Introduction — Soil horizons
A soil horizon is a recognisable, laterally extensive layer within a soil profile, arranged approximately parallel to the ground surface and distinguishable in a cross‑section by measurable differences in physical, chemical or biological properties from adjacent layers. The most commonly used field criteria for delimiting horizons are physical attributes—particularly colour and texture—which can be described comparatively (e.g. “coarser” or “sandier”) or quantified precisely (for example by particle‑size distribution).
Horizon designation follows a hierarchical symbol system: principal (master) horizons are indicated by capital letters and are modified by combinations of lowercase letters and numerals that record secondary characteristics. Multiple national and international notation schemes exist; these are conceptual tools adapted to local conditions, employ different definitions for symbols and suffixes, and therefore are not interchangeable.
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Horizon description underpins most soil classification systems. Some taxonomies (for example the German approach) treat whole horizon sequences as the defining unit, whereas others define soil types by the presence or properties of particular horizons. Many global and national classifications—notably the World Reference Base for Soil Resources (WRB), USDA Soil Taxonomy and the Australian Soil Classification—rely on a set of diagnostic horizons as formal criteria. Diagnostic horizons carry specific names that reflect characteristic formation processes or property assemblages (e.g. cambic, spodic); the WRB currently recognizes 40 such horizons.
Horizons result from pedogenic processes; material lacking evidence of such modification is generally termed a “layer.” Because some soils show minimal or absent horizon development, alternative diagnostic information (e.g. parent material, texture, or chemical criteria) is sometimes required for classification.
Horizon sequence
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Soil profiles are conventionally described as a vertical succession of discrete horizons. A basic and widely used ordering (from the surface downward) comprises: an organic surface layer (O), a mineral topsoil (A), a pedogenically altered subsoil (B), a poorly developed layer of parent material or regolith (C), and, where present, in situ bedrock (R).
The O horizon consists of accumulated organic residues — litter, humus and decomposing biomass — and its occurrence and thickness are important indicators of surface biological activity; many taxonomies mark it explicitly with the capital letter O, although symbols vary between schemes. The A horizon is the mineral topsoil that typically overlies the subsoil and concentrates fine roots and organic–mineral mixtures; it commonly records surface processes such as bioturbation and eluviation and is the usual reference horizon for mineral soils. The B horizon represents the subsoil produced by pedogenic transformations (for example, illuviation of clays, oxides or organic compounds and horizon differentiation) and therefore signals considerable vertical redistribution and chemical or mineral alteration relative to the A and C layers. The C horizon comprises relatively unmodified parent material or regolith that is loose and weakly developed compared with the overlying horizons and forms the transition to underlying bedrock. The R horizon denotes bedrock in place; its depth and character strongly influence soil thickness, drainage, nutrient dynamics, root penetration and the potential for further pedogenesis.
Horizon presence, sequence and expression are not universal: many soils lack one or more of these horizons, classification systems commonly subdivide or add layers, and intensive human activity (e.g., deep ploughing or major earthworks) can obscure or remove natural horizonation. Accurate field description therefore requires consideration of local geomorphology and documented land‑use history to ensure that observed layers are interpreted and named correctly.
Examples of soil profiles
Soil profiles are vertically stratified into horizons that record the balance of organic input, weathering, and translocation processes. Each horizon exhibits characteristic composition, colour and structure that reflect dominant pedogenic activity.
The O horizon is a surface layer dominated by plant litter and organic residues; its upper part is often little decomposed while the lower part is more humified. It serves as the main repository of fresh and partially decomposed organic matter at the soil surface and strongly affects surface biogeochemical cycling and microhabitat conditions.
The A horizon is the mineral-rich topsoil with the highest biological activity and organic matter content. Weathering within this layer forms oxides (notably iron) and secondary clay minerals and typically produces a granular or crumb structure. Intense eluviation from the A can leave a lighter-coloured E horizon at its base; alternatively, prolonged bioturbation and subsequent winnowing can produce an A that functions as a biomantle.
An E horizon, when present, is a subsurface zone depleted in clay, iron, aluminium and soluble organic compounds due to pronounced eluviation—the downward removal and transport of these constituents—resulting in a conspicuously lighter colour than adjacent layers.
The B horizon, or subsoil, accumulates materials translocated from above (illuviation), principally clay minerals, iron and aluminium compounds and some organic matter. It commonly shows iron-dominated colouration, lower organic content than the A, and a well-developed soil structure derived from these pedogenic accumulations.
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The C horizon comprises little‑weathered or unweathered parent material (soft rock or unconsolidated deposits). It retains many physical and chemical attributes of the source material and may concentrate soluble compounds, such as calcium carbonate, by percolating waters.
The R horizon is continuous, indurated bedrock that is partially weathered or unweathered and cannot be excavated by hand; soils that form in situ from this bedrock tend to reflect its lithologic and mineralogical character.
Horizons and layers in the World Reference Base (WRB) are defined and coded according to the WRB Manual (4th ed., 2022), with layer-classification procedures cross-referenced to Chapter 3.3. The manual distinguishes compositional bases used for diagnosis: the fine earth fraction comprises particles ≤ 2 mm and serves as the foundation for several organic-content criteria, whereas the whole soil includes fine earth plus coarse fragments, artefacts, cemented materials and dead plant residues of any size, so percentage thresholds stated “of the whole soil” use this inclusive denominator.
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Surface and near-surface units are treated distinctly. A litter layer is a loose, mainly undecomposed assemblage of recognizable plant tissues and is diagnosed when such tissues exceed 90% of the combined volume of fine earth plus all dead plant residues; plant parts remaining attached to living vegetation are excluded. The conventional soil surface (0 cm) is taken after removal of any litter and beneath living plant cover; the mineral soil surface denotes the top boundary of the uppermost layer composed predominantly of mineral material.
WRB uses the neutral term “layer” for any zone roughly parallel to the soil surface with properties that differ from adjacent zones; if at least one distinguishing property results from pedogenic processes the layer is termed a “soil horizon.” Layers dominated by organic material are classified as organic layers when they contain ≥ 20% organic carbon, measured relative to the fine earth plus dead plant residues (of any length) with particles ≤ 5 mm in diameter, and must not consist of artefacts.
Organotechnic layers are recognized when two compositional conditions are met: (1) artefacts constitute ≥ 35% of the layer by volume when assessed relative to the whole soil, and those artefacts themselves contain ≥ 20% organic carbon; and (2) the non-artefact portion of the fine earth plus dead plant residues (≤ 5 mm) contains < 20% organic carbon. Layers that fail to meet the thresholds for organic or organotechnic status are classified as mineral layers.
WRB nomenclature codes layers with a capital letter as the master symbol, generally followed by one or more lowercase suffix letters that provide additional descriptive detail about the layer’s properties.
