Unit 4: Soil colloids and organic matter

1. Introduction to Soil Colloids

The term "colloid" is derived from the Greek word kolla (glue) and eidos (resemblance). In soil science, the colloidal fraction is the most chemically active portion of the soil, often referred to as the "seat of soil chemical activity."

Definition

Soil colloids are organic and inorganic particles with a diameter less than 0.002 mm (2 microns). They exhibit distinct properties such as high surface area and electric charge, which govern the adsorption and exchange of ions.

2. General Properties of Soil Colloids

A. Size and Surface Area

• Size: Extremely small particles ().
• Specific Surface Area: Due to their small size and plate-like structure, colloids possess an immense external and internal surface area per unit mass.
  • Example: 1g of colloidal clay may have a surface area of 800 , whereas 1g of coarse sand may have a surface area of only 0.01 .

B. Surface Charge

• Soil colloids carry an electric charge (mostly negative, though positive charges exist).
• Electronegativity: The external surfaces of most soil colloids differ from the internal core, carrying a net negative charge. This allows them to attract and hold positively charged ions (cations) like , , , and .

C. Adsorption

• Because of the surface charge and high surface area, colloids attract ions and water molecules to their surfaces.
• Cation Adsorption: The attraction of positively charged ions to the negative surfaces prevents nutrients from leaching away.
• Water Adsorption: Colloids attract water molecules (polarity), forming a hydration shell.

D. Plasticity and Cohesion

• Plasticity: The capability of being molded when wet and retaining shape when dry.
• Cohesion: The tendency of clay particles to stick to one another (related to hydrogen bonding and electrostatic attraction).
• Adhesion: The tendency of clay particles to stick to other objects.

E. Swelling and Shrinkage

• Some colloids (specifically expanding 2:1 clays like smectites) expand when wet as water enters the interlayer spaces and shrink when dry. This results in the formation of deep cracks in soils (e.g., Vertisols).

F. Brownian Movement & Tyndall Effect

• Brownian Movement: The continuous, random zigzag movement of colloidal particles suspended in water, caused by the collision with water molecules.
• Tyndall Effect: The scattering of light by colloidal particles, making a light beam visible when passing through a suspension.

G. Flocculation vs. Dispersion

• Flocculation: The aggregation of individual colloidal particles into clumps or "floccules." This is promoted by polyvalent cations (e.g., , ).
• Dispersion: The repulsion and scattering of particles, often caused by monovalent cations like Sodium (), leading to poor soil structure.

3. Types of Soil Colloids

Soil colloids are broadly classified into two groups based on their chemical composition.

I. Inorganic Colloids (Clay Minerals)

These are mineral-based and constitute the bulk of the colloidal fraction in most soils.

1. Layer Silicate Clays (Phyllosilicates)

These possess a crystalline structure made of sheets.

• Tetrahedral Sheet: Composed of silicon () and oxygen ().
• Octahedral Sheet: Composed of aluminum () or magnesium () and hydroxide ().

Classification based on Layer Structure:

2. Iron and Aluminum Oxide Clays (Sesquioxides)

• Prevalent in highly weathered soils (Tropics/Ultisols/Oxisols).
• Examples: Gibbsite (), Goethite (), Hematite ().
• Properties: Non-sticky, non-plastic, net positive charge at low pH (Acidic soils), low CEC but high Anion Exchange Capacity (AEC).

3. Allophane and Imogolite (Amorphous Clays)

• Derived from volcanic ash (Andisols).
• Non-crystalline (no ordered structural sheets).
• Very high capacity to adsorb phosphate and organic matter.

II. Organic Colloids (Humus)

• Humus is the stable, decomposed organic fraction.
• Structure: Unlike clays, humus is not crystalline. It consists of complex chains of carbon, oxygen, hydrogen, and nitrogen (polymers, phenolic rings).
• Charge: Possesses a very high net negative charge (higher than clays) derived from the dissociation of functional groups (carboxyl and phenolic ).
• Properties: High water holding capacity, dark color, promotes soil aggregation.

4. Origin of Charge on Soil Colloids

The ability of colloids to exchange ions depends on their charge.

A. Isomorphous Substitution (Permanent Charge)

• Occurs during the formation (crystallization) of clay minerals.
• An ion of similar size but lower valence substitutes a central ion in the lattice structure.
  • replaces in the tetrahedral sheet Net negative charge -1.
  • or replaces in the octahedral sheet Net negative charge -1.
• This charge is permanent and independent of soil pH. Common in 2:1 clays (Smectite, Vermiculite).

B. pH-Dependent Charge (Variable Charge)

• Occurs at the broken edges of clay crystals and on functional groups of humus and oxides.
• Mechanism:
  • Acidic conditions (Low pH): ions attach to surface hydroxyls (protonation), reducing negative charge or creating a positive charge.
  • Alkaline conditions (High pH): dissociates from or groups (deprotonation), creating a negative charge.
• Dominant source of charge in Kaolinite, Fe/Al oxides, and Humus.

5. Ion Exchange

This is the reversible process by which cations and anions are exchanged between the solid phase (colloids) and the liquid phase (soil solution).

Cation Exchange Capacity (CEC)

• Definition: The total sum of exchangeable cations that a soil can hold at a specific pH. Expressed in centimoles of charge per kilogram ().
• Importance: Indicator of soil fertility and nutrient retention capacity.
• CEC Hierarchy:
  • Humus () > Vermiculite () > Montmorillonite () > Illite ($30$) > Kaolinite () > Fe/Al Oxides ().

Anion Exchange Capacity (AEC)

• The capacity of soil colloids to hold anions (e.g., , , ).
• Occurs primarily in acidic soils rich in Fe/Al oxides and 1:1 clays (where pH-dependent positive charges develop).

6. Soil Organic Matter (SOM)

Definition

Soil Organic Matter encompasses all organic materials in the soil, including:

1. Living biomass (microorganisms, roots, fauna).
2. Detritus (undecomposed plant/animal tissues).
3. Humus (decomposed, stable organic material).

Decomposition of Organic Matter

Decomposition is a biological process performed by bacteria, fungi, and actinomycetes.

1. The Process

1. Breakdown: Large particles are shredded by soil fauna.
2. Catabolism: Microbes secrete enzymes to break down complex molecules (cellulose, lignin, proteins) into simpler compounds.
3. Mineralization: Conversion of organic compounds into inorganic (mineral) forms (e.g., Organic N ).
4. Immobilization: The reverse of mineralization; microbes consume inorganic nutrients for their own growth, making them temporarily unavailable to plants.

2. The Carbon:Nitrogen (C:N) Ratio

The relative amount of Carbon to Nitrogen determines the rate of decomposition and nutrient availability.

• Wide Ratio (> 30:1): (e.g., sawdust, straw). Microbes are N-starved. They consume available soil nitrogen, leading to Immobilization (Nitrogen deficiency for plants).
• Narrow Ratio (< 20:1): (e.g., legumes, manure). There is sufficient N for microbes; excess N is released into the soil. Leads to Mineralization (Nitrogen availability for plants).

Humus Fractionation

Humus is chemically complex and resistant to further decay. It is fractionated based on solubility in acids and alkalis:

1. Fulvic Acid: Lowest molecular weight, lightest color, soluble in both acid and alkali.
2. Humic Acid: Medium molecular weight, dark color, soluble in alkali but insoluble in acid.
3. Humin: Highest molecular weight, black, insoluble in both acid and alkali (highly resistant).

Functions/Significance of SOM

7. Comparative Summary: Clay vs. Humus