Soil Lecture 6, Soil Classification —

Horizon structure and soil profile basics

• Purpose of the unit: understand soil horizons, how they form, how they are named, and how soils are classified into orders, suborders, etc. Emphasis on field identification, diagnostic horizons, and the move from field observations to the USDA Soil Taxonomy system (and related World Reference Base).

Horizon nomenclature and typical order

• General order of horizons in a well-developed soil profile: O → A → E → B → C → R. In many profiles you will see some horizons missing or abbreviated (e.g., young soils with only A and B; or AB as a flow/transition horizon).
  • O horizon: organic material at the surface (barely to highly decomposed leaves/organic matter).
  • Subdivisions mentioned: Oi (slightly decomposed), Oa/OA (more decomposed, near mineral surface).
  • A horizon: mineral soil with some mixing of organic material; darker due to humus.
  • E horizon: eluviation (leaching) zone; pale color due to loss of clays, oxides, etc.
  • B horizon: illuviation/accumulation horizon; clay, iron, aluminum oxides accumulate here; often the core of soil development.
  • Example notation: Bt means clay accumulation in the B horizon.
  • G feature: gleying (g); strong gley feature indicates saturation in parts of the year leading to redox changes (oxidized/reduced iron forms).
  • C horizon: less weathered parent material; transition toward unweathered material.
  • R horizon: hard, consolidated bedrock.
• Transitional horizons (between main horizons): AB, BE, EB, BC, CB, etc. These record mixed characteristics from adjacent horizons when development is incomplete.
• Special case noted in lecture: AB can form when part of A/E blends with B due to flow of materials; such as A + E + B features in one layer.
• In some profiles, you may see just A and B (young sandy soil) and a sand-derived basal material (the “sandy bar material”).

Examples of soil profiles described in the lecture

• Temperate broadleaf forest profile:
  • O horizon present, with Oi (slightly decomposed) and OA (more decomposed) layers.
  • Clear separation between Oi/Oa and OA; below that a mineral surface with A horizon, then B horizon.
  • Surface boundary at the mineral surface defined as zero depth for the mineral soil; O layers on top.
  • Often an E horizon is present if eluviation is pronounced (not always visible in every profile).
  • In the same profile, A horizon overlays B horizon in the expected order (O → A → B) without overlapping in reverse order.
• Coastal plain soil profile:
  • Well-defined OI, OA, OB horizons at the surface; below that the A horizon, then E horizon, then a transition horizon to B (often labeled eB or AB as a flow layer).
  • Post-deposition processes may mix parts of OA/E into AB; baserich B layers or ferrous line features can appear (ferrous lines indicate iron movement and redox changes).
• Young sandy soil (low development):
  • Only A and B present (perhaps a thin layer of C material present beneath a sandy surface layer). Little soil development; eluviation and illuviation minimal.
• Sandy forest soil (older):
  • O horizon overlying directly an E horizon, with a relatively thin A horizon or weak development between O and C; deep eluviation can remove materials from O and A, leaving a light E horizon.
• Eluviation and leaching concepts:
  • E horizon indicates eluviation (loss) of materials such as clays and oxides from above; eluviation may be pronounced in some profiles, especially with high leaching (eluviation) from O-A and O-A-E layers.
  • E horizon often underlain by a Bt horizon where materials accumulate.

