Valence Charge and Hydrated Ionic Radius
Charge of ion
Selectivity = Size

22. The Effects of Neighboring
Cations

23. pH influences nutrient holding capacity:
Cation Exchange Capacity
• pH influences what cations are
adsorbed to the exchange
complex
• At lower pH values, more H+
and Al3+ ions are adsorbed to the
exchange complex holds than
non-acid nutrient cations
• Acid cations: H+ and Al3+
• Non-acid (or base) cations:
Ca2+, Mg2+, K+, Na+ (plant nutrients)

24. Sources of
Charge
and their
influence
on CEC
Figure 8.14 Influence of pH on the cation exchange
capacity of smectite and humus. Below pH 6.0 the
charge for the clay mineral is relatively constant. This
charge is considered permanent and is due to ionic
substitution in the crystal unit. Above pH 6.0 the
charge on the mineral colloid increases slightly
because of ionization of hydrogen from exposed
hydroxyl groups at crystal edges. In contrast to the
clay, essentially all of the charges on the organic
colloid are considered pH dependent. [Smectite data
from Coleman and Mehlich (1957); organic colloid data
from Helling et al. (1964)]

26. pH and pOH
pH = -log{H+}

28. Acid cations replacing non-acid cations on soil colloids

29. What About Anion Exchange ?
Cl- chlorine
- Essential
NO3 nitrate
Plant
SO4-2 sulfate Nutrients
PO4-3 phosphate
First we need to know about:
Soil pH
And Variable Charge

30. CEC vs
AEC
Figure 8.16 (Left) Effect of increasing the pH of subsoil material from an Ultisol from Georgia on the cation and
anion exchange capacities. Note the significant decrease in anion exchange capacity associated with the
increased soil pH. When a column of the low-pH material (pH = 4.6) was leached with Ca(NO3)2 (right), little
sulfate was removed from the soil. In contrast, similar leaching of a column of the soil with the highest pH
(6.56), where the anion exchange capacity had been reduced by half, resulted in anion exchange of NO32 ions
for SO42 ions and significant leaching of sulfate from the soil. The importance of anion adsorption in retarding
movement of specific anions or other negatively charged substances is illustrated. [Data from Bellini et al.
(1996)]

31. Liming requirements to raise pH to 6.5
• pH
• Texture
• Organic matter content
• Types of clay present
Clay minerals and organic matter influence
CEC most substantially

32. Field Estimates of CEC
Uses Soil Texture and Organic Matter Content
to predict the CEC of a soil
How much of a Soil Colloid (%) ?
What type or types of Colloids present ?

33. Example
A soil contains 20% smectite, 5% Fe/Al oxides, and 4% humus.
Calculate its CEC.
(5% = 0.05 kg per 1 kg soil)
Visit Table 8.3: pH of 7 is neutral; smectite CEC = 100 cmolc/kg
Organic Matter CEC = 200 cmolc/kg
Gibbsite/Goethite (Fe/Al oxide) CEC = 4 cmolc/kg
From the clays: 0.2 kg x 100 cmolc/kg = 20 cmolc
From O.M.: .04 kg x 200 cmolc/kg = 8 cmolc
From oxides: 0.05 kg x 4 cmolc/kg = 0.2 cmolc
Sand does not carry a charge, so…
Total CEC of the soil = 20 + 8 + 0.2 = 28.2 cmolc/kg soil