Soil horizon - Wikipedia. A cross- section of soil top, revealing horizons. A soil horizon is a layer generally parallel to the soil crust, whose physical characteristics differ from the layers above and beneath. Each soil type usually has three or four horizons. Horizons are defined in most cases by obvious physical features, chiefly colour and texture. Horizon free download. Get the latest version now. Innovative Xbox 360 modding tool on the market. Horizon is a realtime, open-source backend for JavaScript apps. Rapidly build and deploy web or mobile apps using a simple JavaScript API. Scale your apps to millions. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material (i. The differentiation of the soil into distinct horizons is largely the result of influences, such as air, water, solar radiation and plant material, originating at the soil- atmosphere interface.
Since the weathering of the soil occurs first at the surface and works its way down, the uppermost layers have been changed the most, while the deepest layers are most similar to the original parent material. Identification and description of the horizons present at a given site is the first step in soil classification at higher levels, through the use of systems such as the USDA soil taxonomy or the Australian Soil Classification. The World Reference Base for Soil Resources lists 4. Most soils, especially in temperate climates, conform to a similar general pattern of horizons, often represented as . 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 . It should be emphasised that no one system is more correct . Also, many subtropical and tropical areas have soils such as oxisols or aridisols that have very different horizons from . 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 that 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 horizon not listed is the O horizon which is grass and animal/plant life. Soil generally consists of visually and texturally distinct layers, also called profiles, which can be summarized as follows from top to bottom: O) Organic matter: Surficial organic deposit with litter layer of plant residues in relatively non- decomposed form. This layer of mineral soil contains the most organic matter accumulation and soil life. This layer eluviates (is depleted of) iron, clay, aluminium, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter colored . A- horizons 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 . This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation. C) Parent rock, also known as substratum: The parent material in sedimentary deposits. Layer of large unbroken rocks. This layer may accumulate the more soluble compounds . This layer denotes the layer of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer. These areas of bedrock are under 5. It is a surface layer dominated by the presence of large amounts of organic material derived from dead plant and/or animal residues which is in varying stages of decomposition. The O horizon is generally absent in grassland regions. The O horizon usually occurs in forested areas and is commonly referred to as the forest floor. The O horizon should be considered distinct from the layer of leaf litter covering many heavily vegetated areas, which contains no weathered mineral particles and is not part of the soil itself. O horizons may be divided into O1 and O2 categories, whereby O1 horizons contain decomposed matter whose origin can be spotted on sight (for instance, fragments of rotting leaves), and O2 horizons contain only well- decomposed organic matter, the origin of which is not readily visible. O horizons may also be divided into three subordinate O horizons denoted as: Oi, Oe, and Oa. P horizon. 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. This layer generally contains enough partially decomposed (humified) organic matter to give the soil a color darker than that of the lower horizons. The A horizons are often coarser in texture, having lost some of the finer materials by translocation to lower horizons and by erosion. This layer is known as the zone in which the most biological activity occurs. Soil organisms such as earthworms, potworms (enchytraeids), arthropods, nematodes, fungi, and many species of bacteria and archaea are concentrated here, often in close association with plant roots. Thus the A horizon may be referred to as the biomantle. These are present only in older, well- developed soils, and generally occur between the A and B horizons. The E horizon often has a pale color that is generally lighter in color than either the horizon above or below it. E horizons are commonly found in soils developed under forests, but are rare in soils developed under grasslands. 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 . In soils that contain gravels, due to animal bioturbation, a stonelayer commonly forms near or at the base of the E horizon. The above layers may be referred to collectively as the . The layers below have no collective name but are distinct in that they are noticeably less affected by surface soil- forming processes. B horizon. The B horizon is commonly referred to as the . In humid regions, B horizons are the layers of maximum accumulation of materials such as silicate clays, iron (Fe) and aluminium (Al) oxides, and organic material. These materials typically accumulate through a process termed illuviation, wherein the materials gradually wash in from the overlying horizons. Accordingly, this layer is also referred to as the . In addition, it is defined as having a distinctly different structure or consistency than the horizon(s) above and the horizon(s) below. The B horizon may also have stronger colors (higher chroma) than the A horizon. In arid and semiarid regions, calcium carbonate or calcium sulfate may accumulate in the B horizon. As with the A horizon, the 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 . B2 horizons have a concentration of clay, minerals, or organics and feature the highest soil 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 little humus. It is usually brownish or reddish due to residual clay and iron oxides. C horizon. This layer is little affected by soil- forming processes and they thus have a lack of pedological development. In other words, the C horizon is the unconsolidated material underlying the solum (A and B horizons). It may or may not be the same as the parent material from which the solum formed. The C horizon forms as the R horizon weathers and rocks break up into smaller particles. The C horizon is below the zones of greatest biological activity and it has not been sufficiently altered by soil genesis to qualify as a B horizon. In dry regions, carbonates and gypsum may be concentrated in the C horizon. While loose enough to be dug with a shovel, C horizon material often retains some of the structural features of the parent rock or geologic deposits from which it formed. The A and B layers usually originated from the C horizon. The upper layers of the C horizon may in time become a part of the solum as weathering and erosion continue. The C Horizon may contain lumps or more likely large shelves of unweathered rock, rather than being made up solely of small fragments as in the solum. It contains rocks with cracks and crevices. Unlike the above layers, R horizons are composed largely of consolidated masses of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer. L horizon. Included are coprogenous earth (sedimentary peat), diatomaceous earth, and marl; and is usually found as a remnant of past bodies of standing water. Subdivisions within master horizons. For example, if three different combinations of structure and colors can be seen in the B horizon, then the profile may include a sequence such as B1- B2- B3. If two different geologic parent materials (e. For example, a soil would have a sequence of horizons designated O- A- B- 2. C if C horizon developed in a different parent material. Where a layer of mineral soil material was transported by humans from a source outside the pedon, the caret symbol (^) is inserted before the master horizon designation. Transition horizons.
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