Master symbols
Master horizon symbols designate the principal organic, mineral, lithic and hydrological units used in soil description and classification. Each symbol identifies a layer by origin, dominant material and diagnostic properties.
H — An organic or organotechnic horizon that lies below the litter and is defined principally by prolonged saturation (more than 30 consecutive days in most years) or by artificial drainage; it typically corresponds to peat or other organic deposits formed in limnic conditions.
O — An organic or organotechnic horizon also below the litter but with only short-term saturation (no more than 30 consecutive days in most years) and without drainage; it represents non‑peat, non‑limnic organic accumulations.
A — A mineral horizon at the soil surface or buried that contains organic matter altered in place and displays developed soil structure or cultivation-induced structural elements in at least half of the fine earth volume; rock structure, when present, occupies less than half of that volume.
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E — An eluviated mineral horizon characterized by the downward loss (vertical or lateral movement) of one or more constituents such as iron, aluminum or manganese compounds, clay minerals, and/or organic matter; it is the layer of relative depletion produced by leaching.
B — A subsurface mineral horizon that originally formed beneath an A or E horizon and in which rock structure occupies less than half of the fine earth volume; B horizons are distinguished by pedogenic secondary features (e.g., aggregate formation, neoformation of clays or oxides) and by illuvial accumulations or depletions (e.g., Fe, Al, Mn, clays, organic matter, silica, carbonates, gypsum), as well as other possible secondary deposits.
C — A mineral parent material layer that may be unconsolidated (easily cut when moist) or consolidated but more fractured than bedrock; it shows either no detectable pedogenesis or only incipient soil development insufficient to qualify as A, E or B.
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R — Consolidated bedrock in which air‑dry samples do not slake within 24 hours when submerged; fractures, if present, occupy under 10% of the total soil volume, and the R layer is primary rock rather than a cemented soil horizon.
I — A perennially frozen layer composed predominantly (≥75% by volume) of ice, occurring beneath any of the H, O, A, E, B or C layers.
W — A designation indicating permanent water at or between soil layers; such water bodies may freeze seasonally but are persistent in the soil profile.
Suffixes (functions and application)
Soil-horizon suffixes modify master- and subordinate-layer symbols to record secondary properties, processes and materials that are not conveyed by the basic horizon letter. Each suffix has a restricted set of horizon types where it may be applied and, where relevant, a specific interpretation (for example identical letters can mean different things in organic versus mineral layers). Suffixes may be combined to describe multiple co‑occurring features, but their sequence follows prescribed rules based on process relationships, dominance and special-case ordering.
Categories of suffixes (key meanings)
- Decomposition state in organic layers: i, e and a denote initial, intermediate and advanced stages of organic decomposition respectively; these appear only on H and O layers and must be ordered according to the prescribed sequence (i, then e or a as applicable).
- Burial, cryogenic and permafrost indicators: b marks a previously formed horizon that has been buried by mineral deposits; @ indicates cryogenic alteration; f denotes permafrost. When these co‑occur with other suffixes, the cryogenic/permafrost markers precede b (e.g., @b, fb) under the ordering rules.
- Accumulations and cements: a suite of suffixes records accumulations of specific materials—k (secondary carbonates), q (secondary silica), y (secondary gypsum), z (readily soluble salts), j (jarosite/schwertmannite), v (plinthite), s (Fe/Mn/Al from illuviation), o (residual pedogenic oxides), g and l (different spatial patterns of Fe/Mn accumulation), t (clay illuviation), n (exchangeable sodium ≥ 6%), r (strong reduction) and φ (Fe/Mn patterns from lateral flow). Where concretions or nodules form, the c suffix is appended immediately after the particular substance suffix (for each agent present). Pedogenic cementation that occupies at least half the volume and is at least moderately strong is indicated by m, which must directly follow the suffix naming the cementing agent (allowed agents: k, l, q, s, v, y, z).
- Organic content and cultivation: h denotes substantial organic matter (with differing genesis in A, B and C horizons); p records cultivation effects on mineral layers; u marks anthropogenic artefacts present within the horizon.
- Physical/structural and density features: ss records slickensides/wedge aggregates (treated as a single double‑letter suffix); w denotes general evidence of structure and oxide/clay development but is omitted in B horizons when any more specific B‑suffix (g, h, k, l, o, q, s, t, v, y) is strongly expressed; x indicates fragic (root‑restricting) characteristics. Bulk‑density markers include β (very low bulk density ≤ 0.9 kg dm−3) and δ (very high bulk density excluding root penetration except via cracks).
- Other special indicators: γ signals a volcanic‑glass fraction ≥ 5% in the 0.02–2 mm size class; λ denotes limnic (water‑deposited) material; σ indicates permanent saturation without redoximorphic features; τ records human-transported natural material; ρ is a relict marker appended directly after the suffix for the relict feature (see combination rules). I and W master layers are not assigned suffixes.
Combination and ordering principles
- Direct adjacency to agent: Concretion (c), relict (ρ) and cementation (m) modifiers must be written immediately after the suffix that names the relevant agent or original feature. If several agents/forms are present, each is followed by its own c, m or ρ.
- Cementation linkage: m is not freestanding; it follows only k, l, q, s, v, y or z and is repeated for each distinct cementing agent present.
- Relict notation: ρ is attached only to the specific suffix that is relict (g, k, l, p, r, @). Multiple relict features each carry their own ρ.
- Process pairs and sequence: When suffixes represent the same soil‑forming process they are written consecutively. For the specific pair of clay illuviation and sodicity, t must precede n (i.e., tn). Other such adjacency rules remain subject to the agent‑adjacency prescriptions above.
- Dominance ordering: Beyond the mandatory local adjacency rules and the special sequences above, all suffixes within a horizon are arranged in descending order of dominance—the most diagnostic or strongly expressed characteristic is written first (examples: Btng, Btgb, Bkcyc illustrate dominance ordering and agent‑modifiers).
- B‑horizon exception for w: In B horizons the general w suffix is suppressed when more specific pedogenic features (g, h, k, l, o, q, s, t, v, y) are prominent; if those specific features are weak and w is present, both the specific suffix(es) and w are used.
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This system allows concise encoding of multiple interacting pedogenic attributes while preserving clarity about causal agents, relative importance and relict status.
Transitional layers
Transitional layers describe situations in which diagnostic attributes of two or more master horizons occur within the same vertical zone of a soil profile. Such occurrences may take the form of true superposition—where horizon properties coexist at the same position—or of discrete intercalations and lamellae occupying the same depth range but as separate bodies.
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Concatenated notation (no separator) is used when the diagnostic features of multiple master horizons are genuinely superimposed in the same vertical position. In this case the master symbols are written together with the dominant symbol placed first; each master symbol may then carry its applicable suffixes. Examples of concatenation include AhBw, BwAh, AhE, EAh, EBg, BgE, BwC, CBw and BsC, CBs. The symbol order signals which characteristic is predominant, while appended suffixes qualify each component.
