A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, i.e. "coarser" or "sandier" or "darker" than the horizons above and below.
The identified horizons are indicated with symbols, which are mostly used in a hierarchical way. Master horizons (main horizons) are indicated by capital letters. Suffixes, in form of lowercase letters and figures, further differentiate the master horizons. There are many different systems of horizon symbols in the world. No one system is more correct—as artificial constructs, their utility lies in their ability to accurately describe local conditions in a consistent manner. Due to the different definitions of the horizon symbols, the systems cannot be mixed.
In most soil classification systems, horizons are used to define soil types. The German system uses entire horizon sequences for definition. Other systems pick out certain horizons, the "diagnostic horizons", for the definition; examples are the World Reference Base for Soil Resources (WRB), Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License. the USDA soil taxonomy, and the Australian Soil Classification. Diagnostic horizons are usually indicated with names, e.g. the "cambic horizon" or the "spodic horizon". The WRB lists 40 diagnostic horizons. In addition to these diagnostic horizons, some other soil characteristics may be needed to define a soil type. Some soils, called "Entisols" in the USDA soil taxonomy, do not have a clear development of horizons.
A soil horizon is a result of soil-forming processes (pedogenesis). Layers that have not undergone such processes may be simply called "layers". According to the fourth edition of the World Reference Base, "A soil layer is a zone in the soil, approximately parallel to the soil surface, with properties different from layers above and/or below it. If at least one of these properties is the result of soil-forming processes, the layer is called a soil horizon".
Horizon sequence
Many soils have an organic surface layer, which is denominated with a capital letter "O" (letters may differ depending on the system). The mineral soil usually starts with an A horizon. If a well-developed subsoil horizon as a result of soil formation exists, it is generally called a B horizon. An underlying loose, but poorly developed horizon is called a C horizon. Hard bedrock is mostly denominated R. Most individual systems defined more horizons and layers than just these five. In the following, the horizons and layers are listed more or less by their position from top to bottom within the soil profile. Not all of them are present in every soil.
Soils with a history of human interference, for instance through major earthworks or regular deep ploughing, may lack distinct horizons almost completely. When examining soils in the field, attention must be paid to the local geomorphology and the historical uses to which the land has been put, in order to ensure that the appropriate names are applied to the observed horizons.
Examples of soil profiles
O) Organic surface layer: Plant litter layer—the upper part is often relatively undecomposed, but the lower part may be strongly humified.
A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. Additionally, due to weathering, oxides (mainly iron oxides) and clay minerals are formed and accumulated. It has a pronounced soil structure. But in some soils, clay minerals, iron, aluminum, organic compounds, and other constituents are soluble or in colloidal suspension and move downwards. When this eluviation is pronounced, a lighter coloured E subsurface soil horizon is apparent below the A horizon. The A horizon may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil. In this case, the A horizon is regarded as a "biomantle".
B) Subsoil: This layer normally has less organic matter than the A horizon, so its colour is mainly derived from iron oxides. Iron oxides and clay minerals accumulate as a result of weathering. In soil, where substances move down from the topsoil, this is the layer where they accumulate. The process of accumulation of clay minerals, iron, aluminum, and organic compounds, is referred to as "illuviation". The B horizon generally has a soil structure.
C) Substratum: Layer of non-indurated poorly weathered or unweathered rocks. This layer may accumulate more potentially soluble compounds like CaCO3. Soils formed in situ from non-indurated material exhibit similarities to this C layer.
R) Bedrock: R horizons denote the layer of partially weathered or unweathered bedrock at the base of the soil profile. Unlike the above layers, R horizons consist largely of continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand. Soils formed in situ from bedrock will exhibit strong similarities to this bedrock layer.
Horizons and layers according to the World Reference Base for Soil Resources
The designations are found in Chapter 10 of the World Reference Base for Soil Resources Manual, 4th edition (2022). The chapter starts with some general definitions:
The fine earth comprises the soil constituents ≤ 2 mm. The whole soil comprises fine earth, coarse fragments, artefacts, cemented parts, and dead plant residues of any size.
A litter layer is a loose layer that contains > 90% (by volume, related to the fine earth plus all dead plant residues) recognizable dead plant tissues (e.g. undecomposed leaves). Dead plant material still connected to living plants (e.g. dead parts of sphagnum mosses) is not regarded to form part of a litter layer. The soil surface (0 cm) is by convention the surface of the soil after removing, if present, the litter layer and, if present, below a layer of living plants (e.g. living mosses). The mineral soil surface is the upper limit of the uppermost layer consisting of mineral material.
A soil layer is a zone in the soil, approximately parallel to the soil surface, with properties different from layers above and/or below it. If at least one of these properties is the result of soil-forming processes, the layer is called a soil horizon. In the following, the term layer is used to indicate the possibility that soil-forming processes did not occur.
