Source: Master horizons and layers H horizons or layers: These are layers of organic material. Organic material is defined by having a certain minimum content of soil organic carbon. In the
WRB, this is 20% (by weight). The H horizon is formed from organic residues that are not incorporated into the mineral soil. The residues may be partially altered by decomposition. Contrary to the O horizons, the H horizons are saturated with water for prolonged periods, or were once saturated but are now drained artificially. In many H horizons, the residues are predominantly
sphagnum mosses (peat mosses). Although these horizons form above the mineral soil surface, they may be buried by redeposited mineral soil and therefore be found at greater depth. H horizons may be overlain by O horizons that especially form after
drainage.
O horizons or layers: These are layers of organic material. Organic material is defined by having a certain minimum content of soil
organic carbon. In the WRB, this is 20% (by weight). The O horizon is formed from organic residues that are not incorporated into the mineral soil. The residues may be partially altered by
decomposition. Contrary to the H horizons, the O horizons are not saturated with water for prolonged periods and not drained artificially. In many O horizons, the residues are
leaves,
coniferous needles,
twigs,
mosses, and
lichens. Although these horizons form above the mineral soil surface, they may be buried by redeposited mineral soil (e.g. after
landslide,
flooding, aeolian
dust deposition) and therefore be found at greater depth.
A horizons: These are mineral horizons that formed at the surface or below an O horizon. All or much of the original rock structure has been obliterated. Additionally, they are characterized by one or more of the following: • an accumulation of
humified organic matter, intimately mixed with the mineral fraction, and not displaying properties characteristic of E or B horizons (see below); • properties resulting from
cultivation,
pasturing, or similar kinds of
disturbance; • a morphology that is different from the underlying B or C horizon, resulting from processes related to the surface. If a surface horizon has properties of both A and E horizons but the dominant feature is an accumulation of humified organic matter, it is designated an A horizon.
E horizons: These are mineral horizons in which the main feature is loss of
clay minerals,
iron,
aluminium,
organic matter or some combination of these, leaving a concentration of
sand and
silt particles, mostly of
quartz. However,
pedogenesis is advanced, because the lost substances first have been formed or accumulated there. All or much of the original rock structure is obliterated. An E horizon is usually, but not necessarily, lighter in colour than an underlying B horizon. In some soils, the colour is that of the sand and silt particles. An E horizon is most commonly differentiated from an underlying B horizon: by colour of higher
value or lower
chroma, or both; by coarser texture; or by a combination of these properties. An E horizon is commonly near to the surface, below an O or A horizon, and above a B horizon. However, the symbol E may be used without regard to the position in the profile for any horizon that meets the requirements and that has resulted from soil genesis.
B horizons: These are horizons that formed below an A, E, H, or O horizon, and in which the dominant features are the obliteration of all or much of the original rock structure, together with one or a combination of the following: • residual concentration of
oxides (especially iron oxides) and/or
clay minerals; • evidence of removal of
carbonates or
gypsum; •
illuvial concentration, alone or in combination, of
clay minerals,
iron,
aluminium,
organic matter,
carbonates,
gypsum or
silica; • coatings of oxides that make the horizon conspicuously lower in
value, higher in
chroma, or redder in
hue than overlying and underlying horizons without apparent illuviation of iron; • alteration that forms clay minerals or liberates oxides or both and that forms a granular, blocky or prismatic
structure if volume changes accompany changes in moisture content; •
brittleness. All kinds of B horizons are or were originally subsurface horizons. Examples of layers that are not B horizons are: layers in which clay films either coat
rock fragments or are found on finely stratified unconsolidated sediments, whether the films were formed in place or by illuviation; layers into which carbonates have been illuviated but that are not contiguous to an overlying genetic horizon; and layers with
gleying but no other pedogenic changes.
C horizons or layers: These are horizons or layers, excluding hard bedrock, that are little affected by
pedogenic processes and lack properties of H, O, A, E or B horizons. Most are mineral layers, but some
siliceous and
calcareous layers, such as shells,
coral, and
diatomaceous earth, are included. The material of C layers may be either like or unlike that from which the overlying
solum presumably formed. Plant roots can penetrate C horizons, which provide an important growing medium. Included as C layers are
sediments,
saprolite, non-indurated
bedrock, and other geological materials that commonly slake within 24 hours when air-dry or drier chunks are placed in water, and that, when moist, can be dug with a spade. Some soils form in material that is already highly weathered, and if such material does not meet the requirements of A, E, or B horizons, it is designated C. Changes not considered pedogenic are those not related to overlying horizons. Layers having accumulations of silica, carbonates, or gypsum, even if indurated, may be included in C horizons, unless the layer is obviously affected by pedogenic processes; then it is a B horizon.
