SES 401 Soil Science II - Comprehensive Study Notes on Soil Colloids, Structure, and Surface Charges

Course Overview and Introduction to Soil Science II

• Course Title: Soil Science II

• Credit Hours: 3(2-1)

• Course Number: SES 401

• Institution: Institute of Soil and Environmental Sciences (ISES), University of Agriculture Faisalabad, Pakistan

• Key Topics Covered:

  • Soil colloids: definition and types.

  • Definition and description of basic structural units.

  • Sources of negative charges on clays and humus.

  • Cation and anion exchange.

  • Cation Exchange Capacity (CEC): Definition, units, and factors affecting it.

  • Soil pH: Definition, the pH scale, and its overall significance.

Classification of Soil Particles by Size

• Soil particles are classified based on their diameter (mm):

  • Sand: $2$ to $0.02\,mm$

  • Silt: $0.02$ to $0.002\,mm$

  • Clay: Less than $0.002\,mm$

Definition and Properties of Soil Colloids

• General Definition: A colloid is a solid substance whose particles are extremely small but possess a very large surface area.

• Specific Characteristics of Soil Colloids:

  • Particle Size: They consist of very small particles, typically $\le 1\,\mu m$.

  • Surface Area: They have a large surface area per unit mass. For example, the external surface area of one gram of colloidal clay is at least 1,000 times greater than that of one gram of coarse sand.

  • Visibility: They are too small to be seen using an ordinary light microscope.

  • Surface Charge: Colloids possess either a positive or negative charge on their surface.

  • Adsorption: Both ions and molecules are adsorbed onto colloidal surfaces.

• Surface Area Exercise: A structure (3x3 cm) is presented to consider the calculation of total surface area across three different structural configurations.

Importance and Functions of Soil Colloids

• Soil colloids are considered the "heart of soil activities." Their importance includes:

  1. Nutrient Retention: They release and retain nutrients through the process of ion exchange.

  2. Soil Structure: They play a critical role in the development of soil structure and modify the effects of soil texture on plant growth and water retention.

  3. Physical Properties/Engineering: They are vital in soil management and the construction of roads, buildings, and airports due to their shrinkage and swelling properties.

  4. Environmental Management: They play an important role in pollution management.

Types of Soil Colloids

• The soil colloidal system is composed of two primary types of colloids:

  1. Inorganic Colloids (Clay): These include silicate clays and various oxides.

  2. Organic Colloids (Humus): Microscopic particles of organic matter.

• Sub-categories of Inorganic Colloids:

  • Layer silicate clays (Phyllosilicates).

  • Iron and aluminum oxide and hydroxide clays.

  • Allophanes and associated amorphous clays.

Structural Units of Layer Silicate Clays (Phyllosilicates)

• The term Phyllosilocates comes from the Greek word phyllon, meaning "leaf."

• Tetrahedral Sheets:

  • Basic Unit: The tetrahedron.

  • Structure: Consists of a central $Si^{4+}$ ion surrounded by four equidistant oxygen atoms, forming a four-sided structure.

  • Formation: Many tetrahedral units link together horizontally to form a tetrahedral sheet, also known as a silica tetrahedral sheet.

• Octahedral Sheets:

  • Basic Unit: The octahedron.

  • Structure: Consists of an $Al^{3+}$ ion surrounded by six equidistant oxygen atoms or hydroxyl ions, forming an eight-sided structure.

Classification and Sequence of Silicate Clay Structures

• Structural Sequence: Tetrahedron/Octahedron $\rightarrow$ Tetrahedral/Octahedral Sheet $\rightarrow$ Combined Layer.

• Anatomy of a Layer:

  • Layers are composed of planes: the O plane, Si plane, and Al or Mg plane.

  • Apical Oxygen: Points into the structure.

  • Basal Oxygens: Form the base of the tetrahedral sheet.

  • Interlayer: The space between layers where adsorbed cations and water molecules reside.

• Groups of Silicate Clays:

  1. 1:1 Type Silicate Clay: Consists of a single silica sheet (tetrahedral) attached to a single alumina sheet (octahedral). Example: Kaolinite.

  2. 2:1 Type Silicate Clay: Consists of one alumina sheet (octahedral) sandwiched between two silica sheets (tetrahedral). Examples: Vermiculite, Smectite, and Mica.

