A soil horizon , often called the horizon for short , is an area in the soil that can be distinguished from areas above and below based on its special properties. The horizons of a soil can be seen in a so-called soil profile (vertical section of the soil in an excavation, less often a borehole) and are almost always horizontal or parallel to the slope . The horizon sequence of a soil is the decisive criterion for the determination of the existing soil type .
Soil horizons are areas within the soil that have uniformly similar characteristics and properties and differ from areas above or below. The processes of soil formation and development ( pedogenesis ) usually influence or penetrate the soil vertically from top to bottom. These are primarily natural processes, i.e. physical or chemical weathering (due to the effects of heat, cold, precipitation), biological activity (plant roots, soil organisms , burrowing animals, microorganisms) or the leaching of substances with the precipitation and seepage water. In populated regions, people now also have a major influence (soil relocation, material input, plowing ...).
The horizon sequence
The soil-forming processes are not equally pronounced everywhere in the soil. Many of them are concentrated on the surface. This is z. B. overgrown (humus formation), better ventilated (activity of the soil organisms) or exposed to the weather (weathering). Humid regions show a constant shift of substances from top to bottom as the precipitation seeps away. So there are vertical gradients of the soil-forming processes and their products, which leads to a vertical sequence of the areas in the soil that they shape. Naturally, horizons always lie more or less one above the other, but not next to one another.
The number of horizons can vary significantly from location to location. In the case of a sand dune that has just blown up , it is 1, since only "pure sand" is present. Already after a short time pioneer plants settle and humus forms. This increases the number of horizons to 2, as an area “sand with humus” has arisen on the surface, in contrast to the “pure sand” subsurface. This makes it clear that the horizon sequence is not fixed, but also varies over time as the soil development progresses.
The sequence of the soil horizons is therefore characteristic of the soil development that has taken place so far. All essential soil chemical and biological processes and properties can be read from it (humus formation and incorporation, acidification, reducing and oxidizing conditions , ...). Addressing and interpreting the existing soil horizons is therefore an essential part of soil mapping , e.g. B. in the context of the forest site survey when deciding on a suitable choice of tree species ( forestry ) or determining the number of arable land . They are used to systematically classify the soils into soil types .
Differentiation of the soil horizons
Soil horizons are named with combinations of letters ( horizon symbols ) that identify essential features and processes.
The soil horizons can be roughly divided as follows - in the soil profile from top to bottom:
- H, L, O horizons : organic horizons
- A horizons : mineral topsoil
- (Enrichment of humus , leaching of substances)
- B horizons : mineral subsoil
- (Mineral conversion, washing in of substances)
- C horizons : mineral subsoil
- (little changed rock, physical weathering )
The more detailed characterization is carried out using trailing lower case letters. There are also additional letters in front to identify special processes and properties that did not result from soil formation. If several pedogenic processes occur that shape a horizon, further combinations of symbols are formed according to certain rules. The following list and description names the essential horizons and is based on the German soil systematics (see Soil Science Mapping Instructions ).
Organic soil horizons
They have a proportion of more than 30% by mass (= approx. 90% by volume) of organic matter.
- H - Peat: Organic horizon made up of remnants of peat- forming plants, formed on the surface. The degradation is inhibited by water, peat is formed. A further differentiation according to the type of peat is possible:
- L - largely undecomposed, fresh litter (Förna, English L itter = litter), formerly (and internationally) also referred to as oil horizon: of the organic matter with at least 30% by mass, less than 10% by volume is fine organic matter .
- O - O rganic support horizon (if no peat!): Of the organic substance with at least 30% by mass, more than 10% by volume is fine organic matter
- Of - next plant residues 10-70 vol .-% organic fine substance occur as a result of decomposition ( F ermentation) on.
- Oh - the organic fine substance ( H umus) as a result of the degradation processes outweighs strong.
- O / C floors
Mineral soil horizons
Organic matter content below 30% by mass
- A horizons
- Aa - humus enrichment between 15 and 30% by mass due to waterlogging ( a peaty topsoil)
- Ae - bleaching, leaching ( leaching horizon ) of humic substances and iron ( e luvial , typical for Podsol )
- Ah - accumulation of humus (<30% by mass, H umus)
- Ai - only slightly powerful accumulation of organic substance ( i nitial)
- Al - leaching of clay particles (clay migration = L essivierung , typical of Parabraunerde and Fahl Earth )
- Ap - regular agricultural cultivation ( p flügen) - topsoil , is also called the plow horizon
- B horizons
- Bh - accumulation of been washed H uminstoffen (strong staining!) And to a lesser extent by sesquioxides, (typical of Podsol , s. Hardpan )
- Bs - accumulation of washed-in s esquioxides (iron, manganese and aluminum compounds ) (typical for Podzol , see Ortstein )
- Bt - accumulation of eingewaschenem tone ( T onanreicherung, typical of Parabraunerde )
- Bv - iron oxidation, the formation of minerals ( V erbraunung, Verlehmung, typical of brown earth )
- C horizons
- Cn - unweathered original rock of the soil formation, loose or solid rock ( n ovus or n eu)
- Cv - source rock (which are slightly weathering v erwittert)
additional geogenic symbols:
Further mineral soil horizons
- G - mineral soil horizons with G round water influential (typical of Gleye , alluvial soils and marshes )
- S - mineral soil horizon with S from condensation influential (typical of Pseudogley )
- Sw - storage aquifer temporarily air arm ( w asserleitend)
- Sd - backwater sole, largely d maybe, making it too slowed infiltration comes
- P - Mineral subsoil horizon,> 45% by mass clay , formed from clay or marl rock ( P from Pelosol )
- T - Mineral subsoil horizon from solution residue from carbonate rocks with more than 75% by mass of carbonate , ( T from Terra rossa and Terra fusca , in Germany only relictic or fossil)
- M - mineral soil horizon made of eroded and continuously sedimented Holocene solum material , typical of floodplain soils and colluvisols ( M from Latin migrare = to wander)
- E - mineral soil horizon, applied by mostly centuries of plague management ( E sch as field name, soil type Plaggenesch )
- R - Mineral mixing horizon by profound bodenvermischende amelioration ( R igolen, deep plowing) originated (soil type, depending on the culture technique Rigosol or Treposol )
Subhydric soil horizons
- F - horizon, formed at the bottom of the water, usually with more than 1% by mass of organic matter
Soil in the tropical rainforest
In the tropical rainforest, chemical weathering dominates due to high temperatures and heavy rainfall. The parent rock is therefore often deeply dissolved, so the C horizon is often at depths of 20 to 100 meters. In addition, there have been hardly any tectonic changes since the Tertiary and therefore the weathering could last for a very long time and so very powerful, deep weathered coverings were created. Frequent tropical soils are after the World Reference Base for Soil Resources (WRB) the Ferralsols and Acrisole and after the USDA Soil Taxonomy the Oxisole and Ultisole .
