5 Soil Science and Soil Colloids

Introduction to Soil Science

  • Course: AGRI 51 - Principles of Soil Science

  • Institution: Division of Soil Science, Agricultural Systems Institute, CAFS, UPLB

Soil Colloids

  • Definition:

    • A colloid is a mixture where one substance consists of microscopically dispersed insoluble particles suspended throughout another substance.

  • Importance of Soil Colloids:

    • Seat of various chemical reactions in soils, crucial for soil fertility.

  • Characteristics:

    • Size: <0.001 mm (microscopic)

    • Chemical reactivity due to:

    • Large specific surface area

    • Presence of electric charges (+/-)

  • Types of Soil Colloids:

    1. Inorganic Colloids (Clays)

    2. Organic Colloids (Humus)

Types of Soil Colloids
I. Inorganic Colloids (Clay Colloids)

  • Composition:

    • Most clay minerals are aluminosilicates containing:

    • Silicon (Si) - 47%

    • Oxygen (O) - 27%

    • Aluminum (Al) - 8.13%

  • Grouping:

    • Structurally

    • Crystalline: Atoms arranged in a repeating order.

    • Amorphous (Non-crystalline): No definite long-range atomic arrangement.

  • Chemically:

    1. Crystalline silicate clays

    2. Crystalline non-silicate clays

    3. Amorphous silicate clays

    4. Amorphous non-silicate clays

Crystalline Silicate Clays

  • Characteristics:

    • Sheet-structured aluminosilicates

    • Different types based on silica to alumina ratio:

    • Kandites: 1 Silicon tetrahedral sheet + 1 Aluminum octahedral sheet (1:1 type)

      • Non-expanding, examples include Kaolinite and Halloysite

    • Smectites: 2:1 expanding type, e.g., Montmorillonite, nontronite, saponite, hectonite, sauconite

      • Expands when wet, shrinks when dry

    • Vermiculites: 2:1 limited expansion type

      • High net negative charge due to aluminum substitution, less expansion

    • Illites: 2:1 non-expanding type, often a weathering product of mica

    • Chlorites: 2:1:1 non-expanding type, magnesium hydroxide or brucite sheet

II. Amorphous Silicate Clays

  • Characteristically high in organic matter, derived from volcanic ashes (e.g., allophane, imogolite).

III. Non-Silicate Clays

  • Crystalline Non-silicate Clays: similar properties as silicate clays but lack silicon.

IV. Amorphous Non-Silicate Clays

  • Iron/Aluminum oxides:

    • Fe Oxides:

    • Hematite (Fe2O3) - red soil

    • Geothite (FeOOH) - yellow/brown soil

    • Al Oxides:

    • Gibbsite (Al(OH)3) - serve as high cation or anion exchange materials

Organic Colloids (Humus)

  • Constituents:

    1. Fulvic Acid:

    • Lowest molecular weight, soluble in both acid and alkali.

    1. Humic Acid:

    • Medium molecular weight and solubility profile.

    1. Humin:

    • Highest molecular weight, insoluble in both acid and alkali.

Conclusion

  • Understanding soil colloids is crucial for grasping soil chemistry and fertility, and it forms the basis for effective soil management practices.