Soil Horizons

Organic horizons

Organic soil horizons are formed from plant and animal remains in the uppermost part of the soil through decomposition, primarily of litterfall, ground litter and surface litter. An organic horizon contains at least 20% organic carbon by weight, corresponding to approximately 35% organic matter by weight.

Depending on whether decomposition has been inhibited due to insufficient oxygen supply or not, two types of organic horizons are distinguished: the H horizon and the O horizon (see items 1 and 2 below).

The degree of decomposition of the organic material may vary from an early stage—where the structure of plant remains and similar materials is still visible—to a stage where the material has been completely transformed into a structureless mass that, when moist, forms a dark brown paste. This variation means that, in some cases, the organic horizons can be subdivided into different sub‑horizons (Of and Oh layers; see the box “Subdivision of the humus layer into humus forms” on this page).
Above the actual O horizon there is usually:

• a litter layer (L horizon) consisting of dead plant and animal remains that are so weakly decomposed that they retain their original living structure,

or

• a so‑called S layer. If litterfall and/or ground litter have become trapped in living mosses or other living plants, it is difficult to distinguish a separate litter layer. In such cases, the designation S layer is used for this layer consisting of litter and living green plants.

These two layers (L and S) are not part of the soil proper.

The gradual transformation of litter into humus usually results in an indistinct boundary between the litter layer/S layer and the uppermost organic soil horizon (O).

1)   H horizon

Decomposition is, or has been, inhibited by insufficient oxygen supply due to a high water content in the upper soil layers during large parts of the growing season. Typical species or species groups whose litter has served as the source material for humus formation include sphagnum mosses, sedges, rushes, horsetails and reeds, and to some extent also haircap mosses. The admixture of mineral soil is usually small.

In everyday language, an H horizon is most often simply referred to as a peat layer; note, however, that the humus form is termed peat‑like mor if the thickness of the humus layer is < 30.5 cm.

If a soil with a developed H horizon is artificially drained (e.g. by ditching), the designation H horizon is retained as long as the “peat character” remains (the decisive factor is thus the moisture conditions under which the humus material was formed).

2)   O horizon

Decomposition is not inhibited by high water content except, at most, temporarily. An O horizon therefore forms on naturally well‑drained sites. The O horizon is usually a mor layer.
The O horizon can be subdivided into the sub‑horizons Of (fermentation layer) and Oh (humus layer). A more detailed description of these is given under Subdivision of the humus layer into humus forms in the section Humus form.

Mineral soil horizons

Mineral soil horizons are designated according to their properties as A, E, B and C. Transitional horizons are denoted AB and BC. To further specify the properties of the horizons, these designations are combined with a suffix, for example h (accumulation of humus), s (accumulation of sesquioxides, i.e. iron and aluminium oxides), g (gley), or p (ploughed). Combinations in use include Ap, Eh, Bs, Bsh, Bh and Cg.

A horizon

This combined mineral soil and humus horizon is generally the uppermost mineral soil horizon. It consists of humus‑mixed mineral soil, where the proportion of mineral soil exceeds 10% by volume. The organic material is well decomposed. In nature, the horizon may form either with or without the influence of burrowing soil animals.

If burrowing soil animals are absent, at most a few “weak” aggregates occur, and the horizon is generally thin (up to a few centimetres). In the presence of burrowing soil animals—primarily where earthworms are abundant—numerous, more stable aggregates form and the horizon becomes thicker. Between the aggregates there is a branched network of coarse pores, and within the aggregates there is also a multitude of finer pores.

Ap horizon

Humus‑mixed mineral soil also forms through human ploughing of land for agricultural purposes; this humus‑mixed mineral soil (topsoil) is designated Ap. The Ap horizon is usually about 20 cm thick.

AB horizon

In soil types with the humus forms Mull and mull‑like moder, the boundary between the A and B horizons may be very diffuse. This transitional horizon is referred to as the AB horizon.

E horizon

A leaching horizon (eluviation horizon; bleached layer) in podzols, usually white to ash‑grey. The horizon forms through mineral weathering and leaching of soluble weathering products. Primarily weathering‑resistant minerals remain, such as quartz.

Eh horizon

In some cases, the E horizon may be mixed with humus and then has a dirty grey to dirty brown colour; such a horizon is designated Eh.

B horizon

A horizon that differs from the A, E and C horizons in colour and/or structure. In Swedish soils, the B horizon is most often an accumulation horizon in which iron, aluminium and/or humus accumulate. The B horizon is often subdivided into several sub‑horizons. To further specify the properties of the B horizon or its sub‑horizons, the designations Bs, Bsh and Bh are used.

Bs horizon

A B horizon in which podzolisation has resulted in a distinct accumulation of iron and aluminium compounds (rust yellow/rust red/rust brown colour), also known as rust soil. It is the iron compounds that give rise to the reddish colour tones.

Bsh horizon

Under certain conditions, a black‑brown layer with strong humus accumulation forms at the top of a Bs horizon—primarily in the soil type Podzol. This sub‑horizon is designated Bsh.

Bh horizon

A B horizon with strong humus accumulation. The colour is often brown‑black and no rust colour is visible. This type of horizon occurs mainly in the soil type Gleysol.

BC horizon

A diffuse transition zone between the B and C horizons.

C horizon

The underlying material/parent mineral soil that is largely unaffected by soil‑forming processes.

Cg horizon

A C horizon with gley features.

More about soil horizons on Wikipedia.