Key horizon-related features and processes

• Eluviation vs. Illuviation:
  • Eluviation: loss of material from a horizon (usually E) due to water movement.
  • Illuviation (in the B horizon): accumulation of materials (clays, oxides, carbonates).
• Oxidation-reduction (redox) features:
  • G (gley) features indicate prolonged saturation and redox cycling, leading to oxidized iron and reduced iron forms in the same profile (redoxomorphic features).
• Depth and thickness thresholds (used in epipedon definitions and general classification):
  • Mollic epipedon: depth of the surface horizon typically greater than 25 ext{ cm} and rich in base cations; dark, fertile topsoil (often developed under grasses, with thick, deep roots and high organic matter).
  • Mollic vs. Umbric: both are thick and dark, but Umbric epipedon is also thick (>25 ext{ cm}) but has low base saturation (more acidic conditions and reduced base cations).
  • Histic epipedon: very high organic matter content; dark color; typically deep organic-rich material (often associated with wetlands or organic-rich soils).
  • Albic horizon: very light-colored horizon due to eluviation; often the boundary beneath an E horizon; associations with low color intensity because of leaching.
  • Endopedons and related horizons (below the surface): horizons formed within the soil (not just at the surface) due to soil-forming processes.
• Endopedons and speculative horizon types described:
  • Arginic endopedon: horizon showing accumulation of clay within the endopedon (argic-related horizons).
  • Kandic endopedon: horizon with potassium-bearing clays accumulating (strongly weathered clays; often hard to distinguish from argic without tests).
  • Calcic endopedon: accumulation of calcium carbonate (carbonate enrichment) within the endopedon, common in drier or calcareous environments.
  • Albic endopedon: very light color horizons within an endopedon (albic-type). The lecture notes mention albic horizons as part of endopedon discussions.
  • Oxyendopedon: endopedon horizon formed under highly weathered minerals with oxides; may relate to oxidically altered minerals in very weathered soils.
  • Candic endopedon: endopedon with accumulation of iron and aluminum oxides where clays are limited (low base saturation) but clay is present in the endopedon.
  • Endopedons with humus accumulation and characteristic horizons (spodic/endopedon types):
  • Spodic endopedon: spodic horizon (spodic layer) forms in illuvial horizons with organic matter and aluminum/iron complexes; often seen in podzols/Spodosols-style settings.
  • Spolic endopedon: a type of endopedon horizon associated with spodic processes in soils where organic matter and cations accumulate into a discrete layer.
• Other diagnostic horizons and materials mentioned:
  • Albic horizon (light color, eluviation features), Albic horizon sometimes associated with endopedons or with epipedons.
  • Cambic horizon: weakly developed horizon reflecting limited clay illuviation and horizon development (more subtle than Bt).
  • Calcic (calcareous) horizon: accumulation of calcium carbonate (CaCO3).
  • Duripan: cemented horizon due to silica cementation; prevents leaching and water movement; common in arid/semi-arid regions and problematic for septic systems.
  • Petrocalcic and Petrogypsic horizons: hard, cemented calcic or gypsum-rich horizons within the soil profile.
  • Fragipan: dense, hardpan-like horizon that is brittle and dense; impedes root growth and water movement; often found in clayey or compacted soils.
  • Endopedons vs. epipedons distinction: epipedons are diagnostic topsoil horizons (surface layer features, such as Mollic, Histic, Umbric), while endopedons are diagnostic horizons formed within the soil profile below the surface.
  • Plaggen epipedon: a man-made, very organic, deep epipedon developed through long-term addition of manures and other organic amendments (notably in Northwest Europe).

USDA soil taxonomy and soil classification concepts (in lecture)

• Why classify soils?
  • To distinguish soils for management, ecology, and production—farmers, ecologists, and land managers need a common framework to predict behavior and management.
  • Analogy used: classifying trees (pine vs broadleaf) or cats (genus vs species) requires a system to group similar soils by properties and origin.
• Historical background (brief and high-level from the lecture):
  • Origins in the Russian Empire; motivation included tax revenue and land assessment for taxation.
  • Vasily Lukashev mentioned in the lecture as part of early soil science history (note: traditional histories credit Dokuchaev as foundational in many texts; the lecture references Lukashev).
  • Charles Kellogg initiated global soil classification concepts aimed at standardizing soil descriptions worldwide.
  • Gates Smith (and a long list of soil scientists) contributed to the development of the first USDA-type soil specification and soil survey.
• Modern framework and references:
  • USDA Soil Taxonomy (official system in the United States): 12 soil orders at the top level. More detailed levels follow: suborder, great group, subgroup, family, and series.
  • As you move down the hierarchy, more specific information is added (e.g., moisture regime, temperature regime, and other physical/chemical properties).
  • World Reference Base (WRB) for Soil Resources: a global reference system; free online access; used internationally alongside USDA taxonomy.
  • Other national systems exist (Brazilian soil taxonomy, etc.), but WRB and USDA Soil Taxonomy are the primary anchors; cross-links exist between them.
• Practical steps in soil classification (as described in the lecture):
  • Identify the diagnostic horizon(s) present or absent in the soil profile.
  • Determine the epipedon(s) (surface diagnostic horizons) to assign the soil to the correct order and suborder, then proceed to suborder, great group, subgroup, family, and finally series as more data become available.
  • Distinguish activities that are field-observable (horizon sequence, color, texture, structure) from laboratory analyses (mineralogy, cation exchange capacity, carbon content, base saturation) that confirm certain diagnostic horizons.
  • Visualization in the field: a field book is used to mark horizons, features, and transitions; many diagnostic horizons require lab work for full confirmation (e.g., clay illuviation, calcic carbonate content).
• Scale and landscape context in soil science:
  • Soils are heterogeneous and vary across landscapes due to topography and soil-forming factors; a landscape can contain multiple soils (polyhedron concept).
  • Within a landscape, a single soil type's 3D body is identified (soil series) at a particular site; typical map unit scales can range from tens of meters to hundreds of hectares.
  • The Nations: the map scales can span from 100 hectares down to 1 hectare or less for detailed study.

