Soil Composition, Formation, and Classification

Posted on Jan 23, 2025 in Geology

Soil: Composition, Formation, and Classification

Soil is a cover or mantle of superficial alteration, consisting of minerals, air, water, organic debris, and living organisms. It develops in the area of interaction between the geosphere, hydrosphere, atmosphere, and biosphere.

Composition

  • Mineral Fraction: Fragments of bedrock, colloidal particles, ions, and minerals.
  • Organic Fraction: Results from the decomposition of biological remains through two main processes:
    • Humification: Decomposes organic remains and transforms them into organic molecules, such as humic acids, which form humus.
    • Mineralization: The maximum decomposition of organic molecules, transforming them into inorganic molecules.

Formation and Evolution

  • Horizon A: Consists of organic matter mixed with mineral matter. Rainwater washes and transports materials such as clay, carbonate, calcium, and iron oxide to deeper layers through eluviation.
  • Horizon B: Accumulation (illuviation) of materials transported from Horizon A.
  • Horizon C: Consists of rock fragments.
  • Horizon R and D: The bedrock from which the soil originates.
  • Horizon E: A layer that has suffered significant leaching.

Factors Influencing Soil Formation

  1. Type of Parent Rock: According to its permeability, composition, and granulometry.
  2. Composition of Biological Cover: Depending on the type of organic matter and plant debris, it will cause either humification or mineralization.
  3. Climate: Thermal and water regimes in the climate control the availability of water, the dominant type of weathering, and the composition of the cover.
  4. Time: A sufficient period is necessary to develop a full soil profile.
  5. Topographic Factors: Depends on the slope of the terrain and altitude. Flat terrain may have more evolved soils, while very steep terrain predominates in surface runoff and underdeveloped soils.

Climate or Zonal Classification

  1. Zonal Soils: Their characteristics fully meet the conditions of the bioclimatic zone (biomes), differentiating between soils of tundra, taiga, temperate moist forest, prairie, Mediterranean, steppe, warm desert areas, damp savanna, and equatorial forest.
  2. Intrazonal Soils: Bioclimatic analogies are only partially met. These include saturated soils (excess water), halomorphic soils (saline substrate), and calcimorphous soils (limestone substrate).
  3. Azonal Soils: Do not show characteristics of the bioclimatic region in which they develop. Notable examples are floodplain soils and sandy soils.

Genetic Classification

Depending on the manner in which the organization’s training process occurs, this classification seeks the soils in the footsteps of the factors that have influenced the training process. Among the most widely used are humification, the intensity of the weathering of the parent rock, and the time the soil takes to evolve.

Analytical Classification (According to Diagnostic Horizon)

  • Mollic Horizon: Rich in humus and dark in color.
  • Ochric Horizon: Light color and little organic matter.
  • Argillic Horizon: Horizon B with an accumulation of clay.
  • Salic Horizon: Accumulation of salts.

Orders

  • Entisols: Young, mineral soils with low organic matter and few horizons.
  • Alfisols: Profile with an argillic horizon (clay) in humid environments.
  • Vertisols: Clay soils.
  • Mollisols: Mollic horizon (rich in organic matter).
  • Aridisols: Very dry and saline soils of arid areas.

Soils of Spain

  • Mediterranean Atmosphere and Sandy-Clay Substrate: The most abundant and least evolved (non-calcium and red-brown soils, reddish semiarid soils, soils originating from clay).
  • Mountainous and Limestone Substrates: Brown limestone and forest lands.
  • Predominantly Silicon Substrate: Southern brown land.
  • Floodplain Soils of Rivers: Meadow soil.
  • Soils from Areas with Recent Volcanic Origin Substrates: Andosols.

Desertification

Desertification occurs when natural phenomena transform productive substrates into unproductive ones. It is soil degradation in arid, semiarid, and subhumid areas, which takes place due to various factors, including climatic variations and human activities not aimed at the sustainability of the ecosystem.