Cation Exchange Capacity (CEC), Anion Exchange Capacity (AEC), Base Saturation, and Buffering Capacity

1. Introduction

• Soils act as reservoirs of plant nutrients due to the presence of charged colloids — mainly clay minerals and organic matter (humus).
  These colloids carry negative and positive charges, which allow them to adsorb, retain, and exchange ions (cations and anions) with the soil solution.
• The ability of a soil to hold and exchange these ions determines its fertility, nutrient retention, and buffering power.

2. Cation Exchange Capacity (CEC)

Definition

• Cation Exchange Capacity (CEC) is the total quantity of exchangeable cations that a soil can adsorb and exchange per unit weight of soil.
• In simpler terms:
  CEC measures the soil’s ability to hold positively charged ions (cations) such as Ca²⁺, Mg²⁺, K⁺, Na⁺, and NH₄⁺ on negatively charged colloid surfaces.

3. Unit of Measurement

• Milliequivalents per 100 grams of soil (meq/100g) or
• Centimoles of charge per kilogram (cmol(+)/kg)
• 1 meq/100g = 1 cmol(+)/kg

4. Mechanism of Cation Exchange

• Clay and humus particles have negative surface charges.
• They attract and hold cations by electrostatic forces, not by chemical bonds.
• These cations are reversibly exchangeable with cations in the soil solution.
• Example: Clay – Ca + 2K⁺ ↔ Clay – (K)₂ + Ca²⁺
• Thus, the cations on colloid surfaces are in dynamic equilibrium with those in the soil solution.

5. Sources of CEC

• Isomorphous substitution in clay minerals (permanent charge).
• Ionization of hydroxyl groups (–OH) at clay edges (variable charge).
• Dissociation of carboxyl (–COOH) and phenolic (–OH) groups in humus.

6. Factors Affecting CEC

7. Typical CEC Values of Soil Components

8. Importance of CEC

• Nutrient Retention: Prevents leaching of essential cations.
• Nutrient Supply: Cations held on colloids can be exchanged and made available to plants.
• Soil Fertility Indicator: High CEC = fertile soil.
• Soil Management: Influences fertilizer efficiency and lime requirements.
• Buffering Effect: Soils with high CEC resist sudden changes in nutrient concentration and pH.

9. Anion Exchange Capacity (AEC)

Definition

• Anion Exchange Capacity (AEC) is the total quantity of exchangeable anions (negatively charged ions) that a soil can adsorb and exchange per unit weight of soil.
• In contrast to CEC, AEC involves positively charged soil surfaces that attract anions such as Cl⁻, NO₃⁻, SO₄²⁻, H₂PO₄⁻, and HCO₃⁻.

10. Origin of AEC

• AEC arises mainly due to protonation of surface hydroxyl (–OH) groups in Fe, Al oxides, and the edges of kaolinite.
• It is therefore pH-dependent and dominant in acidic soils.
• –OH + H⁺ → –OH₂⁺

The protonated surface (–OH₂⁺) attracts anions.

11. Factors Affecting AEC

12. Relative Magnitude of AEC and CEC

13. Importance of AEC

• Retention of Anions: Prevents leaching losses of NO₃⁻, PO₄³⁻, SO₄²⁻ in acidic soils.
• Influence on Fertilizer Efficiency: In strongly acid soils, AEC helps retain applied phosphate and sulfate.
• pH Buffering: AEC contributes to the soil’s ability to buffer against acidification.

14. Relationship Between pH, CEC, and AEC

15. Base Saturation (BS)

Definition

• Base Saturation is the percentage of the Cation Exchange Capacity (CEC) that is occupied by basic cations — primarily Ca²⁺, Mg²⁺, K⁺, and Na⁺.
• Base Saturation (%) = Sum of exchangeable bases / CEC×100

16. Example Calculation

If a soil has:

• Exchangeable Ca²⁺ = 8 meq/100g
• Mg²⁺ = 2 meq/100g
• K⁺ = 1 meq/100g
• Na⁺ = 1 meq/100g
• CEC = 20 meq/100g

Then, Base Saturation = (8+2+1+1) / 20 ×100 = 60% 

So, 60% of the exchange sites are occupied by basic cations; the remaining 40% are held by acidic cations (H⁺ and Al³⁺).

17. Significance of Base Saturation

Interpretation:

• High base saturation → good nutrient reserve, well-limed soils.
• Low base saturation → acidic, leached soils (need lime or amendments).

18. Buffering Capacity of Soil

Definition

• Buffering capacity is the ability of soil to resist changes in pH when acids or bases are added.
• Soils with high CEC (e.g., clays and humus-rich soils) have greater buffering capacity, while sandy soils have poor buffering.

19. Factors Influencing Buffering Capacity

20. Importance of Buffering Capacity

1. Stabilizes Soil pH — protects plants from rapid pH changes.
2. Improves Fertilizer Efficiency — prevents nutrient toxicity/deficiency.
3. Regulates Microbial Activity — maintains favorable environment for microbes.
4. Guides Lime Requirement — high buffering soils need more lime to raise pH.

21. Interrelationships: CEC, AEC, Base Saturation, and Buffering

In general:

High CEC + High Base Saturation → High Fertility and Strong Buffering
Low CEC + Low Base Saturation → Infertile, Poorly Buffered Soil

22. Typical Values in Common Indian Soils

23. Summary Table