Unit 5 - Notes

SOL103

Unit 5: Soil chemistry and exchange phenomena

1. Silicate Clays: Constitution and Properties

Soil colloids (clay and organic matter) are the most chemically active portion of the soil. Among these, crystalline silicate clays are dominant in most mineral soils.

A. Structural Building Blocks

Silicate clays are built from two fundamental structural units composed of oxygen, silicon, and aluminum.

Silicon Tetrahedron ( $SiO_4^{4-}$ )
- Shape: A central silicon ion ( $Si^{4+}$ ) surrounded by four oxygen atoms ( $O^{2-}$ ) in a pyramidal shape.
- Tetrahedral Sheet: Many tetrahedra linked together horizontally by sharing basal oxygen atoms form a continuous sheet.
Aluminum Octahedron ( $Al(OH)_6^{3-}$ )
- Shape: A central aluminum ion ( $Al^{3+}$ ) (or Magnesium $Mg^{2+}$ ) surrounded by six oxygen or hydroxyl ( $OH^-$ ) ions.
- Octahedral Sheet: Many octahedra linked horizontally form a sheet. If the central ion is $Al^{3+}$ , it is a Dioctahedral sheet (Gibbsite-like). If the central ion is $Mg^{2+}$ , it is a Trioctahedral sheet (Brucite-like).

B. Classification of Silicate Clays

Based on the arrangement and number of tetrahedral and octahedral sheets, silicate clays are classified into major groups:

1. 1:1 Type Minerals (e.g., Kaolinite)

Structure: One tetrahedral sheet is chemically bonded to one octahedral sheet.
Bonding: The layers are held together rigidly by hydrogen bonding (between the O of one layer and the OH of the next).
Properties:
- Non-expanding: Water and ions cannot enter between the layers.
- Low Surface Area: Primarily external surface area.
- Low Plasticity and Stickiness: Easy to cultivate.
- Low CEC: 3–15 $cmol_c/kg$ .

2. 2:1 Type Minerals (Expanding)

Structure: One octahedral sheet sandwiched between two tetrahedral sheets.
- Smectite Group (Montmorillonite):
  - Bonding: Layers held loosely by weak Oxygen-Oxygen van der Waals forces and hydrated cations.
  - Properties: Highly expanding (water enters interlayer spaces), high swelling/shrinkage, high plasticity, high specific surface area (internal and external).
  - CEC: High (80–150 $cmol_c/kg$ ).
- Vermiculite Group:
  - Bonding: Similar to smectite but with higher negative charge in the tetrahedral sheet, holding layers closer together with water molecules and $Mg^{2+}$ ions.
  - Properties: Moderate expansion (less than smectite), high nutrient holding capacity.
  - CEC: Very High (100–200 $cmol_c/kg$ ).

3. 2:1 Type Minerals (Non-Expanding)

Illite (Fine-grained Mica):
- Structure: 2:1 structure similar to smectite.
- Specific Feature: Potassium ions ( $K^+$ ) fit snugly into the hexagonal holes of the tetrahedral sheet, firmly binding the layers together.
- Properties: Non-expanding, lower CEC than smectites (20–40 $cmol_c/kg$ ).

4. 2:1:1 Type Minerals (Chlorite)

Structure: A 2:1 framework with an additional octahedral sheet (Mg-dominated brucite sheet) in the interlayer space.
Properties: Non-expanding, properties similar to Illite.

2. Sources of Charge on Soil Colloids

Soil colloids carry electric charges (mostly negative) that attract ions. There are two primary mechanisms for charge generation.

A. Isomorphous Substitution (Permanent Charge)

This is the primary source of constant negative charge in 2:1 clays.

Definition: The substitution of one ion for another of similar size but different valency within the crystal lattice during the mineral's formation.
Mechanism:
- Tetrahedral Layer: $Al^{3+}$ replaces $Si^{4+}$ . (Net charge: $-1$ )
- Octahedral Layer: $Mg^{2+}$ or $Fe^{2+}$ replaces $Al^{3+}$ . (Net charge: $-1$ )
Result: The crystal lattice develops a net negative charge that is independent of soil pH.

B. pH-Dependent Charge (Variable Charge)

This source of charge changes with the pH of the soil solution. It is dominant in 1:1 clays (Kaolinite), Fe/Al oxides, and organic matter (humus).

Ionization of Hydroxyl Groups (Broken Edges):
- At the edges of clay minerals, there are exposed -OH groups ( $Al-OH$ or $Si-OH$ ).
Dissociation of Organic Functional Groups:
- Carboxyl ( $-COOH$ ) and Phenolic ( $-OH$ ) groups in humus.

Mechanism:
- High pH (Alkaline): Groups lose (deprotonation) Negative Charge.
  - $Si-OH + OH^- \rightarrow Si-O^- + H_2O$
- Low pH (Acidic): Groups accept (protonation) Positive Charge.
  - $Al-OH + H^+ \rightarrow Al-OH_2^+$

3. Ion Exchange in Soils

Ion exchange is a reversible chemical process where ions are interchanged between the solid phase (colloid surface) and the liquid phase (soil solution).

