Weathering and Soil Formation

Overview of Weathering and Soil Formation

Definition of Weathering
- Necessary process for soil formation and the availability of essential nutrients for biosphere.
Types of Weathering
- Chemical Weathering: Involves chemical reactions that alter minerals.
- Physical Weathering: Physical breakdown of rocks.
Impact of Weathering
- Produces loose debris and minerals (e.g., clay minerals).
- Releases ions and elements into water, facilitating percolation.

Weathering
- Initial breakdown of pre-existing geological materials, resulting in loose debris.
Role of Water
- Rainwater Movement: Carries clay and ions deeper into the soil system.
- Development of soil zones:
  - Zone of Leaching: Top layer, characterized by:
  - Removal of fine grains and nutrients.
  - Results in porous, nutrient-poor topsoil.
  - Zone of Accumulation: Below leaching zone:
  - Deposits fine materials and nutrients, leading to denser soil layers.
  - Excessive leaching can deplete nutrients in the topsoil.
Biological Action
- Contributions from:
  - Microbes, plants, fungi (e.g., earthworms).
- Organic matter production: named humus.
- Nutrient Cycling:
  - Production of acids (e.g., humic acid), absorption of nutrients, deposition of organic waste, interaction with soil structure.
- Fertile soils consist of:
  - 50% mineral debris and humus.
  - 50% pore spaces and water.

Overview of Soil Horizons
- Different processes occur at varying soil depths, creating identifiable layers.
O Horizon (Organic Layer)
- Thin top layer, predominantly made up of organic debris and humus.
A Horizon (Topsoil)
- Below the O horizon; richer in humus (~30% humus) and minerals (clay, silt, sand).
- Continuous chemical weathering present.
E Horizon (Eluviation Layer)
- Characterized by leaching.
- Often lacking organic material; lighter color compared to A horizon.
B Horizon (Subsoil)
- Area of accumulation from above horizons, often rich in clay and moisture.
C Horizon (Parent Material)
- Made of weathered rock; no significant leaching.
- Represents the transition to unweathered bedrock.

Climate
- High rainfall and warm temperatures enhance soil production via increased weathering and biological activity.
Parent Rock
- Type of rock influences nutrient availability (e.g., basaltic rock contains more iron than granitic rock).
Slope Steepness
- Steeper slopes typically result in thinner soils due to gravitational movement of materials.
Soil Wetness
- Proximity to water table increases organic matter and soil development.
Time
- Soil formation is a slow process; thick soils take thousands of years to develop; young soils are generally thin.
Vegetation Type
- Different root systems can stabilize soil and contribute to the overall soil profile.

Mollisols: Prairie soils, fertile with rich humus content.
Alfisols: Forest soils, typically under broadleaf forest cover.
Gelisol: Soils in polar regions, very young and thin due to permafrost.
Calcrete Formation: In arid regions, calcium carbonate layers form due to low moisture.

Tropical Soils
- Characterized by deep zones of accumulation due to heavy rainfall and leaching.
- Topsoil may be thin; examples include soils in Jamaica with high aluminum ore (bauxite).
Temperate Soils
- More balanced profiles; adequate layers of humus and mineral accumulation.

Fragility of Soils
- Soils are sensitive to environmental changes; human activities can lead to rapid degradation.
Examples of Soil Degradation
- Overgrazing: Leads to loss of protective root systems.
- Clear-Cutting: Results in erosion and nutrient loss.
- Farming Practices: Conventional tilling can lead to nutrient depletion and increased erosion.
Soil Erosion Rates: Can increase by 10 to 100 times due to human activities; as much as 6 tons per acre may be lost annually.
Historical Example: Dust Bowl (1930s)
- Resulted from unsustainable farming practices leading to loss of topsoil and ecological disaster.
- Highlighted the need for responsible soil management practices.