A slash (/) between master symbols indicates that the horizons share a common depth range but are physically separated into distinct parts—for example as lamellae, lenses or lateral intercalations—rather than being fully coextensive. The dominant horizon is listed first. Typical notations are Bt/E (E material interfingering into a Bt) and C/Bt (Bt lamellae within a C layer).
Combination rules are restricted for certain symbols: W (water) must not be combined with any other master symbol, and the symbols H, O, I and R may only appear in combination via the slash notation, not by concatenation.
Layer-sequence notation records soil units from the surface downward using hyphens as separators; the sequence is read left-to-right as the stratigraphic order (top → down). When a lithic discontinuity separates strata, a leading numeral is introduced immediately before layer symbols beginning with the second stratum; that numeral identifies the stratum and applies to every subsequent layer that belongs to the same stratum (e.g., Oi–Oe–Ah–E–2Bt–2C–3R, where “2” prefixes Bt and C to denote the second stratum and “3” prefixes R to denote a third). By convention, I and W layers are not treated as strata and therefore are not assigned leading-stratum numerals when discontinuities are recorded.
Repeated occurrences of the same layer designation are disambiguated by appending trailing figures to the letter code; these within-sequence figures continue sequentially across the entire profile rather than restarting in each stratum. The notation therefore permits mixed use of trailing (within-sequence) numbers and leading-stratum numerals to show both repetition and stratigraphic position (for example, Bw1–Bw2–2Bw3–3Ahb1 shows two early Bw occurrences, a third Bw in the second stratum, and an Ahb in the third). Practically, any code without a leading numeral belongs to the first (uppermost) stratum unless it carries a trailing figure marking a repeated occurrence; any code prefixed by n (nX) belongs to the nth stratum and all other layers in that same stratum should carry the identical leading numeral to maintain consistent stratigraphic correlation.
Master horizons and layers (WRB): The World Reference Base recognises a set of master horizons and non‑soil layers that summarise the main genetic and physical divisions of a soil profile. For organic horizons the WRB applies a quantitative threshold of a minimum of 20% soil organic carbon (by weight) to distinguish O and H master horizons from mineral horizons.
O and H horizons: Both O and H are dominated by organic material formed above the mineral soil and may occur at the surface or become buried beneath mineral deposits. They differ primarily in hydrology and residue type. H horizons consist of organic residues (often mosses) that meet the 20% organic‑carbon threshold and are, or have been, water‑saturated (including situations where drainage was subsequently induced). O horizons also meet the 20% organic‑carbon threshold but comprise surface plant litter and small‑plant debris (leaves, needles, twigs, lichens) and are not subject to prolonged saturation or artificial drainage. Either horizon may be overlain by the other in complex sequences following drainage or burial.
A and E horizons: A horizons are mineral surface layers (or lie beneath an O horizon) in which original rock fabric is largely destroyed and significant humified organic matter is intimately mixed with mineral material; they may also exhibit morphological evidence of cultivation or other surface disturbance, or a surface aspect distinct from underlying horizons. When features characteristic of both A and E occur, dominance of humified organic matter warrants A designation. E horizons are mineral layers defined by eluviation — the removal of clay, Fe/Al oxides and/or organic matter — producing a relative enrichment in sand and silt, lighter colour and often coarser texture. Although typically situated below O or A and above B, the E symbol is applied on the basis of genesis wherever such depletion has occurred.
B horizons: B horizons are subsurface mineral horizons in which rock structure has been substantially obliterated and one or more pedogenic accumulative or alteration features dominate. Diagnostic properties include illuvial concentration of clays, oxides (particularly iron), organic matter, carbonates, gypsum or silica; residual enrichment of oxides; clear oxide coatings that alter colour and chroma; removal of carbonates or gypsum; and neoformation or alteration of clay minerals often associated with distinctive structural units and brittleness. Layers with only gleying, or with clay coatings restricted to rock fragments or finely stratified sediments, or with isolated, non‑contiguous carbonate illuviation, are excluded from B status.
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C horizons: C horizons comprise material little modified by soil forming processes and lacking the diagnostic properties of H, O, A, E or B horizons. They include unconsolidated sediments, saprolite and non‑indurated bedrock, and may contain siliceous or calcareous accumulations (shells, diatomite, marl, etc.). Typical C materials slake within 24 hours when air‑dry fragments are immersed and can be excavated with a spade when moist; many C horizons are root‑penetrable and act as a growth medium. Accumulations of silica, carbonates or gypsum are treated as C unless there is clear evidence that they have been modified by pedogenesis, in which case a B designation applies. Highly weathered parent material that nonetheless lacks A, E or B diagnostics is also classed as C.
R, I, L and W layers: R denotes hard bedrock that remains coherent and resistant to slaking (air‑dry fragments do not slake within 24 hours) and is impractical to excavate with hand tools; it generally permits little root penetration except in limited fractures. I denotes ice lenses or wedges that consist of at least 75% ice by volume and act as discrete separators between soil layers. L indicates lacustrine (limnic) deposits formed in standing water bodies and may be organic (peat, coprogenous earth) or mineral (diatomaceous earth, marl). W denotes the presence of water within or above the soil where water is present permanently or cyclically within a 24‑hour period; shallow permanent or tidal inundation and floating organic soils are indicated by this symbol.
Transitional horizons and layers
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Transitional horizons are described by combining the two relevant master-horizon symbols as capital letters, placing the symbol for the dominant set of properties first (e.g., AB or BA) to indicate a blended layer in which A- and B-type characteristics are integrated but one type predominates. By contrast, when A- and B-type materials occur as separate, intermingled bodies within the same horizon interval rather than as a single blended zone, the symbols are linked with a slash (e.g., A/B or B/A) to signal spatial coexistence of discrete bodies.
Lowercase suffixes that denote subordinate morphological or compositional features are appended after the combined-capital notation, thereby qualifying the composite horizon (for example, AhBw indicates an A–B transitional horizon with subordinate humic features on the A component and weak development on the B component). The special master symbols I, L and W are explicitly excluded from use in transitional-horizon designations and must not appear in combined or intermixture notations.
Subordinate characteristics are appended to primary horizon symbols to record specific physical, chemical, pedogenic, cryogenic or anthropogenic attributes that do not warrant a separate master horizon. Many single‑letter suffixes have context‑dependent meanings and established horizon restrictions, so careful application is required.
Degrees of organic decomposition are indicated only for H and O horizons by the suffixes a (highly decomposed), e (moderately decomposed) and i (slightly decomposed). Several letters have distinct meanings when applied to L (litter/organic) horizons versus mineral horizons: for L horizons c denotes coprogenous (dung‑derived) material, d denotes diatomaceous earth and m denotes marl; when applied to mineral horizons, c signals concretions or nodules, d denotes a dense, root‑restrictive layer (and must not be combined with mineral m), and m denotes strong cementation or induration (massive structure).
