Properties and Types of Soil Colloids

• Increasing pH (Deprotonation): As the soil becomes more alkaline, $\text{H}^{+}$ ions are pulled away from the surface hydroxyl groups, leaving behind a negative charge.
• Decreasing pH (Protonation): As the soil becomes more acidic, $\text{H}^{+}$ ions are added to the surface, which can create a positive charge.

Exam Tip: If a question asks which type of charge is most prevalent in organic matter (humus) or highly weathered oxides, remember that these materials rely heavily on pH-dependent charge rather than isomorphous substitution.

Types of Soil Colloids

Soil colloids are categorized into four main groups based on their chemical composition and structural organization.

1. Crystalline Silicate Clays (Phyllosilicates)

These are the most dominant colloids in most soils. They consist of layered sheets made of tetrahedral sheets (composed of $\text{Si}$ and $\text{O}$) and octahedral sheets (composed of $\text{Al}$, $\text{Mg}$, or $\text{Fe}$ with $\text{O}$ or $\text{OH}$). Based on how these sheets are stacked, they are divided into 1:1 and 2:1 clays.

1:1 Silicate Clays

A 1:1 clay consists of one tetrahedral sheet and one octahedral sheet per layer. These layers are held together by strong hydrogen bonding, which prevents water or ions from entering the space between layers.

• Example: Kaolinite is the most common 1:1 clay.
• Properties: Because the layers are so tightly bound, they do not expand when wet (non-expanding). They have low Cation Exchange Capacity (CEC) because they have very little isomorphous substitution and rely mostly on charge at the broken edges of the crystals.

2:1 Silicate Clays

A 2:1 clay consists of two tetrahedral sheets sandwiching one octahedral sheet. These are further divided into expanding and non-expanding groups.

Expanding 2:1 Clays: In these minerals, the layers are loosely bound, allowing water and cations to enter the interlayer space. This leads to significant shrink-swell behavior.

• Smectite (e.g., Montmorillonite): Known for very high CEC and high plasticity/stickiness.
• Vermiculite: An alteration product of micas with very high CEC, making it one of the most nutrient-rich clays.

Non-expanding 2:1 Clays: In these minerals, the layers are tightly bound, restricting the entry of ions or water.

• Fine-grained Mica: Contains $\text{Al}^{3+}$ substituted for $\text{Si}^{4+}$ in the tetrahedral sheet, creating a strong charge that holds potassium ($\text{K}^{+}$) tightly in the interlayer, making it nutrient-poor.
• Chlorite: A 2:1:1 structure where a "hydroxy sheet" (containing $\text{Mg}$, $\text{Al}$, or $\text{Fe}$) sits in the interlayer, "locking" the structure and restricting swelling.

2. Noncrystalline Silicate Clays

Unlike the neat, stacked sheets of crystalline clays, noncrystalline clays have an amorphous, poorly-structured arrangement of $\text{Si}$, $\text{Al}$, and $\text{O}$.

• Examples: Allophane and Imogolite.
• Properties: These often form from volcanic ash and possess high nutrient and water-holding capacities.

3. Iron and Aluminum Oxides

These colloids are essentially modified octahedral sheets that lack a tetrahedral $\text{Si}$ sheet. They are common in highly weathered soils.

• Examples: Gibbsite ($\text{Al}$ oxide), Goethite ($\text{Fe}$ oxide), and Hematite ($\text{Fe}$ oxide).
• Properties: They do not expand and can strongly adsorb anions.

4. Organic Particles (Humus)

Humus is the amorphous, non-crystalline fraction of organic matter produced during decomposition.

• Properties: It is arguably the most chemically active fraction of the soil. It possesses a very large net negative charge and a massive potential for Cation Exchange Capacity (CEC), allowing it to adsorb vast amounts of nutrients and water.

Summary of Colloidal Properties

The following table summarizes the key differences between the major soil colloids: