Soil horizon

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 and in terms relative to the surrounding material, i.e. "coarser" or "sandier" than the horizons above and below.
The identified horizons are indicated with symbols, which are mostly used in a hierarchical way. Master 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, 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 do not have a clear development of horizons.
A soil horizon is a result of soil-forming processes. Layers that have not undergone such processes may be simply called "layers".

Horizon sequence

Many soils have an organic surface layer, which is denominated with a capital letter "O". 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 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 and move downwards. When this eluviation is pronounced, a lighter coloured E subsurface soil horizon is apparent at the base of 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 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 of hard rock that cannot be excavated by hand. Soils formed in situ from bedrock will exhibit strong similarities to this bedrock layer.

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%, i.e. rock structure, if present, in < 50%.
E:
Mineral horizon;
has lost by downward movement within the soil one or more of the following: Fe, Al, and/or Mn species; clay minerals; organic matter.
B:
Mineral horizon that has formed below an A or E horizon;
rock structure, if present, in < 50% ;
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, 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% ;
not resulting from the cementation of a soil horizon.
I:
≥ 75% ice, 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 .
b: Buried horizon; first, the horizon has formed, and then, it was buried by mineral material .
c: Concretions and/or nodules ; only used if following another suffix that indicates the accumulated substance.
d: Drained .
e: Organic material in an intermediate state of decomposition .
e: Saprolite .
f: Permafrost .
g: Accumulation of Fe and/or Mn oxides predominantly inside soil aggregates, if present, and loss of these oxides on aggregate surfaces, or loss of Fe and/or Mn by lateral subsurface flow and pale colours in ≥ 50% of the exposed area ; transport in reduced form .
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 .
i: Organic material in an initial state of decomposition; .
j: Accumulation of jarosite and/or schwertmannite .
k: Accumulation of secondary carbonates .
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 .
m: Pedogenic cementation in ≥ 50% of the volume; cementation class: at least moderately cemented; only used if following another suffix that indicates the cementing agent .
n: Exchangeable sodium percentage ≥ 6% .
o: Residual accumulation of large amounts of pedogenic oxides in strongly weathered horizons .
p: Modification by cultivation ; mineral layers are designated A, even if they belonged to another layer before cultivation .
q: Accumulation of secondary silica .
r: Strong reduction .
s: Accumulation of Fe oxides, Mn oxides and/or Al by vertical illuviation processes from above ..
ss: Slickensides and/or wedge-shaped aggregates .
t: Accumulation of clay minerals by illuviation processes ..
u: Containing artefacts or consisting of artefacts .
v: Plinthite .
w: Formation of soil structure and/or oxides and/or clay minerals .
x: Fragic characteristics .
y: Accumulation of secondary gypsum .
z: Presence of readily soluble salts .
@: cryogenic alteration.
α: Presence of primary carbonates .
β: Bulk density ≤ 0.9 kg dm-3 .
γ: Containing ≥ 5% volcanic glasses in the fraction between > 0.02 and ≤ 2 mm .
δ: High bulk density, so that roots cannot enter, except along cracks .
λ: Deposited in a body of water .
ρ: Relict features .
σ: Permanent water saturation and no redoximorphic features
τ: Human-transported natural material .
φ: Accumulation of Fe and/or Mn in reduced form by lateral subsurface flow with subsequent oxidation .

I and W layers have no suffixes.
Combination of suffixes:
1. The c follows the suffix that indicates the substance that forms the concretions or nodules; if this is true for more than one suffix, each one is followed by the c.
2. The m follows the suffix that indicates the substance that is the cementing agent; if this is true for more than one suffix, each one is followed by the m.
3. The ρ follows the suffix that indicates the relict features; if this is true for more than one suffix, each one is followed by the ρ.
4. If two suffixes belong to the same soil-forming process, they follow each other immediately; in the combination of t and n, the t is written first; rules 1, 2 and 3 have to be followed, if applicable.
Examples: Btn, Bhs, Bsh, Bhsm, Bsmh.
5. If in a B horizon the characteristics of the suffixes g, h, k, l, o, q, s, t, v, or y are strongly expressed, the suffix w is not used, even if its characteristics are present; if the characteristics of the mentioned suffixes are weakly expressed and the characteristics of the suffix w are present as well, the suffixes are combined.
6. In H and O layers, the i, e or a is written first.
7. The @, f and b are written last, if b occurs together with @ or f : @b, fb.
8. Besides that, combinations must be in the sequence of dominance, the dominant one first. Examples: Btng, Btgb, Bkcyc.