Soil formation is a complex process influenced by various factors. From physical and chemical weathering to biological activity, these processes work together to create the diverse soils we see today. Understanding how soils form is crucial for agriculture, construction, and environmental management.

Soil profiles reveal the intricate layers that develop over time. These horizons, from the organic-rich topsoil to the underlying bedrock, tell a story of soil development. Classification systems help us organize this complexity, providing a common language for soil scientists and land managers worldwide.

Soil Formation and Components

Process of soil formation

• Pedogenesis involves physical weathering breaking down rocks into smaller particles, chemical weathering altering mineral composition, and biological activity adding organic matter and influencing soil structure
• Parent material, climate, topography, organisms, and time affect soil formation (CLORPT factors)
• Organic matter contribution through decomposition of plant and animal residues forms humus and drives nutrient cycling
• Time allows accumulation of weathered materials, development of distinct soil horizons, and influences soil maturity and profile depth (young vs. mature soils)

Components of soil

• Mineral particles categorized by size: sand (2.0 - 0.05 mm), silt (0.05 - 0.002 mm), clay (< 0.002 mm)
• Organic matter includes living organisms, dead plant and animal residues, and humus
• Water forms soil solution and occupies pore spaces crucial for nutrient transport
• Air in pore spaces not filled with water enables root respiration and microbial activity
• Typical soil composition: ~45% mineral matter, ~5% organic matter, ~25% water, ~25% air

Soil Profiles and Classification

Soil horizons and characteristics

• O horizon: organic layer at surface with leaf litter and partially decomposed matter
• A horizon: topsoil rich in organic matter and zone of maximum biological activity
• E horizon: zone of eluviation where materials are leached downward
• B horizon: subsoil with accumulation of clay, iron, and aluminum oxides
• C horizon: partially weathered parent material or sediments with minimal soil development
• R horizon: solid bedrock underlying soil profile

Soil classification systems

• USDA Soil Taxonomy: hierarchical system with 6 levels (Order, Suborder, Great Group, Subgroup, Family, Series)
• 12 soil orders: Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, Vertisols
• Classification based on soil properties, diagnostic horizons, and soil temperature and moisture regimes
• Guides agricultural practices, aids urban planning, supports conservation, facilitates communication among soil scientists
• Other systems: World Reference Base for Soil Resources (WRB), various national systems