Soil Composition, Properties, Formation, and Risks

Posted on Nov 29, 2024 in Geology

Theme 6: Soil Processes and Risk

1. Characteristics of a Soil Profile

The study of soils requires the opening of a cost sheet to find bedrock. Soils are structured in horizontal layers, called horizons, which together constitute the soil profile.

  • O Horizon: The outermost, superficial layer, rich in humus (organic matter decomposition) and living organisms. Also called the vertical washing area, because water-soluble compounds and mineral fragments are dragged toward the horizon below. Provides nutrients for plants and is the arable layer in agricultural soils.
  • B Horizon: Intermediate layer, clearer but less weathered, with less humus. Fewer plant roots reach this layer, and it has lower atmospheric gas content. Also known as the zone of illuviation or precipitation.
  • C Horizon: Composed of weathered bedrock fragments without humus, limited atmospheric gases, and few roots. It is a transition zone.

Below the C horizon is the unaltered bedrock.

2. Components of Soil

Soils are composed of solid, liquid, and gaseous substances.

  • Solid Phase: Consists of inorganic and organic matter.
  • Liquid Phase: Primarily an aqueous solution of common salts. It is the vehicle transporting chemical substances.
  • Gaseous Phase: Formed by atmospheric gases: O2 and CO2.

Solid soil components are classified into inorganic and organic:
Inorganic: Originate from the chemical and physical weathering of rocks in the Earth’s surface layer. The main materials are inorganic silicates (clays and sands). Hydroxides and oxides are also abundant.

  • Clays: Exhibit expansive behavior.
  • Metal Oxides and Hydroxides: Iron oxide (hematite) gives soil a reddish color. Aluminum hydroxide is another example.
  • Carbonates: (Calcite, dolomite) are characteristic of arid and semiarid areas, appearing whitish.
  • Sulfates: Gypsum is found in arid climates.
  • Chlorides: High chloride content is detrimental to plants.

Organic: Transformed remains of living organisms, fermentation products, and humus.

  • Plants: The main source of organic matter in soil. Plant remains decompose on the surface and gradually incorporate into the soil.
  • Animals
  • Microorganisms: Contribute through excretion of metabolic substances.

Mineralization: Decomposers (bacteria, fungi) utilize dead biomass as an energy source, converting organic matter into inorganic compounds.

3. Properties of Soil

  • Texture: Refers to the size of mineral particles. Clay is the finest fraction, silt is intermediate, and sand is the coarsest.
  • Structure: The grouping of mineral particles into larger units called aggregates.
  • Porosity: The space not occupied by solid particles, expressed as a percentage of volume. Porosity = (pore volume / total soil volume).
  • Water Content: Depends on rainfall, water retention, and drainage.
  • Temperature: Determined by heat input, especially solar radiation, and heat loss components. This property influences the soil’s climate and moisture.
  • Color: Dark color indicates high organic matter; light color indicates low organic matter. Reddish hues suggest iron or manganese oxides; white indicates calcium carbonates.
  • Depth
  • Salinity: Soils can contain various soluble salts. High salt accumulation, through a process called salinization, can be detrimental.

4. Soil Types: Soil Classification

Fully-developed soils include laterite, loess, steppe-desert soils (chernozems), gray forest soils, podzols, and tundra soils.
Intrazonal soils include alkaline soils and peat soils (torbera).
Immature soils include skeletal soils and alluvial soils.
Climate is usually the determining factor in soil type classification.

5. Factors Determining Soil Formation

  • Building Factors: Climate, weathering, organisms, time.
  • Destroying Factors: Impacts (pollution, deforestation, etc.), erosion.
  • Fixed Factors: Terrain slope and bedrock.

When building factors dominate, the soil develops. If destroying factors prevail, the soil becomes thinner.

6. Risks: Erosion and Desertification

Erosion and desertification are significant risks, leading to land degradation and loss of fertile soil.
Soil Erosion:
Different types of erosion exist: water erosion, glacial erosion (by ice), wind erosion (aeolian), biological erosion (caused by living organisms), and anthropic erosion (caused by humans).
Erosivity: The erosive capacity of the predominant geological agent (water, wind) in a given area.
Erodibility: The vulnerability of soil to degradation, depending on soil type, terrain, and vegetation.