1.1 Prelab Soil Formation and Factors Influencing Soil Properties

Objectives

  • Differentiate the soil-forming factors that influence the type of soil formed.
  • Identify visible differences among representative soil profiles from various locations in the Philippines and globally.
  • Define and characterize major soil horizons.

Definition of Soil

  • Soil is defined as:
    • A natural body formed on the Earth's surface from the weathering of rocks.
    • Composed of a mixture of inorganic and organic matter, water, and air.
    • Differentiated into horizons with varying physical, chemical, and biological properties compared to the parent material below (Weil & Brady, 2017; Jenny, 1994).

Soil Formation Process

  • The process of soil formation is known as Pedogenesis.
  • Pedogenesis is regulated by five factors:
    1. Climate
    2. Organisms
    3. Relief
    4. Parent Material
    5. Time
  • Formula by Hans Jenny: s = f(cl, o, r, p, t).
  • Soil formation leads to the development of layers called soil horizons.

Understanding Soil Profiles

  • Soil Profile: A vertical section of soil showing all horizons.
  • Soil Horizon: A layer differing in properties (physical, chemical, biological) from adjacent layers.

Factors of Soil Formation

  • Each factor contributes to the unique properties of soils in different regions:

    • Climate:

    • Influences leaching rates and horizon development.

    • Temperature: Higher temperatures increase weathering rates significantly (every 10°C doubles reaction rates).

    • Rainfall: Higher rainfall promotes leaching and accelerates weathering processes.

    • Organisms:

    • Affects soil through organic material accumulation and decomposition, influencing horizon thickness and color.

    • Bioturbation: Mixing of soil by living organisms (e.g., vegetation).

    • Different vegetation impacts soil characteristics:

      • Tropical forests develop thin surface horizons; grasslands create thick, dark horizons due to organic contribution.

    • Relief/Topography:

    • Position in the landscape impacts soil formation speed.

    • Soils at higher elevations (summit, shoulder) develop faster than those on backslope positions.

    • Deep water tables indicate well-drained soils; shallow water tables can lead to poor drainage classified by redoximorphic features (color changes due to water saturation).

      • Mottles: Indicate poor drainage conditions (red and gray colors reflect iron oxidation/reduction).

    • Parent Material:

    • Underlying geological material affects soil horizon formation:

      • Residual Parent Material: Formed in place (e.g., igneous rocks).
      • Transported Parent Material: Classified by transport mechanisms (water, wind, gravity, ice).

    • Time:

    • Refers to the development extent of soil rather than age in years.

    • Old soils have well-developed horizons; young soils show minimal development.

    • Factors that speed up development include:

      • Permable parent material, warm humid climates, forest vegetation, well-drained slopes.

    • Retarding factors:

      • Impermeable parent materials, cold-arid climates, less vegetation, steep slopes.

Human Impact on Soil Formation

  • Man as a Factor:
    • Human activities influence soil formation, termed anthrosolization (e.g., intensive farming practices, irrigation).
    • Classification of anthropogenic soils is a subject of ongoing debate among scientists.

Master Horizon Designations

  • Horizons refer to distinct soil layers:
    • O: Organic matter dominated.
    • A: Organic-rich surface zone.
    • E: Zone of maximum eluviation.
    • B: Zone of illuviation.
    • C: Weathered parent material.
    • R: Consolidated bedrock.

Transitional Horizons

  • AB: Between A and B.
  • BA: Between B and A.
  • AC: Between A and C.
  • EB: Between E and B.
  • BE: Between B and E.
  • BC: Between B and C.

Subhorizon Symbols

  • Notations for specific soil characteristics:
    • a: Highly decomposed organic matter.
    • i: Slightly decomposed organic matter.
    • m: Hardpan layers.
    • n: Sodium accumulation.
    • p: Layer disturbed by plowing.

References

  • Weil, R.R., & Brady, N.C. (2017). The Nature and Properties of Soils, 15th ed.
  • Jenny, H. (1994). Factors of Soil Formation: A System of Quantitative Pedology.
  • Additional sources include works by Dudziak, Crutzen, and Yaalon.