Key Soil Formation Processes and Pedogenesis Mechanisms

Fundamental Soil Formation Processes (Pedogenesis)

The following mechanisms drive the development and differentiation of soil profiles:

• Eluviation and Illuviation

  1. Eluviation: The migration of material (clay, humus, oxides) out of an upper horizon and into underlying horizons.
  2. Illuviation: The entry and accumulation of soil material, originating from upper layers, leading to features such as the argillic horizon.

• Chemical and Soluble Processes

  • Leaching: The loss of soluble anions and cations. Elements differ in their mobility; alkali metals and some anions exhibit higher rates of mobility.
  • Enrichment: The increase of specific compounds in certain parts of the soil profile. Example: If a sandy soil is irrigated with water containing clay in suspension, clay enrichment occurs.
  • Decalcification: The loss or migration of carbonates throughout the soil profile. This process, common in wetlands, requires the presence of water and carbon dioxide.
  • Calcification: The accumulation of carbonates, typically in the C horizon but also in others. It occurs primarily in arid or semi-arid climates.

• Salt and Sodium Dynamics

  • Salinization: The accumulation of soluble salts (Ca, Mg, K, Na, Cl, CO₃, SO₄). This occurs where vegetation is scarce and rapid organic matter decomposition leads to saturation of soluble salts.
  • Desalinization: The removal of salts by washing the soil, using high doses of water to dissolve and remove the salts, provided an adequate drainage system exists.
  • Alkalinization: The accumulation of sodium (Na) ions in the soil exchange complex.
  • Dealkalinization (Desodification): The removal of Na ions from the exchange complex. This requires chemical amendment followed by washing using high doses of water.
  • Sodification: Excess sodium accumulates in the ground, gradually entering the exchange complex, causing particle dispersion and soil compaction.

• Physical and Biological Mixing

  • Clay Translocation (Lessivage): The mechanical transfer of clay particles into the B horizon, resulting in clay enrichment.

  • Pedoturbation: Soil Mixing Processes

    A process of mixing within the ground, categorized into several types:

    • Provoked by soil fauna (e.g., ants, earthworms, moles, rodents, and humans).
    • Mixing caused by plants (phytoturbation).
    • Mixing resulting from freezing and thawing cycles (cryoturbation).
    • Mixing resulting from clay migration to lower horizons.
    • Displacement caused by constant movement within the ground.

• Climate-Driven Processes

  • Laterization (Ferralitization)

    Occurs under high temperature and high humidity. Abundant vegetation grows, generating large amounts of organic matter (OM) which is quickly decomposed. Acids dissolve iron (Fe) and aluminum (Al) oxides, leading to the formation of Fe₂O₃ (known as laterites or plinthite).

  • Podzolization

    Prevalent in cool and wet conditions. Formation of organic matter on the surface generates organic acids that cause the leaching of substances (bases, colloids, and iron and aluminum oxides). This results in an eluvial horizon (E) of bluish-gray ash, consisting predominantly of SiO₂.

  • Gleyzation

    Occurs under low temperature and high humidity. Abundant herbaceous vegetation leads to slow decomposition of organic matter and a large accumulation of peat. Below the peat is a water-saturated layer where Gleyzation occurs, transforming Fe₂O₃ into bluish-gray FeO.