جمعه 27 فروردین 1389
A soil horizon is a specific layer in the land area which measures parallel to the soil 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
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(Redirected from O horizon)
Soil samples illustrating horizons (subsoil on right)
A soil horizon is a specific layer in the land area which measures parallel to the soil 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. Soils vary in the degree to which horizons are expressed. Relatively new deposits of soil parent material, such as alluvium, sand dunes, or volcanic ash, may have no horizon formation, or only the distinct layers of deposition. As age increases, horizons generally are more easily observed. The exception occurs in some older soils, with few horizons expressed in deeply weathered soils, such as the oxisols in tropical areas with high annual precipitation.
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 World Reference Base for Soil Resources lists 40 diagnostic horizons.
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 assy horizons and these are described by soil scientists when seeking to classify soils (Soil-Net). 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.
Albeluvisol Albic horizon
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 one horizon not listed is O horizon which is grass and animal/plant life.) Soil has three main horizons (A, B, and C), which will be explained below along with other layers.
Soil generally consists of visually and texturally distinct layers, which can be summarized as follows from top to bottom:
O) Organic matter: Litter layer of plant residues in relatively undecomposed form.
A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. This layer eluviates (is depleted of) iron, clay, aluminum, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter colored "E" subsurface soil horizon is apparent at the base of the "A" horizon. 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 "biomantle".
C) parent rock: Layer of big unbroken rocks. This layer may accumulate the more soluble compounds .
The “O” stands for Organic. This layer is also known as humus. With its 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.