Suffixes that describe mineral‑horizon structure and root restriction include x for fragipan characteristics and i (in its mineral sense) for slickensides. Buried and cryogenic conditions are recorded separately: b marks a buried genetic horizon (applies to mineral horizons but not to cryoturbated horizons), f denotes frozen soil (not to be used in I and R horizons), and the symbol @ records evidence of cryoturbation and may be appended to any horizon.
Illuviation and related B/C diagnostics employ a set of discriminating suffixes: t records an illuvial accumulation of clay and can appear on B and C horizons, s denotes illuvial accumulation of sesquioxides and is restricted to B horizons, and w indicates development of colour or structure and is also used on B horizons. Residual versus illuvial pedogenic oxide accumulations are distinguished by o (residual pedogenic sesquioxides; no horizon restriction) versus s (illuvial sesquioxides, B horizons). Pedogenic silica is marked by q (no horizon restriction).
Chemical accumulations are explicitly coded: k indicates pedogenic carbonate accumulation, y denotes pedogenic gypsum, and z records salts more soluble than gypsum; these three suffixes have no horizon restriction. Hydromorphic and redox‑related features are shown by r for strong reduction, n for accumulation of exchangeable sodium, l for gleying or mottling from upward groundwater movement, and g for stagnic conditions; none of these are limited to particular horizons.
Human influence and mechanical disturbance are indicated separately: u flags urban or other artefactual materials and may be appended to H, O, A, E, B and C horizons, while p records ploughing or other mechanical disturbance and may be used without horizon restriction (ploughed mineral or eluviated horizons are conventionally designated Ap when appropriate).
Additional mineral‑pedological markers used as suffixes include h for accumulation of organic matter within mineral horizons, j for jarosite, and v for plinthite. Because several letters perform multiple roles depending on horizon class (notably c, d, m and i), the correct interpretation of a suffix depends on both its letter and the master horizon to which it is appended.
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Discontinuities and vertical subdivisions
Numeric prefixes placed before horizon symbols mark lithic discontinuities, i.e., breaks between distinct parent‑material bodies; by convention the prefix 1 is omitted, so unprefixed symbols belong to the uppermost lithologic unit. Numeric suffixes attached to horizon letters indicate internal subdivisions of a single horizon type, with increasing suffix numbers indicating successive layers from top to bottom within that horizon. Successive horizon notations are joined by hyphens to record the vertical sequence of the profile (for example: Ah‑E‑Bt1‑2Bt2‑2BwC‑3C1‑3C2).
Reading that example token by token: Ah denotes a surficial A horizon enriched in humified organic matter; E is an eluviated (leached) horizon immediately beneath Ah; Bt1 is the first subdivision of an argillic (clay‑accumulation) B horizon in the upper lithologic unit; 2Bt2 is a Bt2 subdivision that belongs to a different lithologic unit indicated by the prefix 2 (the lithic discontinuity separates Bt1 and 2Bt2); 2BwC is a transitional horizon within the same second unit showing Bw characteristics grading into C (parent material); and 3C1 and 3C2 are two successive subdivisions of the C horizon within a third lithologic unit (prefix 3).
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Thus a compact horizon string simultaneously conveys the number and order of lithologic units (implicit unit 1 plus explicit units 2 and 3), the assortment of horizon types present, and any within‑horizon subdivisions. This convention makes abrupt lithologic contacts and distinct parent‑material bodies explicit and preserves detailed internal horizon structure, information that is essential for accurate mapping, classification and interpretation of soil genesis and landscape evolution.
Master horizons and layers
The master horizons and layers designate major vertical units in a soil profile distinguished by origin, composition and dominant pedogenic processes. Each horizon records different interactions among biological activity, chemical translocation and physical weathering, and together they determine soil function for ecosystems and land use.
O horizon
A surface layer dominated by organic matter derived from plant residues (leaf litter, mosses, peat) rather than mineral sediment. It forms where decomposition is slow—e.g., wetlands, bogs and some forest floors—thereby influencing carbon storage, surface hydrology and flammability.
A horizon
The biologically active mineral surface horizon enriched in humified organic matter. Rooting, faunal mixing and microbial transformation darken and aggregate this layer, increasing nutrient availability, porosity and biological activity relative to deeper mineral horizons.
E horizon
A leached mineral horizon from which clay, iron, aluminium and/or organic colloids have been depleted by percolating water (eluviation). It is typically paler and coarser in texture than adjacent layers and signals substantial downward movement of fine particles and solutes.
B horizon
A subsurface horizon where materials mobilized from above accumulate or where secondary alteration occurs (illuviation of clay, oxides, silica, carbonates or gypsum; cementation; structural development). The B horizon thus records dominant pedogenic pathways (e.g., clay translocation, calcification, gypsification, redox-driven oxide concentration) that affect water retention, nutrient status and root penetration.
C horizon
Unconsolidated sediments or weakly weathered parent rock that show little evidence of soil formation. The C horizon largely preserves the original lithology and exerts primary control on texture, mineralogy, drainage and the rate of subsequent horizon development.
L horizon
Sediments of limnic (lake-derived) origin deposited within the soil column. These lacustrine layers, often with distinct particle-size distributions and geochemical signatures, reflect past or present standing-water conditions and are distinct from terrestrial organic horizons in origin and consolidation.
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W and Wf indicators
A persistent layer of liquid water (W) or perennially frozen water (Wf) within or beneath the soil profile. W indicates saturated, anoxic conditions that promote redox transformations; Wf denotes continuous ground frost that suppresses biological activity, root growth and hydraulic conductivity. Ephemeral surface water or transient ice above the soil surface is not classified here.
M horizon
Anthropogenic subsoil layers composed of manufactured or heavily altered materials (e.g., compacted fills, construction debris, bituminous residues, concrete). These engineered units disrupt natural pedogenesis, limit rooting and infiltration, and must be identified separately in surveys and management plans.
R horizon
Indurated or strongly cemented bedrock encountered in the profile that is effectively impermeable to roots and most soil-forming processes. The R horizon defines the lower limit of most soils, constrains vegetation depth and has direct implications for landscape evolution, excavation and engineering suitability.
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Together these horizons provide a framework for interpreting soil genesis, ecosystem function and practical concerns such as land management, carbon cycling and construction planning.
Transitional horizons and layers are denoted by combinations of the master-horizon capital letters, but the form of combination encodes whether the constituent horizon properties are physically integrated or occur as separate bodies. When a single horizon displays blended characteristics of two master horizons throughout its thickness, the letters are written together (e.g., AB or BA), with the letter for the dominant set of properties placed first. This concatenated notation therefore signifies a composite, unified horizon in which traits of both masters are interwoven.