Diagnostic horizons, horizon symbols, and examples mentioned

• Epipedons (topsoil diagnostic horizons):
  • Mollic epipedon: dark, thick, fertile topsoil; depth > 25 ext{ cm}; high base saturation; formed under grasses (deep, vigorous roots; high organic matter).
  • Ollic epipedon: described in the lecture as a type of epipedon with distinctive features; typically dark and organic-rich.
  • Umbric epipedon (umbric): dark, often acid soils with low base saturation; typically found in wetter, more leached regions.
  • Histic epipedon: very high organic matter content; often associated with wetlands or organic-rich soils.
  • Plaggen epipedon: very deep, organic-rich, formed by long-term manuring and biomass addition (European Northwest example).
  • Albic epipedon (noted as albic horizon in epipedons): pale horizon due to eluviation/low iron oxide content.
• Endopedons (horizons formed within the profile, not at the surface):
  • Arginic endopedon: clay illuviation within the endopedon, contributing to argic properties.
  • Kandic endopedon: similar to argic but with potassium-bearing clays; often a diagnostic lower horizon.
  • Calcic endopedon: carbonate accumulation within the endopedon (CaCO3).
  • Candic endopedon: horizon with argillic-like characteristics but with certain chemical signatures (low base saturation) and clay coatings.
  • Albic endopedon: pale endopedon horizon with eluviation characteristics; light color relative to surrounding horizons.
  • Oxyendopedon: endopedon with weathered minerals and oxidized features; often rich in oxides.
  • Spodic endopedon (spodic horizon): humus/iron/aluminum illuvial horizon forming in podzol-type environments; associated with acidic, sandy to sandy-loam textures.
  • Spolic endopedon: related form of spodic horizon within the endopedon (contextual naming from the lecture).
• Other commonly occurring diagnostic horizons (as discussed):
  • Albic horizon: very light color due to eluviation; the boundary beneath E can be sharp or diffuse.
  • Cambic horizon: weakly developed horizon; limited clay movement; a transitional horizon with some color/structural development.
  • Calcic horizon: accumulation of CaCO3; characteristic white/gray patches or accumulation zones.
  • Duripan: cemented horizon due to silica; very hard; restricts water movement; common issue for septic systems.
  • Petrocalcic and Petrogypsic horizons: cemented carbonate or gypsum horizons; very hard and resistant to water movement.
  • Fragipan: dense, brittle horizon that constrains root growth and drainage; typically limits deeper rooting.
• Economic and practical implications:
  • The presence of duripan or fragipan can limit percolation or root penetration and influence uses such as septic systems or building foundations.
  • Diagnostic horizons help in predicting soil behavior for agriculture, groundwater protection, and land-use planning.

Historical and practical context for exam preparation

• Key figures and milestones:
  • Charles Kellogg: initiated early ideas of a global soil classification system.
  • Gates Smith: contributed to the first computation/output of the USDA-style soil classification system.
  • Vasily Lukashev (as mentioned in the lecture): part of early soil science history in the Russian Empire (context of tax-related soil evaluation).
  • The USDA Natural Resources Conservation Service (NRCS): hosts reference materials and official soil taxonomy documentation; free access to standards and field guides.
• Global references:
  • USDA Soil Taxonomy: 12 orders at the top level, with progressively finer tiers (order → suborder → great group → subgroup → family → series).
  • World Reference Base (WRB) for Soil Resources: global reference framework with cross-links to USDA taxonomy.
  • Other national systems exist (e.g., Brazilian taxonomy), but WRB and USDA serve as primary anchors for international comparisons.
• Practical workflow in the field and lab (student-oriented summary):
  • Begin in the field identifying horizon sequence and distinctive features (color, texture, structure, presence/absence of features like gleying).
  • Identify diagnostic horizons (epipedons, endopedons, and key horizons like albic, spodic, argic, kandic, calcic, duripan, etc.).
  • Use a field book to document horizons and transitions (e.g., AB, BE, EB, BC, CB).
  • Collect samples for lab analysis to confirm properties that are not easily determined in the field (mineralogy, cation exchange capacity, base saturation, carbonate content, etc.).
  • Place the soil into a hierarchical classification (order → suborder → great group → subgroup → family → series) based on the diagnostic horizons identified and the soil properties tested.

Key takeaways for exam-style recall

• Remember the horizon sequence and basic meanings: O (organic), A (top mineral horizon), E (eluviation), B (illuviation/accumulation), C (parent material), R (bedrock).
• Be able to recognize and name common diagnostic horizons/horizon features: mollic/umbric/histic/albic epipedons; albic, cambic, calcic, duripan, fragipan; argic, kandic, calcic, candic, spodic, spolic endopedons; duripan, petrocalcic, petrogypsic.
• Understand the difference between epipedons (surface horizons used to classify into orders) and endopedons (subsurface horizons indicative of processes within the soil body).
• Know the purpose and structure of USDA Soil Taxonomy: 12 orders at the top, with suborder/great group/subgroup/family/series as you add detail.
• Recognize how soil classification supports farming, ecology, and land-use planning, and why a common framework matters across regions.
• Be aware of the historical development of soil classification and the major reference systems (USDA taxonomy and WRB).

If you want, I can convert these notes into flashcards (horizon terms, diagnostic horizons, depth thresholds, and key processes) or tailor a condensed exam-focused sheet with definitions and quick-reference horizon symbols.