A soil horizon is a specific layer in the soil which parallels the land surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time periods. Identification and description of the horizons present at a given site is the first step in classifying a soil at higher levels, through the use of systems such as the USDA Soil Taxonomy or the Australian Soil Classification.
The term 'horizon' describes each of the distinctive layers that occur in a soil. Each soil type has at least one, usually three or four different horizons and these are described by soil scientists when seeking to classify soils. Horizons are defined in most cases by obvious physical features, colour and texture being chief among them. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, ie, 'coarser' or 'sandier' than the horizons above and below.
Most soils conform to a similar general pattern of horizons, often represented as an 'ideal' soil in diagrams. Each main horizon is denoted by a capital letter, which may then be followed by several alphanumerical modifiers highlighting particular outstanding features of the horizon. While the general O-A-B-C-R sequence seems fairly universal, some variation exists between the classification systems in different parts of the world. In addition, the exact definition of each main horizon may differ slightly - for instance, the US system uses the thickness of a horizon as a distinguishing feature, while the Australian system does not. It should be emphasised that no one system is more correct - as artificial constructs, their utility lies in their ability to accurately describe local conditions in a consistent manner.
The following horizons are listed by their position from top to bottom within the soil profile. Not all of these layers are present in every location - for instance, P horizons only form in areas which have been waterlogged for long periods of time. 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.
The "O" stands for Organic, with this surface layer being dominated by the presence of large amounts of organic material in varying stages of decomposition. The O Horizon should be held distinct from the layer of leaf litter covering many heavily vegetated areas - these contain no weathered mineral particles and are thus not part of the soil itself. If desired, O horizons may be divided into O1 and O2 categories, whereby O1 horizons contain decomposed matter whose origin can be spotted by sight (for instance, fragments of rotting leaves), and O2 horizons contain only well-decomposed organic matter whose origin cannot be immediately seen.
These horizons are also heavily organic, but are distinct from O Horizons in that they form under waterlogged conditions. The "P" designation comes from their common name, peats. They may be divided into P1 and P2 in the same way as O Horizons.
The technical definition of an A Horizon may vary, but it is most commonly described in terms relative to deeper layers. A Horizons may be darker in colour than deeper layers and contain more organic material, or they may be lighter but contain less clay or sesquioxides. The A is a surface horizon, and as such is also known as the zone in which most biological activity occurs. Soil organisms such as worms, nematodes, fungi, and many species of bacteria exist here, often in close association with plant roots. However, since biological activity extends far deeper into the soil, it cannot be used as a chief distinguishing feature of an A Horizon.
A horizons may be divided into A1, A2, and A3 horizons under the Australian system. A1s contain relatively higher levels of humic matter and will have a darker colour than the A2. A3s are a transitional layer between A and B, whose characteristics are more similar to the overlying A horizons than the B horizons below.
B Horizons are commonly referred to as 'subsoil', and consist of mineral layers which may contain concentrations of clay or minerals such as iron or aluminium, or organic material. In addition, they are defined by having a distinctly different structure or consistence to the A horizon above and the horizons below. They may also have ‘stronger’ colours (ie higher chroma) than the A horizon.
As with A horizons, B horizons may be divided into B1, B2, and B3 types under the Australian system. B1 is a transitional horizon of the opposite nature to an A3 - dominated by the properties of the B horizons below it, but containing some A-horizon characteristics. B2 horizons have a concentration of clay, minerals, or organics and feature the strongest pedological development within the profile. B3 horizons are transitional between the overlying B layers and the material beneath it, whether C or D horizon.
The A3, B1, and B3 horizons are not tightly defined, and their use is generally at the discretion of the individual worker.
"E" being short for eluviated, this designation is most commonly used in the United States to label a horizon that has been significantly leached of its mineral and/or organic content, leaving a pale layer largely composed of silicates. These are present only in older, well-developed soils, and generally occur between the A and B Horizons. In regions where this designation is not employed, leached layers are classified firstly as an A or B according to other characteristics, and then appended with the designation "e" (see the section below on horizon suffixes).
The above layers may be referred to collectively as the 'solum'. The layers below have no collective name but are distinct in that they are noticeably less affected by surface soil-forming processes.
C Horizons are simply named so because they come 'after' A and B within the soil profile. These layers are little affected by soil forming processes, and their lack of pedological development is one of their defining attributes. C Horizons may contain lumps of unweathered but weak rock, rather than being comprised solely of small fragments as in the solum. 'Ghost' rock structure may be present within these horizons.
D horizons are not universally distinguished, but in the Australian system refer to 'any soil material below the solum that is unlike the solum in general character, is not C horizon, and cannot be given reliable designation... [it] may be recognised by the contrast in pedologic organization between it and the overlying horizons' (MacDonald et al, 1990, p. 106).
R horizons basically denote the layer of partially-weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer, while depositional will often appear very distinct.
In addition to the main descriptors above, several modifiers exist to add necessary detail to each horizon. Firstly, each major horizon may be divided into sub-horizons by the addition of a numerical subscript, based on minor shifts in colour or texture with increasing depth (e.g., B21, B22, B23 etc).
Suffixes describing particular physical features of a horizon may also be added. These vary considerably between countries, but a limited selection of common ones employed in Australia is listed here:
The US system employs largely similar suffixes, with a few important differences. For instance, 'e' under the US system denotes a horizon containing 'organic material of intermediate decomposition' rather than a bleached horizon. A full list of suffixes is available online as part of the USDA Soil Survey Manual