By contrast, when the two master horizons persist as distinct, intermingled bodies within the same vertical or horizontal interval, their symbols are separated by a slash (e.g., A/B or B/A). The slash indicates coexistence of discrete horizon materials rather than true integration. In both notational systems the capital letters refer to standard master horizons; the difference between concatenation and slashed notation is strictly one of physical arrangement and dominance of properties, not of the basic horizon types themselves.
Horizon suffixes provide concise qualifiers that describe material composition, pedogenic processes, physical impedances and anthropogenic or cryogenic modification within soil profiles, and several are restricted to particular master horizons. Within organic O horizons, the letters a, e and i indicate progressive stages of decomposition—highly humified material (a), intermediate breakdown (e) and minimally decomposed litter or fibric peat (i)—and are used to interpret organic matter dynamics and carbon storage.
A small set of suffixes applies only to L (litter/undecomposed) horizons and denotes distinctive depositional materials: co for coprogenous (fecal-derived) deposits, di for diatomaceous siliceous accumulations, and ma for marl (carbonate-rich lacustrine or mineral sediment). The suffix b records a buried genetic horizon (not applied to C horizons) and is central to reconstructing former surfaces and sedimentation history; p and u mark human influence, with p identifying plowing or similar mechanical disturbance and u indicating the presence of manufactured artifacts or constructional materials.
Cold-climate and permafrost-related suffixes capture freezing and freeze–thaw effects: f denotes permanently frozen ground with continuous subsurface ice, ff denotes perennially frozen ground lacking continuous ice lenses (so-called “dry” permafrost), and jj records cryoturbation from freeze–thaw mixing. These distinctions are important for hydrology, rootability and paleopedogenic interpretation.
Physical root- and water-restrictive features are indicated by several codes: d denotes a densic, compacted layer limiting roots and water movement; x signals fragipan properties—brittle, low-permeability layers that impede infiltration and root growth; r identifies shallow weathered bedrock or saprolite; m indicates pervasive pedogenic cementation producing indurated horizons; c denotes discrete concretions or nodules formed by localized mineral precipitation; and q records secondary, pedogenic silica accumulation (coatings, nodules or duripans). Slickensides (ss) mark shear surfaces produced by shrink–swell movement and v identifies plinthite—iron-rich, humus-poor accumulations that harden on exposure.
Suffixes for chemical accumulations distinguish magnitude and type of precipitated minerals: k and kk denote pedogenic CaCO3 accumulation below and at/above ~50% by volume respectively, with implications for pH and calcrete formation; y and yy analogously indicate gypsum accumulation and gypsum-dominated horizons (~≥50%); z flags concentrations of salts more soluble than gypsum (e.g., chlorides, nitrates) that drive salinity and osmotic stress; j specifies jarosite, an iron sulfate indicative of strongly acidic, sulfate-rich oxidation; and se records sulfides that may generate acid upon exposure. Exchangeable sodium is recorded by n and signals sodicity with attendant dispersion and hydraulic degradation.
Illuvial and residual indicators identify translocation and intense weathering: t marks clay illuviation (argilluviation) typical of B horizons and associated textural differentiation; s denotes illuvial sesquioxides coupled with organic coatings; o denotes residual sesquioxide concentration resulting from advanced leaching and laterization. Gleying (g) records prolonged reducing, waterlogged conditions producing grey–mottled redoximorphic features. Finally, w (used only with B) denotes weak color or structure development within an otherwise B-designated horizon, reflecting incipient pedogenesis.
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Together, these suffixes enable precise description of horizon materials and processes, facilitating interpretation of soil genesis, hydrology, landscape evolution, agricultural suitability and environmental risk.
Other horizon modifiers in soil-profile notation distinguish changes in parent material from internal differentiation within a single genetic horizon by means of numerals placed before or after horizon symbols. A numeric prefix, written immediately before the horizon letters, identifies the ranked lithologic unit (or discontinuity) to which that horizon belongs; by convention the numeral 1 is omitted, so an unprefixed horizon denotes the first lithologic unit. A numeric suffix, written immediately after the horizon letters, subdivides a master horizon into internally distinct parts within the same lithologic unit, conveying intra-horizon variability without implying a change in parent material.
These conventions apply equally to compound or transitional horizons (for example, BC), which may also carry prefixes and/or suffixes to indicate their position within a particular lithologic unit or their internal subdivisions. In the example sequence A, E, Bt1, 2Bt2, 2BC, 3C1, 3C2, A and E are upper master horizons of the first lithologic unit (no prefix shown); Bt1 is a subdivision of a Bt horizon in that same first unit. The label 2Bt2 denotes a Bt subdivision occurring in the second lithologic unit, and 2BC is the BC transitional horizon within that same second unit. Finally, 3C1 and 3C2 are two subdivisions of the C horizon in the third lithologic unit, indicating deeper material that is compositionally distinct from the units above.
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Horizons
Soil horizons are distinct layers within the soil profile that record the effects of biological activity, weathering and material translocation. The combined depth affected by these surface processes—the solum—consists of the A, any E, and B horizons and represents the active zone of pedogenesis; layers beneath the solum are progressively less modified by surface soil‑forming processes.
At the surface, organic horizons accumulate plant and other organic residues. The O horizon comprises largely organic material in various stages of decomposition and is commonly divided into an O1 of recognisably undecomposed fragments and an O2 of more thoroughly decomposed organic debris; it is defined by a high organic carbon content (≥20%). Peaty or waterlogged organic deposits are treated separately as P horizons (peat/peaty soils), formed under saturated conditions and similarly subdivided; their organic carbon threshold is lower (roughly 12–18%, varying with clay content).
The A horizon (topsoil or biomantle) is the principal biologically active mineral horizon, enriched in humified organic matter and hosting dense communities of soil fauna and microorganisms often associated with plant roots. Morphologically and functionally diverse, A horizons are commonly distinguished into A1 (dark, maximally biologically active), A2 (bleached or paler), and A3 (transitional to the B). Surface processes operating in the A horizon, including runoff, also mediate lateral transport of sediment and dissolved pollutants (an example being agricultural runoff observed during storms).
Leached or eluviated layers that have lost silicate and organic constituents form E horizons—typically pale and sand- or silica‑dominated—and occur in well‑developed, older soils between A and B. Some classification systems (notably Australian) do not apply a discrete E category, instead classifying strongly leached material as A or B and appending an “e” suffix; bioturbation at the base of an E horizon may produce a stonelayer. A classical morphological example is an Albic Luvisol with a dark surface underlain by a bleached (albic) subsurface that tongues into a clay‑enriched Bt horizon.
The B horizon is the zone of accumulation and alteration—subsoil—where secondary minerals (clays, iron oxides) and textural or structural changes concentrate, producing colours and consistencies distinct from adjacent horizons. When material has been translocated into this zone it is described as illuvial. Although plant roots can extend into the B, organic matter is typically scarce there. In Australian practice the B is sometimes subdivided into B1 (transitional with some A‑like features), B2 (marked accumulation of clays or oxides) and B3 (transition to underlying material); the application of transitional subclasses (A3, B1, B3) is partly subjective in the field.
