Soil Profiles and Horizons

Soil Profiles and Horizons

• Soil is a vital resource supporting plant growth and providing a habitat for many organisms
• Soil is made up of layers called horizons
• These horizons vary in colour, texture, and composition
• Together, these layers form the soil profile

Soil Horizons

• O Horizon:
  • Organic layer, composed of decomposed plant matter and humus
  • Found on the surface
  • Main function is to retain water and nutrients for plant growth
  • Supports plant growth by acting as a reservoir for water and nutrients
• A Horizon:
  • Topsoil
  • Rich in organic material and minerals
  • Site of most root activity
  • Rich in organic matter and nutrients which are crucial for plant development and root growth
  • Contains most of the organisms involved in the decomposition process
• E Horizon:
  • Eluviation (leaching) layer
  • Materials like clay, iron, or organic matter are removed
  • Materials are washed downward due to water movement
  • The eluviation layer is where water movement removes materials upwards
• B Horizon:
  • Subsoil
  • Leaching materials accumulate here
  • Often rich in clay and minerals
  • Accumulates materials leached from the overlying layers
  • Frequently shows signs of accumulation of material lost from overlying layers
• C Horizon:
  • Parent material
  • Partially weathered rock or unconsolidated sediments
• R Horizon:
  • Bedrock
  • Unweathered rock layer beneath the soil

Visualizing the Soil Profile

• A graphic representation of soil horizons in order from top to bottom. Illustrates the layers and their main characteristics
• A diagram shows the different soil layers and their characteristics
• The sequence of layers in a soil profile is a good guide to understanding its development
• The diagram visually clarifies the transition between layers and their distinct characteristics

Review Questions

• Q1: O Horizon's main function: The organic matter layer helps retain water and nutrients for plant growth
• Q2: Why is the A Horizon important for plant growth?: Rich in organic matter and nutrients, critical for root growth and plant development. Its high biological activity supports root growth.
• Q3: Which horizon accumulates leached materials?: The B Horizon collects materials leached from the overlying layers. It accumulates these leached materials.
• Q4: Difference between C and R Horizons: The C Horizon is partially weathered parent material, whereas the R Horizon is unweathered bedrock.
• Q5: Eluviation process in the E Horizon: Materials (e.g., minerals, organics) are washed downward into lower layers due to water movement. This is a process where water percolates through layers, removing certain substances.