Топырақ горизонттары: қабаттары мен сипаттамалары

Soil layer whose physical characteristics differ from the layers above and beneath

A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, i. e. 'coarser' or 'sandier' than the horizons above and below. The identified horizons are indicated with symbols, which are mostly used in a hierarchical way. Master horizons (main horizons) are indicated by capital letters. Suffixes, in form of lowercase letters and figures, further differentiate the master horizons. There are many different systems of horizon symbols in the world. No one system is more correct—as artificial constructs, their utility lies in their ability to accurately describe local conditions in a consistent manner. Due to the different definitions of the horizon symbols, the systems cannot be mixed. In most soil classification systems, horizons are used to define soil types. The German system uses entire horizon sequences for definition. Other systems pick out certain horizons, the "diagnostic horizons", for the definition; examples are the World Reference Base for Soil Resources (WRB), the USDA soil taxonomy and the Australian Soil Classification. Diagnostic horizons are usually indicated with names, e. g. the "cambic horizon" or the "spodic horizon". The WRB lists 40 diagnostic horizons. In addition to these diagnostic horizons, some other soil characteristics may be needed to define a soil type. Some soils do not have a clear development of horizons. A soil horizon is a result of soil forming processes (pedogenesis). Layers that have not undergone such processes may be simply called "layers".

Many soils have an organic surface layer, which is denominated with a capital letter "O" (letters may differ depending on the system). The mineral soil usually starts with an A horizon. If a well developed subsoil horizon as a result of soil formation exists, it is generally called a B horizon. An underlying loose, but poorly developed horizon is called a C horizon. Hard bedrock is mostly denominated R. Most individual systems defined more horizons and layers than just these five. In the following, the horizons and layers are listed more or less by their position from top to bottom within the soil profile. Not all of them are present in every soil. 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.

O) Organic surface layer: Plant litter layer—the upper part is often relatively undecomposed, but the lower part may be strongly humified. A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. Additionally, due to weathering, oxides (mainly iron oxides) and clay minerals are formed and accumulated. It has a pronounced soil structure. But in some soils, clay minerals, iron, aluminum, organic compounds, and other constituents are soluble and move downwards. When this eluviation is pronounced, a lighter coloured E subsurface soil horizon is apparent at the base of the A horizon. The A horizon 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". B) Subsoil: This layer normally has less organic matter than the A horizon, so its colour is mainly derived from iron oxides. Iron oxides and clay minerals accumulate as a result of weathering. In soil, where substances move down from the topsoil, this is the layer where they accumulate. The process of accumulation of clay minerals, iron, aluminum, and organic compounds, is referred to as illuviation. The B horizon has generally a soil structure. C) Substratum: Layer of non indurated poorly weathered or unweathered rocks. This layer may accumulate more soluble compounds like CaCO3. Soils formed in situ from non indurated material exhibit similarities to this C layer. R) Bedrock: R horizons denote the layer of partially weathered or unweathered 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 from bedrock will exhibit strong similarities to this bedrock layer.

The designations are found in Chapter 10 of the World Reference Base for Soil Resources Manual, 4th edition (2022).

A horizon that combines the characteristics of two master horizons is indicated with both capital letters, the dominant one written first. Example: AB and BA. If discrete, intermingled bodies of two master horizons occur together, the horizon symbols are combined using a slash (/). Example: A/B and B/A. The master horizon symbols may be followed by the lowercase letters indicating subordinate characteristics (see below). Example: AhBw. The I, L and W symbols are not used in transitional horizon designations.

This is the list of suffixes to the master horizons. After the hyphen, it is indicated to which master horizons the suffixes can be added. a: Highly decomposed organic material—H and O horizons. b: Buried genetic horizon—mineral horizons, not cryoturbated. c: Concretions or nodules—mineral horizons. c: Coprogenous earth—L horizon. d: Dense layer (physically root restrictive)—mineral horizons, not with m.
d: Diatomaceous earth—L horizon. e: Moderately decomposed organic material—H and O horizons. f: Frozen soil—not in I and R horizons. g: Stagnic conditions—no restriction. h: Accumulation of organic matter—mineral horizons. i: Slickensides—mineral horizons. i: Slightly decomposed organic material—H and O horizons. j: Jarosite accumulation—no restriction. k: Accumulation of pedogenic carbonates—no restriction. l: Mottling due to upmoving groundwater (gleying)—no restriction. m: Strong cementation or induration (pedogenic, massive)—mineral horizons. m: Marl—L horizon. n: Pedogenic accumulation of exchangeable sodium—no restriction. o: Residual accumulation of sesquioxides (pedogenic)—no restriction. p: Ploughing or other human disturbance—no restriction; ploughed E, B, or C horizons are referred to as Ap. q: Accumulation of pedogenic silica—no restriction. r: Strong reduction—no restriction. s: Illuvial accumulation of sesquioxides—B horizons. t: Illuvial accumulation of clay minerals—B and C horizons. u: Urban and other human made materials (artefacts—H, O, A, E, B and C horizons. v: Occurrence of plinthite—no restriction. w: Development of colour or structure—B horizons. x: Fragipan characteristics—no restriction. y: Pedogenic accumulation of gypsum—no restriction. z: Pedogenic accumulation of salts more soluble than gypsum—no restriction. @: Evidence of cryoturbation—no restriction.