Below the solum, the C horizon comprises weakly weathered or unconsolidated parent material and lacks the diagnostic features of surface pedogenesis. C horizons derive either from deposited sediments (loess, alluvium, colluvium) or in‑situ weathering of bedrock and may retain carbonate enrichments leached from above; where no lithologic discontinuity exists the C horizon may closely resemble the solum’s parent material (illustrated by profiles with broken rock fragments overlying bedrock, such as at Sandside Bay, Caithness). Some classification systems include a D horizon for material beneath the solum that is markedly unlike the overlying soil and cannot be classed as C; this provision appears in the Australian scheme for anomalous or unassignable substrata.
Two additional horizon types are recognised in some systems. The R horizon denotes essentially unweathered or only partially weathered bedrock that cannot be excavated by hand, forming the base of shallow profiles. L (limnic) horizons record sediments deposited in standing water—diatomite, marl or coprogenous peat—and are not used in the Australian classification. Photographic and field examples—ranging from agricultural runoff at an Iowa field (A‑horizon processes and pollutant transport), to an anthropogenic roadside profile near Bengaluru, to shallow soils over bedrock at Sandside Bay—illustrate how horizon expression varies with landscape context and land use.
Transitional horizons — notation and interpretation
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Transitional horizons are indicated by compound symbols whose form conveys both the kinds of properties present and their spatial relationships within the horizon. When two capital letters are written together (e.g., AB or BA), they denote a single, integrated horizon in which characteristics of both horizon types are intermixed; the first letter identifies the dominant suite of properties while the second indicates the subordinate influence. In contrast, a slash between symbols (e.g., A/B or B/A) signals that the horizon contains distinguishable parts or subunits, some showing one set of properties and some the other; the ordering still reflects which property set is considered primary for interpretation or mapping.
Thus the juxtaposed and slash notations represent different physical situations: the former describes a continuum or homogeneous mixture with a clear dominance order, whereas the latter records internally separable parts with different horizon attributes. In practice, symbols must be read precisely as written so that field descriptions, profile sketches, and classification decisions correctly reflect whether mixed characteristics are integrated and dominant or spatially discrete and separable.
Horizon suffixes
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Soil horizons may be subdivided vertically to capture gradual changes in properties; numeric subscripts (for example B21, B22, B23) indicate a sequence of closely related sub‑units within a principal horizon rather than discrete, independently formed layers. Because soil characteristics often vary markedly over short distances, field descriptions should avoid over‑segmentation: very thin bands are frequently of limited interpretive value and may reflect local heterogeneity rather than distinct pedogenic stages. In practice (Australian guidance), layers thinner than about 5 cm are usually recorded as segregations or pans within an existing horizon rather than being given separate horizon status.
Observations from natural exposures such as cut faces or collapsed slopes are valuable for seeing horizon architecture and collecting samples, but these settings can display ephemeral or mechanically disturbed features. Consequently, horizon boundaries and diagnostic modifiers inferred from such exposures require careful, conservative interpretation.
Suffixes appended to horizon symbols record specific pedogenic attributes or disturbances. Commonly used lower‑case modifiers (Australian usage) include indicators for burial (b); mineral nodules or concretions (c); root‑restrictive layers (d); strong bleaching (e) and sporadic bleaching (j); bioturbation or faunal accumulations in surface horizons (f); gleying from reduction–hydro‑conditions (g); organic enrichment (h); carbonate concentration (k); intense cementation or induration (m); tillage disturbance (p, applied to A horizons); secondary silica (q); weathered but diggable rock (r); accumulation of iron/aluminium oxides (s); clay accumulation (t); weak horizon development (w); fragipan presence (x); gypsum accumulation (y); and salts more soluble than gypsum (z).
Suffixes and numeric subscripts are recorded together with the principal horizon symbol to communicate both position and diagnostic characteristics (for example, a buried B horizon enriched in carbonates can be notated Bk b; an A horizon showing faunal enrichment that has been tilled could be recorded as Af p).
Buried soils
Soil formation is conventionally understood as an in‑place (in situ) transformation of rock or sediment through physical breakdown, chemical alteration, and incorporation of organic and mineral matter, yielding distinct pedogenic horizons. However, soil development at a site may proceed through multiple episodes: an established profile can be blanketed by later wind‑borne or waterborne deposits (aeolian, fluvial or marine), after which a new soil assemblage develops on top of the burial surface. The result is a vertically stacked sequence of soil generations recorded at a single locality.
Such superimposed profiles are especially common in coastal and other depositional landscapes where repeated sedimentation alternates with intervals of pedogenesis. Recognizing these buried and relict horizons is therefore essential for reconstructing landscape evolution, because they preserve a record of alternating depositional and soil‑forming phases.
Soil classification and description accommodate buried sequences by prefixing horizon labels with numerals that indicate generational order (for example, an original near‑surface sequence O–A–B–C may be overlain by second‑generation horizons written as 2A, 2B1, 2B2, 2C, where the leading “2” denotes the buried profile and additional digits mark internal subdivisions). Accurate notation and identification of these horizons are critical for reliable soil mapping, stratigraphic correlation and paleoenvironmental interpretation; overlooking stacked profiles can produce erroneous conclusions about soil age, developmental history and the timing and nature of depositional events.
Diagnostic soil horizons
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Diagnostic horizons are distinctive layers within the soil profile whose occurrence and properties are used as primary, standardized criteria for delineating soil taxa. Taxonomic assignment depends on the presence of one or more such horizons occurring within specified depth intervals; classification schemes therefore require that horizons meet prescribed criteria at particular depths before a unit can be recognized. In practice, however, horizon-based rules are usually combined with other soil attributes — for example texture, structure, chemistry, color, consistency and mineralogy — which together determine and rank soil taxa.
To ensure reproducible classification, diagnostic horizons must be defined by explicit morphological, physical and/or chemical thresholds so that different observers can consistently judge whether a horizon qualifies. Assigning a pedon to a taxonomic unit requires a systematic, horizon-by-horizon inspection: each layer is evaluated against the diagnostic definitions, and the resultant set of qualifying horizons and their depth positions determine the final taxonomic placement.
Because classification systems adopt different sets of diagnostic horizons and definitions, comparative work requires applying each system’s specific criteria rather than assuming equivalence. Consequently, accurate soil mapping, interpretation and taxonomic consistency depend on rigorous field inspection of horizons, clear and unambiguous diagnostic criteria, and faithful application of the required-depth rules.
