3.2 Soil horizons and profiles

Soil horizons and profiles are the building blocks of understanding Earth's skin. They reveal the complex interplay of physical, chemical, and biological processes that shape our planet's surface over time. These layers tell a story of soil formation, offering clues about climate, geology, and land use history.

From the organic-rich O horizon at the surface to the weathered C horizon near the bedrock, each layer has unique characteristics. By studying these horizons, we can unlock secrets about soil health, fertility, and environmental conditions, crucial for agriculture, engineering, and ecosystem management.

Soil horizons and development

Formation and significance of soil horizons

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• Soil horizons develop through physical, chemical, and biological processes over time
• Represent stages in soil formation providing information about soil history, composition, and environmental conditions
• Crucial for understanding pedogenesis (soil formation and development process)
• Serve as indicators of soil health, fertility, and potential land use capabilities
• Reflect complex interactions between climate, parent material, topography, organisms, and time
• Aid in assessing soil quality and suitability for various agricultural and engineering purposes
• Provide insights into past and present environmental conditions affecting the soil

Processes shaping soil horizons

• Weathering breaks down parent material into smaller particles and releases nutrients
• Leaching moves dissolved materials and fine particles downward through the soil profile
• Translocation redistributes soil components between horizons (clay particles, iron oxides)
• Organic matter accumulation and decomposition influence the formation of surface horizons
• Bioturbation mixes soil materials through root growth and animal activity
• Mineral transformations alter the chemical and physical properties of soil components
• Redox reactions occur in waterlogged conditions, leading to distinctive soil features (mottling, gleying)

Major soil horizons

Surface and near-surface horizons

• O horizon comprises fresh and partially decomposed organic matter at the soil surface
  • Thickness varies depending on vegetation type and decomposition rate
  • Subdivided into Oi (litter), Oe (partially decomposed), and Oa (highly decomposed) layers
• A horizon (topsoil) rich in organic matter and maximum biological activity
  • Dark color due to humus content
  • Important for nutrient cycling and water retention
  • Often has granular or crumb structure promoting good aeration and root growth
• E horizon characterized by eluviation (leaching) of clay, iron, and aluminum oxides
  • Light-colored layer due to loss of darkening agents
  • Often found in forest soils or areas with high precipitation
  • May be absent in some soil profiles due to mixing or erosion

Subsurface horizons and parent material

• B horizon (subsoil) accumulates materials leached from upper horizons
  • Enriched in clay, iron, and aluminum compounds
  • Often has blocky or prismatic structure
  • May contain distinct features like clay films or iron concretions
• C horizon consists of partially weathered parent material
  • Retains much of the original rock structure
  • Less affected by soil-forming processes than overlying horizons
  • Important for understanding the soil's mineralogical origin
• R horizon represents underlying bedrock
  • Unweathered and coherent rock material
  • Marks the lower boundary of the soil profile
  • Influences soil depth and drainage characteristics

Transitional and specialized horizons

• Transitional horizons (AB, BC) exhibit properties of two adjacent horizons
  • Important for understanding gradual changes in the soil profile
  • Reflect the continuous nature of soil development processes
• Calcic horizons accumulate calcium carbonate in arid or semi-arid environments
• Spodic horizons form in acidic, sandy soils through the accumulation of organic matter, aluminum, and iron
• Argillic horizons show significant clay accumulation through illuviation
• Fragipans are dense, brittle subsurface layers that restrict water and root penetration

Interpreting soil profiles

Analyzing horizon characteristics

• Thickness of horizons indicates intensity and duration of soil-forming processes
• Color provides clues about organic matter content, iron oxidation state, and drainage conditions
  • Dark colors often indicate high organic matter (A horizon)
  • Red or yellow colors suggest iron oxide presence (well-drained conditions)
  • Gray or mottled colors indicate poor drainage or reducing conditions
• Texture reflects particle size distribution and influences water-holding capacity and nutrient retention
  • Sand (2.0-0.05 mm), silt (0.05-0.002 mm), clay (<0.002 mm)
• Structure describes the arrangement of soil particles into aggregates
  • Types include granular, blocky, prismatic, and platy
  • Affects water movement, root growth, and soil aeration

Identifying soil-forming processes

• Eluviation and illuviation processes reflected in movement of materials between horizons
  • Clay accumulation in B horizon (argillic horizon formation)
  • Organic matter and sesquioxide movement in podzolization
• Organic matter accumulation and decomposition rates evident in depth and darkness of A horizon
  • Mollic epipedons in grassland soils vs. thinner A horizons in forest soils
• Weathering intensity inferred from degree of parent material alteration in lower horizons
  • Development of distinct horizon boundaries
  • Presence of secondary minerals (clay minerals, iron oxides)
• Bioturbation effects visible in mixing and homogenization of soil materials
  • Root penetration creating channels and pores
  • Animal burrowing leading to horizon mixing (krotovinas)
• Redoximorphic features indicate periods of water saturation and oxygen depletion
  • Mottling patterns of red, yellow, and gray colors
  • Gleying in permanently waterlogged soils

Soil profiles: Climate vs geology

Climate-driven soil profile variations

• Arid and semi-arid regions exhibit minimal horizon development
  • Accumulation of salts or carbonates in subsurface horizons (calcic or salic horizons)
  • Often have thin A horizons due to low organic matter input
• Humid tropical regions display deeply weathered soil profiles
  • Thick B horizons rich in clay and oxides (oxisols)
  • Often lack distinct A horizons due to rapid organic matter decomposition
  • Intense leaching leads to nutrient-poor, acidic soils
• Temperate forest soils show well-developed horizons
  • Prominent A horizon with organic matter accumulation
  • Evidence of clay translocation in B horizon (alfisols, ultisols)
  • Spodosols in coniferous forests with distinct E and spodic horizons
• Permafrost-affected soils in polar regions have cryoturbated profiles
  • Disrupted horizon sequences due to freeze-thaw cycles
  • Accumulation of organic matter in surface horizons (cryosols)

Geologic influences on soil profiles

• Young landscapes (recently glaciated areas) have weakly developed horizons
  • Retain many characteristics of parent material
  • Entisols or Inceptisols with minimal horizon differentiation
• Volcanic soils exhibit unique profiles with andic properties
  • Rapid weathering of volcanic ash forms amorphous minerals (allophane, imogolite)
  • High organic matter retention and phosphorus fixation capacity
• Soil profiles on steep slopes may be truncated or have colluvial materials
  • Influence of erosion and deposition processes
  • Thinner A horizons on upper slopes, thicker accumulations on lower slopes
• Limestone-derived soils often have high clay content and neutral to alkaline pH
  • Terra rossa soils with red, clay-rich B horizons
  • Potential for karst topography development
• Floodplain soils show stratified profiles due to periodic sediment deposition
  • Buried horizons and abrupt textural changes
  • Often have high fertility due to nutrient-rich alluvial deposits