Soil Formation and Layers Overview

Soil Formation

• Soil is a layer of broken rock, organic matter, and water covering Earth's surface.
• Bedrock breaks down into smaller pieces through weathering.
• Organisms like bacteria, fungi, and insects live in the weathered material and contribute to humus formation when they die.
• Humus improves soil quality, encouraging more organisms to live and die, creating more humus.
• Soil quality improves over time.

Soil Layers

• Soil is made up of layers called horizons.
• The O horizon: Organic debris like leaf litter.
• The A horizon (topsoil): Mineral particles mixed with organic matter.
• The B horizon (subsoil): Accumulated compounds draining down from above.
• The C horizon: Partly weathered parent material.
• The R horizon: Bedrock.
• Polar soils have good drainage but are shallow, with little distinct layers. They are found in high latitudes.
• Temperate soils range widely, influenced by climate and topography. Desert soils are thin, have little organic matter, but high salt content. Tropical soils have little humus due to leaching.

Soil Types

• Soil types depend on climate, plants, animals, time, and the parent rock.
• Soil texture depends on the proportion of clay, silt, and sand particles in the soil.
• A soil textural triangle is used to classify soil types based on percentages of each particle.

Practice Questions

• Question 1: A soil with 60% clay, 10% silt, and 30% sand is a clay loam.
• Question 2: A soil with 20% clay, 60% silt, and 20% sand is a silt loam.
• Question 3: A soil with 30% clay, 10% silt, and 60% sand is a sandy clay loam.
• Question 4: Regolith on the moon isn't considered true soil because it lacks organic matter and biological activity—vital components of soil formation.
• Question 5: Residual soil forms from the weathering of the underlying rock at the site. Transported soil is moved from another location.
• Question 6: Factors influencing soil conditions:
  • Climate: Affects weathering rates, leaching, and plant life, impacting soil composition.
  • Topography: Affects water drainage, erosion, and sunlight exposure.
  • Parent rock: Influences the mineral composition and texture of the soil.