Sources of Charge and Ion Exchange in Soils

1. Introduction

Soil particles, particularly clay minerals and organic matter (humus), carry electrical charges on their surfaces.
These charges enable soils to attract and hold ions (nutrient elements) through a process called ion exchange.

This capacity to adsorb and exchange cations or anions directly affects:

• Soil fertility
• Nutrient availability
• Soil buffering capacity
• Soil pH and chemical reactions

In simple terms: Charged soil colloids act like a reservoir of nutrients, holding them electrostatically and releasing them for plant uptake.

2. Soil Colloids: The Charge Carriers

The sources of charge in soils come mainly from two types of colloids:

• Inorganic colloids → Clay minerals (especially silicate clays)
• Organic colloids → Humus (decomposed organic matter)

Both have large surface areas and reactive sites, which make them highly active in ion exchange processes.

3. Types of Electrical Charges in Soil

There are two major types of charges found on soil colloids:

Let’s understand each in detail 👇

4. Sources of Charge in Soils

i) Permanent Charge (Structural Charge)

• Origin: Caused by isomorphous substitution within the crystal lattice of silicate clays.
• Process: When a cation of lower valence replaces one of higher valence in the mineral structure, an overall negative charge is produced.

Examples:

• In tetrahedral sheet: Al₃ + replaces Si⁴ + ⇒ −1 charge per substitution
• In octahedral sheet: Mg₂ + replaces Al³⁺⇒ −1 charge per substitution

Result:

• Creates permanent negative charges that do not change with pH.
• Found mainly in 2:1 type clays (e.g., montmorillonite, illite, vermiculite).

Significance:

• Responsible for cation adsorption and exchange.
• Provides high cation exchange capacity (CEC) and nutrient-holding ability.

ii) Variable Charge (pH-Dependent Charge)

Origin:

• Produced by ionization or protonation of surface hydroxyl (–OH) or carboxyl (–COOH) groups present on:
  • Clay mineral edges, and
  • Humus particles (organic matter).

Mechanism:

• At low pH (acidic conditions): Abundant H⁺ ions protonate the surface → positive charge forms: –OH + H⁺ → –OH₂⁺ 
• At high pH (alkaline conditions): H⁺ ions are lost → negative charge forms: – OH → –O⁻ + H⁺ Hence:
• Acidic soil → more positive charge → may adsorb anions (e.g., Cl⁻, NO₃⁻, SO₄²⁻).
• Alkaline soil → more negative charge → greater CEC (cation retention).

Examples:

• Oxides of Fe, Al (e.g., goethite, gibbsite)
• Edges of kaolinite crystals
• Humus colloids (carboxyl and phenolic groups)

iii) Charges on Organic Matter (Humus)

• Humus contains functional groups such as:
  • Carboxyl (–COOH)
  • Phenolic (–OH)
  • Alcoholic (–OH)
• These groups dissociate hydrogen ions depending on soil pH:

• –COOH ⇌–COO⁻ + H⁺  
• –OH ⇌–O⁻ + H⁺

• As pH rises, more groups dissociate → more negative charges → higher CEC.

CEC of Humus: Very high — typically 200–400 meq/100 g (much higher than clay minerals).

5. Summary of Sources of Charge

6. Ion Exchange in Soils

Definition; Ion exchange is the reversible process by which cations or anions are adsorbed onto and exchanged between the surface of soil colloids and the soil solution.

7. Types of Ion Exchange

Mechanism of Ion Exchange

Cation Exchange Process

• Soil colloids (clay/humus) carry negative charges → attract cations from soil solution.
• The adsorbed cations are loosely held by electrostatic forces — they can be exchanged with other cations in the soil solution. Example: Clay – Ca + 2K⁺  ⇌ Clay – (K)₂ + Ca₂⁺
• This reaction is reversible.
• The exchange is equivalent (maintains electrical neutrality).

Anion Exchange Process

• Occurs on positively charged surfaces (mainly Fe, Al oxides and hydroxides).
• Adsorbed anions are exchanged with anions in the soil solution.
• Example: Al – OH₂⁺ Cl⁻ + NO₃⁻ ⇌ Al – OH₂⁺ NO₃⁻+Cl⁻
• Prominent in acidic tropical soils (low pH).

9. Principles of Ion Exchange

1. Reversibility: Ion exchange is a reversible process; cations can replace each other.
2. Equivalence: Exchange is based on electrical equivalence, not on weight. (e.g., 1 Ca²⁺ ↔ 2 Na⁺)
3. Ratio Law: The ratio of adsorbed ions is proportional to the ratio of ions in solution.
4. Selectivity (Preference): Colloids have preference for certain cations, depending on their charge and hydrated radius. General Order of Preference: Al³⁺  > Ca²⁺ > Mg²⁺ > K⁺ =NH₄⁺> Na⁺ 
   • Higher valence → stronger adsorption.
   • Smaller hydrated radius → stronger attraction.
5. Mass Action Law: High concentration of a particular ion in solution favors its adsorption.

10. Cation Exchange Capacity (CEC)

Definition; The total quantity of exchangeable cations that a soil can adsorb and exchange is known as its Cation Exchange Capacity (CEC).

Units:

• meq/100g of soil (milliequivalents per 100 grams), or
• cmol(+)/kg (centimoles of charge per kilogram).

Typical CEC Values

Factors Affecting CEC

• Type of clay mineral
• Amount of clay and organic matter
• Soil pH (increases with pH)
• Degree of weathering (less weathered = higher CEC)

11. Base Saturation

Definition

• The percentage of the soil’s CEC occupied by basic cations (Ca²⁺, Mg²⁺, K⁺, Na⁺).
• Base Saturation (%) = Sum of exchangeable bases / CEC × 100

Significance:

• High base saturation → fertile soil (neutral to slightly alkaline).
• Low base saturation → acidic, leached soils.

Importance of Ion Exchange in Soil Science

• Nutrient Supply: Provides a reservoir of exchangeable nutrients available for plant uptake.
• Soil Fertility Indicator: High CEC = high nutrient-holding capacity → fertile soil.
• Soil Buffering: Prevents rapid changes in soil pH and nutrient levels.
• Soil Conditioning: Influences structure, flocculation, and permeability.
• Reclamation and Fertilization: Helps explain reactions during lime, gypsum, or fertilizer application.

• Summary Table