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Diagnostic horizons in the World Reference Base for Soil Resources (WRB)
WRB diagnostic horizons encapsulate the key pedogenic processes and anthropogenic modifications that define soil behaviour, use and classification. They are identified by morphological, chemical and physical criteria that signal eluviation/illuviation, organic matter accumulation, cementation, salinisation, redox dynamics, temperature regimes and human influence. Below is a concise synthesis of these horizons, grouped by functional similarity and with their principal diagnostic and management implications.
Eluviation and illuviation horizons
– Albic: a leached subsurface layer with a bleached appearance produced by the selective removal of clay, iron and organic matter; indicates strong downward water flux and commonly overlies illuvial B horizons, with consequences for low nutrient availability and acidity.
– Argic: an illuvial clay-enriched B horizon formed by argilluviation; marked by increased fine clay and blocky structure, it controls rootable depth, water retention and is diagnostic for several temperate soil taxa.
– Spodic: an organic–Al/Fe accumulation horizon produced by podzolisation on sandy substrates; strongly acidic and nutrient-poor, it profoundly limits fertility and rooting in cool, humid landscapes.
– Nitic: a dense, high-clay subsurface horizon of tropical/subtropical origin with shiny ped faces and gradual clay increase; reflects intense clay translocation and produces plastic, impermeable subsoils.
– Cambic: an incipiently developed B horizon showing modest structure and colour change without marked illuviation; a marker of early soil development used to separate weakly weathered soils from more mature profiles.
– Protovertic: an early-stage vertic horizon exhibiting initial shrink–swell features (cracks, slickensides) that indicate emerging dominance of high-swelling clays and potential for future structural instability.
Organic and surface epipedons (natural and anthropogenic)
– Mollic: a thick, dark, base-rich, granular surface horizon typical of grassland-derived soils that confers high fertility and tilth.
– Chernic: a chernozem-like humus-rich surface similar to mollic but regionally distinct; important for high-productivity steppe soils.
– Umbric: a dark, organic-enriched surface resembling mollic but with low base saturation and greater acidity, typical of forested humid climates.
– Sombric: an acidic, dark leached surface akin to umbric but distinguished by specific base-status thresholds and leaching history.
– Folic: a thin, relatively fresh organic-enriched surface that fails to meet mollic/histic thickness or humification criteria but signals recent organic inputs or disturbance.
– Histic: a predominantly organic, saturated horizon (peat/muck) characteristic of wetlands and peatlands, central to carbon storage and wetland delineation.
– Hortic: a strongly modified surface horizon resulting from intensive cultivation or imported amendments for horticultural production; indicative of altered fertility and management history.
– Plaggic: a thick, anthropogenic surface horizon formed by long-term addition of organic wastes (plaggen cultivation), producing deep, fertile, culturally stratified topsoils.
– Anthraquic: an organic-rich, reduced horizon reflecting prolonged human inputs under waterlogged conditions (charcoal, wastes); useful for reconstructing land use and controlling wetland fertility.
– Pretic: a horizon preserving relict anthropogenic or palaeoenvironmental features, valuable for archaeological and land‑use interpretation.
Redox, waterlogging and sulfur-related horizons
– Hydragric: a horizon defined by prolonged saturation and reducing conditions with redoximorphic colour patterns and organic accumulation; diagnostic of hydric soils and wetlands.
– Thionic: a horizon influenced by elevated reduced-sulfur compounds (sulfides) that can generate sulphuric acidity upon oxidation; occurs in tidal marshes and sulfidic sediments and requires special management to avoid acidification.
– Cryic: a temperature-defined horizon indicating persistently cold soils near freezing; marks limited pedogenesis, permafrost influence and cryoturbation in cold regions.
Salts, sulfates and carbonate accumulations
– Salic: a horizon with measurable soluble-salt accumulation and salt efflorescence, indicating salinity problems that constrain crop choice and reclamation needs.
– Gypsic: a subsurface accumulation of gypsum (CaSO4) in seams, nodules or soft masses formed under arid conditions; affects plant assemblages and soil chemistry.
– Petrogypsic: a cemented, rock-like gypsic hardpan where gypsum has been indurated into a continuous layer, severely restricting rooting and drainage.
– Calcic: a B horizon enriched in secondary calcium carbonate, often forming nodules or continuous layers in semi-arid climates; raises pH and limits deep rooting.
– Petrocalcic: an indurated carbonate hardpan in which secondary CaCO3 has consolidated into a continuous, rock-like layer, signalling long-term aridity and limiting subsurface exchange.
Silica and iron cementation, plinthite and hardpans
– Duric: a horizon cemented by opaline silica (duripan) producing an impermeable or restrictive layer that impedes root growth and water movement.
– Petroduric: a silica-cemented, rock-like duripan equivalent that indicates advanced pedogenic silica accumulation and severe physical limitation to land use.
– Petroplinthic: an indurated ironstone layer formed by hardened plinthite; its presence reflects repeated wet–dry cycles and produces an impermeable crust.
– Plinthic: a subsurface layer of iron-rich, humus-poor material (plinthite) that hardens irreversibly under alternating wet–dry conditions, restricting drainage and root penetration.
– Pisoplinthic: a plinthic variant containing ironstone pisoliths (rounded concretions) that mark localized iron accumulation under seasonal waterlogging.
– Petrogypsic, petrocalcic and petroduric horizons together exemplify cemented hardpans that are diagnostic of prolonged, specific geochemical regimes and that impose major mechanical and hydrological constraints.
Structural and compacted layers
– Fragic: a brittle, dense fragipan that breaks into irregular blocks and restricts root growth and percolation, often producing perched water tables in temperate loess or glacial landscapes.
– Cohesic: a compact, poorly structured horizon with marked cohesive strength arising from clay mineralogy, cementation or compaction; significant for engineering and agricultural limitations.
– Panpaic: a term for compacted or accumulation layers (natural or anthropogenic) that functionally operate as impermeable or semi-impermeable horizons and preserve stratigraphic evidence of deposition or cultivation.
Sodicity, natric and saline–alkali conditions
– Natric: a sodic subsurface horizon with elevated exchangeable sodium producing columnar or prismatic structure, abrupt textural contrasts and reduced permeability; forms under arid conditions or through inappropriate irrigation and often requires amelioration.
– Irragric: a horizon modified by long-term irrigation practices that alter salinity, sodicity and redox status, used to identify irrigation-induced soil degradation and reclamation needs.
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Colour- and parent-material-related horizons
– Ferric: a horizon enriched in iron oxides (hematite, goethite) giving red–yellow hues and often firm structure; signals oxidising pedogenesis and strong iron precipitation under well-drained conditions.
– Ferralic: an intensely weathered, oxide-dominated horizon typical of humid tropical soils with low-activity clays and very low cation-exchange capacity; diagnostic of long-term leaching and limits to fertility.
– Limonic: a silt-dominated horizon characteristic of loess-derived profiles; often agriculturally productive where chemical constraints are absent and indicative of Quaternary dust deposition.