The following layers are distinguished (see Chapter 3.3 of the WRB Manual): Organic layers consist of organic material: Have ≥ 20% organic carbon, not consisting of artefacts (related to the fine earth plus the dead plant residues of any length and a diameter ≤ 5 mm) and do not form part of a litter layer. Organotechnic layers consist of organotechnic material: Have ≥ 35% (by volume, related to the whole soil) artefacts containing ≥ 20% organic carbon; and < 20% organic carbon, not consisting of artefacts (related to the fine earth plus the dead plant residues of any length and a diameter ≤ 5 mm). Mineral layers are all other layers.
The designation consists of a capital letter (master symbol), which in most cases is followed by one or more lowercase letters (suffixes).
Master symbols
H: Organic or organotechnic layer, not forming part of a litter layer; water saturation > 30 consecutive days in most years or drained; generally regarded as peat layer or organic limnic layer.
O: Organic horizon or organotechnic layer, not forming part of a litter layer; water saturation ≤ 30 consecutive days in most years and not drained; generally regarded as non-peat and non-limnic horizon.
A: Mineral horizon at the mineral soil surface or buried; contains organic matter that has at least partly been modified in-situ; soil structure and/or structural elements created by cultivation in ≥ 50% (by volume, related to the fine earth), i.e. rock structure, if present, in < 50% (by volume).
E: Mineral horizon; has lost by downward movement within the soil (vertically or laterally) one or more of the following: Fe, Al, and/or Mn species; clay minerals; organic matter.
B: Mineral horizon that has (at least originally) formed below an A or E horizon; rock structure, if present, in < 50% (by volume, related to the fine earth); one or more of the following processes of soil formation: formation of soil aggregate structure formation of clay minerals and/or oxides accumulation by illuviation processes of one or more of the following: Fe, Al, and/or Mn species; clay minerals; organic matter; silica; carbonates; gypsum removal of carbonates or gypsum. B horizons may show other accumulations as well.
C: Mineral layer; unconsolidated (can be cut with a spade when moist), or consolidated and more fractured than the R layer; no soil formation, or soil formation that does not meet the criteria of the A, E, and B horizon.
R: Consolidated rock; air-dry or drier specimens, when placed in water, will not slake within 24 hours; fractures, if present, occupy < 10% (by volume, related to the whole soil); not resulting from the cementation of a soil horizon.
I: ≥ 75% ice (by volume, related to the whole soil), permanent, below an H, O, A, E, B or C layer.
W: Permanent water above the soil surface or between layers, may be seasonally frozen.
Suffixes
This is the list of suffixes to the master symbols. In brackets is indicated to which master symbols the suffixes can be added. The suffixes e and i have different meanings for organic and mineral layers.
a: Organic material in an advanced state of decomposition [a like advanced] (H, O). b: Buried horizon; first, the horizon has formed, and then, it was buried by mineral material [b like buried] (H, O, A, E, B). c: Concretions and/or nodules [c like concretion]; only used if following another suffix (k, q, v, y) that indicates the accumulated substance. d: Drained [d like drained] (H). e: Organic material in an intermediate state of decomposition [e like intermediate] (H, O). e: Saprolite [e like saprolite] (C). f: Permafrost [f like frost] (H, O, A, E, B, C). g: Accumulation of Fe and/or Mn oxides predominantly inside soil aggregates, if present, and loss of these oxides on aggregate surfaces (A, B, C), or loss of Fe and/or Mn by lateral subsurface flow and pale colours in ≥ 50% of the exposed area (E); transport in reduced form [g like stagnic]. h: Significant amount of organic matter; in A horizons at least partly modified in situ; in B horizons predominantly by illuviation; in C horizons forming part of the parent material [h like humus] (A, B, C). i: Organic material in an initial state of decomposition; [i like initial] (H, O). j: Accumulation of jarosite and/or schwertmannite [j like jarosite] (H, O, A, E, B, C). k: Accumulation of secondary carbonates [k like German Karbonat] (H, O, A, E, B, C). l: Accumulation of Fe and/or Mn in reduced form by upward-moving capillary water with subsequent oxidation: accumulation predominantly at soil aggregate surfaces, if present, and reduction of these oxides inside the aggregates [l like capillary] (H, A, B, C). m: Pedogenic cementation in ≥ 50% of the volume; cementation class: at least moderately cemented; only used if following another suffix (k, l, q, s, v, y, z) that indicates the cementing agent [m like cemented]. n: Exchangeable sodium percentage ≥ 6% [n like natrium] (E, B, C). o: Residual accumulation of large amounts of pedogenic oxides in strongly weathered horizons [o like oxide] (B). p: Modification by cultivation (e.g. ploughing); mineral layers are designated A, even if they belonged to another layer before cultivation [p like plough] (H, O, A).