Delimitation of the terms horizon and layer
The term geogenic or anthropogenic layer must be distinguished from the term soil horizon : A layer is the result of geogenic or anthropogenic processes, e.g. B. from natural sedimentation or artificial embankment. It can be recognized by a change in material not caused by soil formation (e.g. sand over clay ). Soil horizons, on the other hand, are the result of pedogenesis (soil formation). A soil can consist of one or more layers (firm or loose). As a rule, several horizons have formed in one layer. Only thin layers or layers with not yet advanced pedogenesis sometimes only have a single horizon. On the other hand, pedogenesis can lead to the originally existing layer boundaries in the soil profile no longer being recognizable and several layers appearing like a single horizon.
The following example is intended to explain the difference between a horizon (pedogen) and a layer (geogen). In the foothills of the Alps, we often find a layer of loess over a layer of neogene deposits. The loess contains calcium carbonate , the neogene sediments are often carbonate-free. Pedogenesis took place in both layers. In the loess layer there is humus accumulation , carbonate leaching, weathering of primary silicates with the formation of clay minerals and oxides (especially iron oxides ) and finally lessiviation (clay shift). If the loess layer was not too thick, it becomes completely carbonate-free in the course of the Holocene and no longer has a C horizon. Then the horizon sequence is Ah / Al / Bt. Soil formation also takes place in the neogene layer, which often began long before the Holocene. Silicate weathering occurs with the formation of clay minerals and oxides. Clay minerals from the thin loess layer are often washed into the upper part of the neogene layer. The neogene layer thus has the horizon sequence Btv / Bv / ilCv. The horizon sequence of the entire soil is: Ah / Al / Bt / IIBtv / IIBv / IIilCv. The Roman numerals II denote the second layer.
Horizon designations in other countries
Like Germany, numerous other countries have their systems for naming horizons with letter abbreviations. The systems of the following countries are published in the following documents:
- Austria: Austrian Soil System 2000 in the revised version from 2011 (ÖBS)
- Switzerland: Classification of Soils in Switzerland (KlaBS)
- USA: Field Book for Describing and Sampling Soils
- Australia: Australian Soil and Land Survey Field Handbook
The horizon designations from the Guidelines for Soil Description of the FAO are used internationally .
- W. Amelung, H.-P. Blume , H. Fleige, R. Horn, E. Kandeler , I. Kögel-Knabner , R. Kretschmar, K. Stahr , B.-M. Wilke: Scheffer / Schachtschabel textbook of soil science. 17th edition. Heidelberg 2018. ISBN 978-3-662-55870-6 .
- Blume H.-P., Felix-Henningsen P., Fischer WR (1996 -): Handbuch der Bodenenkunde. Loose-leaf collection, Ecomed Verlag, Landsberg, ISBN 3-609-72232-0
- Ad-hoc Working Group Soil: Soil Science Mapping Instructions , Ed .: Federal Institute for Geosciences and Raw Materials in cooperation with the State Geological Services, 5th edition, 438 p .; 41 figs., 103 tabs., 31 lists, Hannover 2005. ISBN 3-510-95920-5
- Site mapping working group: Forest site survey, 7th edition, 400 pages, IHW-Verlag, Eching 2016. ISBN 978-3930167807 .
- Kuntze, Roeschmann, Schwerdtfeger: Bodenkunde, 5th edition, UTB - Verlag, ISBN 3-8252-8076-4
- Working group for soil systematics of the German Soil Science Society. Current status of the scientific processing of the German soil systematics
- Soil horizons and soil profile. Learning and working environment on the subject of "soil" in the classroom. Westfälische Wilhelms-Universität Münster
- Soil horizons. TRIANET, Program of the European Union Socrates-Comenius
- Floor worlds. Website of the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)
- The rain forest of the always humid tropics and the deciduous forest by Nadine Peiler
- O. Nestroy et al .: Systematic classification of soils in Austria. Austrian Soil System 2000 in the revised version from 2011.
- Soil Science Society of Switzerland: Classification of Soils in Switzerland. 3rd edition 2010.
- PJ Schoeneberger, DA Wysocki, EC Benham and Soil Survey Staff: Field Book for describing and sampling soils. Version 3.0. NRCS, NSSC, Lincoln, Nebraska, 2012.
- National Committee on Soil and Terrain: Australian soil and land survey field handbook. Third edition. CSIRO, Melbourne, 2009.
- FAO Guidelines for Soil Description. Rome, 2006.