– Tsitelic: a regionally used, colour-descriptive horizon name denoting distinctive yellowish–brown colours related to particular iron states or mineral assemblages.
– Terric: a horizon that closely matches the regional typical soil matrix, used to indicate continuity with local soil types and to assist in mapping variants.
Vertisols and shrink–swell behaviour
– Vertic: a horizon dominated by high shrink–swell clays that produce deep cracks, slickensides and self‑mulching behaviour; these traits drive distinctive hydrological and management challenges including seasonally variable waterlogging and surface cracking.
Implications for classification and land management
Collectively, these diagnostic horizons allow WRB users to infer dominant pedogenic processes, past and present environmental conditions (climate, hydrology, parent material and anthropogenic activity), and practical constraints such as rooting depth, drainage, nutrient status and reclamation needs. Recognition of hardpans (silica, carbonate, gypsum, plinthite), salt- and sodium-affected layers, redox‑imposed horizons and anthropogenic epipedons is essential for accurate soil mapping, land-use planning and remediation strategies across diverse bioclimatic zones.
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Diagnostic horizons in the USDA Soil Taxonomy
Diagnostic horizons record the dominant near-surface and subsurface pedogenic processes and provide primary criteria for soil classification. Surface diagnostic horizons (epipedons) are distinguished by organic matter, color, texture, chemistry, or anthropogenic modification and reflect vegetation, hydrology, climate, and land use. Subsurface diagnostic horizons indicate redistribution, precipitation, cementation, or physical hardening of materials within the profile and are essential for interpreting soil development and landscape history.
Surface diagnostic horizons (epipedons)
- Anthropic epipedon: a surface layer substantially altered by prolonged human activity (cultivation, additions of organics or artifacts), producing a topsoil that departs markedly from the natural parent material and signals cultural influence on pedogenesis.
- Plaggen epipedon: an extreme anthropic form produced by centuries of repeated additions (sod, manure, compost), yielding a thick, dark cultivated horizon historically common in parts of Europe.
- Mollic epipedon: a thick, dark, strongly bioturbated, organically enriched surface horizon developed under base-rich conditions (typical of grassland-derived fertile soils).
- Umbric epipedon: similar in appearance to mollic but with lower base saturation and greater acidity due to stronger leaching or less base-rich parent material.
- Melanic epipedon: a very dark, fluffy, highly porous surface horizon rich in well-preserved organic matter, commonly forming on volcanic ash (tephra)-derived parent materials.
- Histic epipedon: an organic-rich horizon formed under prolonged saturation and reducing conditions (peat or muck), characteristic of marshes, bogs, and other waterlogged settings.
- Folistic epipedon: an organic-dominated surface horizon where organic inputs accumulate without continuous saturation; thinner and less decomposed than histic horizons and typical beneath forest litter in cool or moist climates.
- Ochric epipedon: a light-colored, thin, or low-organic surface horizon that lacks the thickness or darkness of other epipedons; often the default surface where richer epipedons are absent, and commonly associated with limited organic accumulation or erosion.
Subsurface diagnostic horizons and hardened layers
Subsurface horizons reflect processes such as eluviation, illuviation, cementation, salt or carbonate accumulation, silica precipitation, and mechanical compaction.
- Albic horizon: an eluviated, pale subsurface layer depleted in clay, organic matter, and oxides relative to overlying horizons, indicative of strong leaching.
- Argillic horizon: a horizon with illuvial accumulation of clay, often with increased structural development and clay coatings on peds, demonstrating fine-particle translocation.
- Kandic horizon: characterized by accumulation of low-activity clays (e.g., kaolinite) and oxide coatings, producing texture change and reduced cation-exchange capacity relative to more active clay horizons.
- Nitisolic/nitic horizon: a structurally stable clay horizon with specific clay properties and strong aggregation produced by advanced pedogenic processes.
- Spodic horizon: an illuvial accumulation of organic matter complexed with Al and/or Fe beneath acid, sandy forest soils, frequently occurring under an albic horizon.
- Sombric horizon: a dark, leached subsurface horizon stained by translocated organic matter and acidic conditions but lacking the base-rich attributes of mollic horizons.
- Cambic horizon: a weakly developed subsurface zone showing alteration in color, structure, or weathering products but without distinctive accumulations (clay, carbonates) required for higher-order diagnostics.
- Oxic horizon: a deeply weathered horizon dominated by sesquioxides and very low-activity clays, characteristic of old tropical profiles with intense leaching (e.g., Oxisols).
- Calcic horizon: accumulation of secondary calcium carbonate as coatings, nodules, or soft concretions, typical where leaching is limited in arid to semi-arid climates.
- Petrocalcic horizon: a calcic horizon that has been cemented into a hard, continuous caliche layer restricting roots and water movement.
- Gypsic horizon: accumulation of gypsum (CaSO4) as crystals, coatings, or nodules formed by evaporative concentration of sulfates.
- Petrogypsic horizon: a gypsum-rich layer lithified into a hardened pan by cementation.
- Salic horizon: a horizon with elevated soluble salts and high electrical conductivity that produces saline conditions affecting vegetation and use.
- Anhydric horizon: a horizon reflecting extreme drying, desiccation, or chemical depletion of pore water, indicating atypical hydrological or saline/alkaline processes.
- Natric horizon: a clay-accumulation horizon enriched in exchangeable sodium, often with columnar or prismatic structure and reduced permeability due to sodium dispersion.
- Duripan: a subsurface layer cemented by silica or similar agents into a dense, relatively impermeable pan common in arid and semi-arid environments.
- Fragipan: a dense, brittle, coarse-structured layer formed by physical reworking or compaction that impedes root growth and drainage and fractures brittly when dry.
- Ortstein: a hardened, humus- and iron-rich layer produced by podzolization or related processes, typical of cool boreal or periglacial soils.
- Petroplinthic horizon: an indurated, iron-rich layer formed from plinthite that hardens upon repeated wetting and drying to produce ironstone-like masses (pisoliths/ferricrete) in seasonally wet tropical or subtropical settings.
- Placic horizon: a thin, sheet-like, often dark and glossy laminar crust formed by concentration of iron/aluminum and organics near the surface that impedes root and water movement.
- Glossic horizon: a transitional zone with marked textural or color contrast forming tongues or intergrades, reflecting lateral or vertical translocation and complex pedogenic interactions.
- Agric horizon: a subsurface or plow-affected zone altered by tillage and long-term cultivation, showing mixed structure and anthropogenic inputs that distinguish it from naturally developed horizons.
Collectively, these diagnostic epipedons and subsurface horizons encode the dominant soil-forming processes—organic accumulation and decomposition, eluviation–illuviation, mineral weathering, salt and carbonate precipitation, cementation, and anthropogenic modification—and thus underpin interpretation and classification within the USDA Soil Taxonomy.