Understanding Soil Composition, Factors, and Horizons

Posted on Feb 24, 2025 in Geology

Soil, whether single-layer or composed of multiple layers, consists of three primary elements: solid components, liquids, and gases. The solid parts are comprised of minerals, particles from rock erosion, and organic matter.

Factors Influencing Soil Formation

The alteration of the terrestrial mineral medium is influenced by climate and living organisms. Soil formation begins with the physical disintegration of rocks through contact with the atmosphere, allowing vegetation to establish. Key factors include:

  • Substrate Rock: The parent rock influences the soil’s texture, color, and structure.
  • Climate: Precipitation affects biological and chemical activity, influencing dissolution and leaching. Excessive rainfall can impoverish the soil, while temperature affects bacterial activity.
  • Topography: Steep slopes favor erosion, while plants contribute to soil fertility through their root systems.
  • Living Organisms: Bacteria and fungi decompose organic matter, forming humus, which gives soil its brown or dark color. Some bacteria fix nitrogen, making it available to plants.
  • Human Impact: Human activities can alter and degrade soil or improve it through fertilization and reforestation.
  • Time: Soil formation requires centuries.

Soil Profile and Horizons

Soils are structured in profiles, with distinct horizons. In depth, the C horizon is found, built from weathered parent rock. The surface horizon consists of organic matter, a decomposition zone, and a leaching zone where substances are carried away by precipitation to lower layers (horizon B).

Types of Soil

Zonal Soils

  1. Oceanic Climate Soils: These soils are well-developed and rich in organic matter. Constant rainfall leads to leaching of surface layers, resulting in acidic and siliceous soil. Vegetation is often dominated by species like eucalyptus or pine. Lime is used to counteract acidity, and the land is often dedicated to pasture. Rankers are found in higher areas with steeper slopes, supporting pastures and forests. Humid brown soil is excellent for corn crops and meadows.
  2. Mediterranean Climate Soils:
    • Silicate Rocks: The soil is brown and poor due to acidity and lack of humus, often used for grazing.
    • Limestone Rocks: A climatic arxil horizon forms due to calcium carbonate dissolution. The red Mediterranean soil is rich in nutrients and suitable for various crops. Rocky outcrops can hinder mechanization, leading to marginal forests and crops. Black soils are fertile and used for cultivation in regions like Badajoz and Pamplona.
    • Chalky Soils: In dry, sub-desert areas, the soil is often bare due to salinization problems from evaporation. Rainfed agriculture is limited.

Azonal and Intrazonal Soils

Azonal soils have not had sufficient time to form, often located on slopes. Intrazonal soils are found in areas with specific characteristics, such as Renzino on acalareas, alluvial soils along rivers, endorheic waterlogged areas, and saline soils in marshes. Volcanic soils are often unproductive.