A. Cation Exchange

The interchange of a cation in the soil solution with a cation adsorbed on the surface of a negatively charged colloid.

Mechanism: Cations are held by electrostatic attraction. The bond is relatively weak, allowing exchange.
Cation Selectivity (Lyotropic Series):
The strength with which cations are held depends on valence and hydrated ionic radius.
- Aluminum is held tightest; Sodium is held loosely.

B. Principles Governing Cation Exchange

Reversibility: The reaction can go in either direction.
Charge Equivalence: Exchange takes place on a charge-for-charge basis (e.g., one $Ca^{2+}$ replaces two $K^+$ ).
Ratio Law: The ratio of cations on the colloid varies directly with the ratio of cations in the solution.

4. Cation and Anion Exchange Capacity

A. Cation Exchange Capacity (CEC)

Definition: The total sum of exchangeable cations that a soil can adsorb. It represents the magnitude of the negative charge per unit weight of soil.
Units: Centimoles of charge per kilogram of soil ( $cmol_c/kg$ or $cmol(+)/kg$ ). Formerly meq/100g.
Typical Values:
- Sands: 1–5 $cmol_c/kg$
- Kaolinite: 3–15 $cmol_c/kg$
- Montmorillonite: 80–150 $cmol_c/kg$
- Humus: 200+ $cmol_c/kg$
Significance:
- Nutrient Buffer: High CEC soils hold more nutrients ( $K^+, Ca^{2+}, Mg^{2+}$ ) preventing leaching.
- Lime Requirement: High CEC soils require more lime to raise pH than low CEC soils.

B. Anion Exchange Capacity (AEC)

Definition: The total sum of exchangeable anions that a soil can adsorb.
Mechanism: Occurs on positively charged sites (usually at low pH via protonation of hydroxyl groups).
Relevance: Important for retaining anions like Nitrate ( $NO_3^-$ ), Sulfate ( $SO_4^{2-}$ ), and Phosphate ( $H_2PO_4^-$ ).
Occurrence: Most common in highly weathered tropical soils rich in Fe/Al oxides (Gibbsite, Goethite) and kaolinite under acidic conditions.

5. Soil Reaction (pH)

Soil reaction refers to the acidity or alkalinity of the soil, expressed as pH.

Definition: $pH = -log[H^+]$
Range:
- Acidic: pH < 7.0
- Neutral: pH = 7.0
- Alkaline: pH > 7.0

Types of Soil Acidity

Active Acidity:
- The $H^+$ ion concentration in the soil solution.
- Measured by standard water-pH tests.
- Directly affects plant root environment immediately.
- Very small quantity compared to total acidity.
Exchangeable (Salt-Replaceable) Acidity:
- $Al^{3+}$ and $H^+$ ions adsorbed on the colloid surfaces.
- Can be released into solution by exchange with unbuffered salts (e.g., KCl).
- Aluminum hydrolysis generates acidity: $Al^{3+} + H_2O \rightarrow Al(OH)^{2+} + H^+$ .
Residual (Reserve) Acidity:
- $H^+$ and $Al^{3+}$ bound in non-exchangeable forms within clay lattices or organic matter.
- The largest fraction of soil acidity.

6. Buffering Capacity of Soils

Definition

Buffering capacity is the ability of the soil to resist a change in pH when acid or base is added.

Mechanism

The soil acts as a buffer system through equilibrium between the solution (Active Acidity) and the colloids (Reserve Acidity).

Equilibrium Equation:

\text{Reserve Acidity (Colloid-H)} \rightleftharpoons \text{Active Acidity (Solution-H)}

If a base (Lime) is added:
- The base neutralizes the $H^+$ in the solution (Active Acidity).
- To restore equilibrium, $H^+$ ions are released from the colloid surface (Reserve Acidity) into the solution.
- Result: The pH change is minimized.
If an acid is added:
- The concentration of $H^+$ in solution increases.
- The excess $H^+$ is adsorbed onto the colloid surfaces (moving from Active to Reserve).
- Result: The pH drop is minimized.

Factors Affecting Buffering Capacity

CEC / Clay Content: High clay content (high CEC) = High Buffering Capacity. Sands have very low buffering capacity.
Organic Matter: Humus has high charge density, contributing significantly to buffering.

7. Base Saturation

Base saturation connects CEC and Soil Reaction. It indicates the proportion of the CEC occupied by basic cations versus acidic cations.

Basic Cations: $Ca^{2+}, Mg^{2+}, K^+, Na^+$
Acidic Cations: $H^+, Al^{3+}$

Formula

\text{Percent Base Saturation (BS\%)} = \frac{\sum (\text{Exchangeable } Ca + Mg + K + Na)}{CEC} \times 100

Relationship with pH

High Base Saturation (> 80%): Indicates a neutral to alkaline pH. The colloids are dominated by bases.
Low Base Saturation (< 50%): Indicates acidic soils. The colloids are dominated by $H^+$ and $Al^{3+}$ .

Importance

Fertility Indicator: High base saturation generally implies high availability of basic nutrient cations.
Liming Guide: Soil pH is essentially a function of base saturation. To raise pH, one must increase the base saturation (by adding lime, $CaCO_3$ ).