Numerical prefixes are used to denote lithic discontinuities. By convention, 1 is not shown. Numerical suffixes are used to denote subdivisions within a horizon. The horizons in a profile are combined using a hyphen ( ). Example: Ah E Bt1 2Bt2 2BwC 3C1 3C2.

O: Organic soil materials (not limnic). A: Mineral; organic matter (humus) accumulation. E: Mineral; some loss of Fe, Al, clay, or organic matter. B: Subsurface accumulation of clay, Fe, Al, Si, humus, CaCO3, CaSO4; or loss of CaCO3; or accumulation of sesquioxides; or subsurface soil structure. C: Little or no pedogenic alteration, unconsolidated earthy material, soft bedrock. L: Limnic soil materials. W: A layer of liquid water (W) or permanently frozen water (Wf) within or beneath the soil (excludes water/ice above soil). M: Root limiting subsoil layers of human manufactured materials. R: Bedrock, strongly cemented to indurated.

A horizon that combines the characteristics of two master horizons is indicated with both capital letters, the dominant one written first. Example: AB and BA. If discrete, intermingled bodies of two master horizons occur together, the horizon symbols are combined using a slash (/). Example: A/B and B/A.

a: Highly decomposed organic matter (used only with O). aa: (proposed) Accumulation of anhydrite (CaSO4). b: Buried genetic horizon (not used with C horizons). c: Concretions or nodules. co: Coprogenous earth (used only with L). d: Densic layer (physically root restrictive). di: Diatomaceous earth (used only with L). e: Moderately decomposed organic matter (used only with O). f: Permanently frozen soil or ice (permafrost); continuous subsurface ice; not seasonal ice. ff: Permanently frozen soil ("dry" permafrost); no continuous ice; not seasonal ice. g: Strong gley. h: Illuvial organic matter accumulation. i: Slightly decomposed organic matter (used only with O). j: Jarosite accumulation. jj: Evidence of cryoturbation. k: Pedogenic CaCO3 accumulation (<50% by vol.). kk: Major pedogenic CaCO3 accumulation (≥50% by vol.). m: Continuous cementation (pedogenic). ma: Marl (used only with L). n: Pedogenic, exchangeable sodium accumulation. o: Residual sesquioxide accumulation (pedogenic). p: Plow layer or other artificial disturbance. q: Secondary (pedogenic) silica accumulation. r: Weathered or soft bedrock. s: Illuvial sesquioxide and organic matter accumulation. se: Presence of sulfides (in mineral or organic horizons). ss: Slickensides. t: Illuvial accumulation of silicate clay. u: Presence of human manufactured materials (artifacts). v: Plinthite. w: Weak color or structure within B (used only with B). x: Fragipan characteristics. y: Accumulation of gypsum. yy: Dominance of gypsum (≈≥50% by vol.). z: Pedogenic accumulation of salt more soluble than gypsum.

Numerical prefixes are used to denote lithologic discontinuities. By convention, 1 is not shown. Numerical suffixes are used to denote subdivisions within a master horizon. Example: A, E, Bt1, 2Bt2, 2BC, 3C1, 3C2.

A horizon that combines the characteristics of two horizons is indicated with both capital letters, the dominant one written first. Example: AB and BA. If distinct parts have properties of two kinds of horizons, the horizon symbols are combined using a slash (/). Example: A/B and B/A.

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. In the Australian system, 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 features of a horizon may also be added. The Australian system provides the following suffixes:

b: buried horizon. c: presence of mineral concretions or nodules, perhaps of iron, aluminium, or manganese. d: root restricting layer. e: conspicuously bleached. f: faunal accumulations in A horizons. g: gleyed horizon. h: accumulation of organic matter. j: sporadically bleached. k: accumulation of carbonates, commonly calcium carbonate. m: strong cementation or induration. p: disturbed by ploughing or other tillage practices (A horizons only). q: accumulation of secondary silica. r: weathered, digable rock. s: sesquioxide accumulation. t: accumulation of clay minerals. w: weak development. x: fragipan. y: accumulation of calcium sulfate (gypsum). z: accumulation of salts more soluble than calcium sulfate.

Soil formation is often described as occurring in situ: Rock breaks down, weathers and is mixed with other materials, or loose sediments are transformed by weathering. But 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.

Many soil classification systems have diagnostic horizons. A diagnostic horizon is a horizon used to define soil taxonomic units (e. g. to define soil types). The presence or absence of one or more diagnostic horizons in a required depth is used for the definition of a taxonomic unit. In addition, most classification systems use some other soil characteristics for the definition of taxonomic units. The diagnostic horizons need to be thoroughly defined by a set of criteria. When allocating a soil (a pedon, a soil profile) to a taxonomic unit, one has to check every horizon of this soil and decide, whether or not the horizon fulfils the criteria of a diagnostic horizon. Based on the identified diagnostic horizons, one can proceed with the allocation of the soil to a taxonomic unit. In the following, the diagnostic horizons of two soil classification systems are listed.