R layers: These consist of hard
bedrock underlying the soil.
Granite,
basalt,
quartzite, and indurated
limestone or
sandstone are examples of bedrock that are designated R. Air-dry or drier chunks of an R layer, when placed in water, will not slake within 24 hours. The R layer is sufficiently coherent when moist to make hand digging with a spade impractical. The bedrock may contain cracks, but these are so few and so small that few roots can penetrate. The cracks may be coated or filled with soil material.
I layers: These are
ice lenses and wedges that contain at least 75 per cent ice (by volume) and that distinctly separate layers (organic or mineral) in the soil.
L layers: These are sediments deposited in a body of water. They may be organic or mineral. Limnic material is either: (i) deposited by precipitation or through action of aquatic organisms, such as
algae, especially
diatoms; or (ii) derived from underwater and floating aquatic plants and subsequently modified by aquatic animals. L layers include coprogenous earth or sedimentary
peat (mostly organic),
diatomaceous earth (mostly siliceous), and
marl (mostly calcareous).
W layers: These are either water layers in soils or water layers submerging soils. The water is present either permanently or cyclic within the time frame of 24 hours. Some organic soils float on water. In other cases, shallow water (i.e. water not deeper than 1 m) may cover the soil permanently, as in the case of shallow lakes, or cyclic, as in
tidal flats. The occurrence of
tidal water can be indicated by the letter W in brackets: (W).
Transitional horizons and layers A horizon that combines the characteristics of two master horizons is indicated with both capital letters, the dominant one written first. Example: AB and BA. If discrete, intermingled bodies of two master horizons occur together, the horizon symbols are combined using a slash (/). Example: A/B and B/A. The master horizon symbols may be followed by the lowercase letters indicating subordinate characteristics (see below). Example: AhBw. The I, L and W symbols are not used in transitional horizon designations.
Subordinate characteristics This is the list of suffixes to the master horizons. After the hyphen, it is indicated to which master horizons the suffixes can be added. •
a: Highly decomposed organic material—H and O horizons. •
b: Buried genetic horizon—mineral horizons, not cryoturbated. •
c:
Concretions or
nodules—mineral horizons. •
c: Coprogenous earth—L horizon. •
d: Dense layer (physically root restrictive)—mineral horizons, not with m. •
d:
Diatomaceous earth—L horizon. •
e: Moderately decomposed organic material—H and O horizons. •
f:
Frozen soil—not in I and R horizons. •
g:
Stagnic conditions—no restriction. •
h: Accumulation of
organic matter—mineral horizons. •
i:
Slickensides—mineral horizons. •
i: Slightly decomposed organic material—H and O horizons. •
j:
Jarosite accumulation—no restriction. •
k: Accumulation of pedogenic
carbonates—no restriction. •
l: Mottling due to upmoving
groundwater (
gleying)—no restriction. •
m: Strong
cementation or induration (pedogenic, massive)—mineral horizons. •
m:
Marl—L horizon. •
n: Pedogenic accumulation of exchangeable
sodium—no restriction. •
o: Residual accumulation of
sesquioxides (pedogenic)—no restriction. •
p:
Ploughing or other human disturbance—no restriction; ploughed E, B, or C horizons are referred to as Ap. •
q: Accumulation of pedogenic
silica—no restriction. •
r: Strong
reduction—no restriction. •
s:
Illuvial accumulation of sesquioxides—B horizons. •
t: Illuvial accumulation of
clay minerals—B and C horizons. •
u: Urban and other human-made materials (
artefacts—H, O, A, E, B and C horizons. •
v: Occurrence of
plinthite—no restriction. •
w: Development of colour or
structure—B horizons. •
x:
Fragipan characteristics—no restriction. •
y: Pedogenic accumulation of
gypsum—no restriction. •
z: Pedogenic accumulation of salts more soluble than gypsum—no restriction. •
@: Evidence of
cryoturbation—no restriction.
Discontinuities and vertical subdivisions Numerical prefixes are used to denote lithic discontinuities. By convention, 1 is not shown. Numerical suffixes are used to denote subdivisions within a horizon. The horizons in a profile are combined using a hyphen (-). Example: Ah-E-Bt1-2Bt2-2BwC-3C1-3C2. == Horizons and layers according to the USDA
Field book for describing and sampling soils (2024) ==