جمعه 27 فروردین 1389
Soil horizon part 2
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
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. This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation.
The A Horizon is the top layer of the soil horizons or 'topsoil'. 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 color 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 earthworms, potworms (enchytraeids), arthropods, nematodes, fungi, and many species of bacteria and archaebacteria are concentrated here, often in close association with plant roots. Thus the A-horizon may be referred to as the "biomantle". However, since biological activity extends far deeper into the soil, it cannot be used as a chief distinguishing feature of an A Horizon.
“E” being short for eluviated, this designation is most commonly used in Stellenbosch University 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.
The B Horizon is commonly referred to as ‘subsoil’, and consist of mineral layers which may contain concentrations of clay or minerals such as iron or aluminum, or organic material which get there by leaching. Accordingly, this layer is also known as the "illuviated" horizon or the "zone of accumulation". In addition it is defined by having a distinctly different structure or consistence to the A horizon above and the horizons below. They may also have ‘stronger’ colors (is higher chroma) than the A horizon.
As with A horizon, B horizon 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.
Plant roots penetrate through this layer, but it has very little humus. It is usually brownish or red because of the clay and iron oxides (rust) washed down from A Horizon.
The C Horizon is simply named so because it comes ‘after’ A and B within the soil profile. This layer is little affected by soil forming processes (weathering), and the lack of pedological development is one of the defining attributes. The C Horizon may contain lumps or more likely large shelves of unweathered rock, rather than being comprised solely of small fragments as in the solum. ‘Ghost’ rock structure may be present within these horizons. C Horizon also contains parent material.
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 Horizon (bedrock)
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.
Horizon numbering and suffixes
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). While this can add necessary depth to a field description, workers should bear in mind that excessive division of a soil profile into narrow sub-horizons should be avoided. Walking as little as ten metres in any direction and digging another hole can often reveal a very different profile in regards to the depth and thickness of each horizon. Over-precise description can be a waste of time, and as a rule of thumb, layers thinner than 5 cm (2 inches) or so are best described as pans or segregations within a horizon rather than as a distinct layer.
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:
Thus, a bleached A2 horizon would be described as ‘A2e’.
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.
Main article: Paleosol
While soil formation is generally described as occurring in situ, as rock breaks down and is mixed with other materials, the process is often far more complicated. For instance, a fully-formed profile may have developed in an area only to be buried by wind- or water-deposited sediments which later formed into another soil profile. This sort of occurrence is most common in coastal areas, and descriptions are modified by numerical prefixes. Thus, a profile containing a buried sequence could be structured O, A1, A2, B2, 2A2, 2B21, 2B22, 2C with the buried profile commencing at 2A2.