  3. 2:1:1 or 2:2 Type Clay Minerals: Consists of a 2:1 layer with a magnesium-dominated tri-octahedral sheet. Example: Chlorite. Brucite is the associated Mg mineral.

Specific Minerals and regional Context

• Iron (Fe) and Aluminum (Al) Oxides:

  • Also known as sesqui-oxides.

  • Dominant in soils formed from Fe and Al rich parent material, typically in tropical and semitropical regions where silica has been leached away.

  • Characteristics: Poor crystalline structure; low nutrient holding capacity due to low CEC ($0-4\,cmol_c\,kg^{-1}$).

  • Iron Examples: Limonite, Goethite, Hematite, Magnetite.

  • Aluminum Examples: Gibbsite, Boehmite, Alumina.

• Clay Minerals in Pakistani Soils: The sequence of dominance is recorded as:

  • Illite $\gg$ Kaolinite > Montmorillonite > Chlorite > Vermiculite.

• Allophanes and Amorphous Minerals:

  • Most Common: Allophane.

  • Composition: Hydrated $Al_2O_3$, $Fe_2O_3$, and $SiO_2$.

  • Source: Found in volcanic soils (Andisols) in regions like New Zealand, Japan, and Hawaii.

  • Properties: CEC ranges from $75-150\,cmol_c\,kg^{-1}$ (highly pH dependent). High surface area depends on the degree of crystallinity.

Organic Soil Colloids (Humus)

• Composition: Composed of carbon ($C$), hydrogen ($H$), and oxygen ($O$) rather than the Al, Si, and O found in silicate clays.

• Properties:

  • Size: $0.1-1.0\,\mu m$.

  • Shape: Variable and amorphous (lacks a clear crystalline shape).

  • Surface Area: $500-800\,m^2\,g^{-1}$.

  • CEC: High capacity of $200-750\,cmol_c\,kg^{-1}$.

  • Stability: Not as stable as inorganic soil colloids.

Sources of Negative Charges on Soil Colloids

• Permanent (Constant) Charge:

  • Isomorphic Substitution: The process by which one element replaces another element of similar size but different charge within the crystal structure without disrupting it.

  • Mechanism: A lower-valent cation (e.g., $Mg^{2+}$, $Fe^{2+}$, $Zn^{2+}$) replaces a higher-valent cation (e.g., $Al^{3+}$ in octahedral sheets, or $Al^{3+}$ replacing $Si^{4+}$ in tetrahedral sheets). This results in a net decrease in positive charge, leading to an excess of negative charge.

  • Occurrence: Main source of charge in 2:1 type minerals; can also occur in 1:1 minerals.

  • Broken Edges: Charges produced at the edges of silica crystals where oxygen ions have unsatisfied bonds.

• Variable (pH-Dependent) Charge:

  • Protonation/Deprotonation: Dependent on the activity of $H^+$ in the soil solution.

  • Mechanism: Addition of $H^+$ (protonation) or loss of $H^+$ (deprotonation) from surface functional groups.

  • Functional Groups:

    • Inorganic: Hydroxyl ($-OH$) groups.

    • Organic (Humus): Carboxyl ($-COOH$), Phenolic ($-C_6H_4OH$), and Amine ($-NH_2$) groups.

Chemical Reactions and pH Effects

• pH Effects on Inorganic Colloids:

  • High pH (> 7): $\text{Al-OH} + OH^- \rightarrow \text{Al-O}^- + H_2O$ (Results in negative charge).

  • Intermediate pH: Neutral charge.

  • Very Low pH (< 7): $\text{Al-OH} + H^+ \rightarrow \text{Al-OH}_2^+$ (Results in positive charge).

• pH Effects on Organic Colloids:

  • At high pH, the hydrogen from the carboxyl group ($R-COOH$) dissociates: $R-COOH \rightarrow R-COO^- + H^+$.

  • The loss of the hydrogen ion into the solution leaves a carboxylate ion with one negative charge.

• Comparison of Smectite vs. Humus:

  • Smectite: Below pH 6, the charge is mostly constant (permanent) due to isomorphic substitution. Above pH 6, pH-dependent charge increases slightly due to the ionization of hydrogen from hydroxyl ions at the crystal edges.

  • Humus: The charge is entirely considered to be pH dependent.