The black earth (also: Tschernosem from synonymous Russian чернозём [ t͡ʃɪrnʌˈzjom ]) is a type of soil that forms under certain conditions on calcareous loose material such as loess . It is the dominant soil in the steppe belt of the northern hemisphere and is one of the world's most fertile locations. Occurrences can also be found in Central Europe . It is named after the mighty topsoil, colored black with humus . The soil type is divided into class T (black earth). Its abbreviation is TT.
On lime-rich loose material, the settlement of pioneer plants leads to the formation of a thin humus horizon (soil type loose syrup ). As soon as this is more than 2 cm thick, the soil moves on to the next stage of soil development , the pararendzina . This can result in different types of soil: brown earths , parabroun earths or black earths.
According to the classical doctrine, five factors must apply simultaneously for a Pararendzina to develop into a black earth:
- Lime-rich and loose raw material (ideal: loess )
- Grassy vegetation ( steppes )
- Continental climate
- Balanced water balance ( semi-arid to semi-humid )
- Deep and intensive soil mixing by soil animals ( bioturbation )
According to this, lush steppe vegetation develops in spring under favorable humidity and temperature conditions (especially grasses), which provides a lot of organic material for humus formation. In the dry, warm summer, the production of organic matter decreases. At the same time, however, the degradation ( mineralization ) is inhibited by the drought. The short, damp autumn is followed by a long, very cold winter, in which the conversion of the organic matter rests. In the long term, more organic matter is produced than broken down, which means that humus accumulates in the soil.
The extreme climatic fluctuations also affect the soil animals. Due to the high availability of vegetable food, numerous species occur. Endogean earthworms , steppe marmots and common hamsters are very important here ( i.e. those living in the upper area of the mineral soil) . During the hot and dry summer and the icy, snowy winter, these have to retreat deep into the ground. This causes the typical intensive digging activity, which homogenizes the entire topsoil ( bioturbation ). Humous material is worked deep into the soil over time. The deep black topsoil thus reaches an average thickness of 60–80 cm. In the transition area to the starting material there are always isolated, deeper corridors that fill with humic material after the task. These are characteristic of black earth and are called Krotowinen from Russian Krot ( mole ).
The water conditions are also crucial. Seen over the entire year, the rainfall is so low that it allows steppe plants to grow, but there is no deep seepage. Rather, it stagnates at a shallow depth. This means that lime and nutrients remain in the soil. In the depths of the soil, where the seepage water of the annual rainy season regularly "gets stuck", secondary limescale precipitations (limestone concretions, "loess pebbles") can occur.
Further development to black earth parabrown earth
Under the above conditions, black soils show no tendency to transform into other types of soil. The black earth seems to be the final stage of soil development on loess in steppe climates ( climax soil ).
If the climate becomes permanently more maritime and thus more humid, not only does the vegetation change (forest formation). Soil-forming processes are also used, which are dormant under steppe climates. The soil type continues to develop or degrades. The processes that begin arise primarily from the now regular flow of seepage water ( leaching ) and more moisture present:
- New clay mineral formation and shifting of clay ( lessivation )
- Weathering of the raw material ( browning and silting )
- End of humus accumulation, long-term decrease in humus (mineralization)
- End of deep homogenization (formation of new soil horizons)
Degraded black earths (formerly gris earth) belong to the parabrown earths . Black earth pale earths are the exception, but can also occur.
This process can be observed in the black earths in Central Europe . The further development of the local black earths to parabroun earths in the Holocene essentially follows the development of precipitation or temperature or, in summary, the course of the climatic water balance and the seepage water. The Atlantic and Subboreal were , with some fluctuations, drier and warmer. Soil development was slower. With the subatlantic (around the turn of the ages ) higher precipitation and more seepage water and thus accelerated soil development can be expected. The decalcification could already have been completed in Central Europe at the beginning of the Neolithic in the upper decimeters. When clay is shifted, diffusely distributed clay is transported colloidally into deeper soil horizons and deposited there again with oriented clay layers. In the case of black soils, clay-humus complexes are relocated. This unmasked the light quartz grains and brightened the ground. Black clay deposits in the BHT horizon are a characteristic feature of the de-activated black earth . These prove the black earth past.
The black earths found in Central Europe today would therefore be a relic of earlier climatic conditions. Accordingly, they were formed during the Boreal almost 10,000 years ago, when the weather in Central Europe was much drier and more continental.
Alternative theories of origin
Some researchers question the current theory, since the C14 age of black earths, 3000 to 7000 years ago ( BP ), seems to be too young for a preneolithic formation (i.e. in the boreal climatic phase of Central Europe). Accordingly, a fixation of the carbon under the conditions of the steppe at a turnover rate of the organic substance of 30 to 100 years does not provide an explanation for the old age of the carbon. The current ties between the black earths and the steppes show the preservation, but not the educational conditions. According to more recent knowledge, black earths are also deep black because they contain significant proportions (10–40% of the organic substance) of pyrogenic carbon . This arises from incomplete combustion or carbonization and is also known as black carbon .
For the Neolithic (transition from hunter and gatherer cultures to sedentary farmers), continuous settlement with a fire economy (slash and burn) in the fields can be assumed to last up to 3600 years. This economic method with the formation and the entry of black carbon is proven by investigations on lake sediments and came to a standstill in the Bronze Age (Germany: approx. 2200 to 800 BC). Furthermore, the diggers are said to have only appeared with the increased food supply through agriculture. The bioturbation visible in the form of burrows (crotovines) is a clear profile feature, but does not bring about the homogenization of the topsoil. The preservation of the primary loess stratification is u. a. occupied in the Magdeburg and Hildesheimer Börde. In addition to the general soil development with decalcification, browning and shifting of clay, the incineration residues would have been continuously introduced into the Neolithic settlement areas. These are shifted into the ground over the entire period and incorporated into the grave passages (crotovines). The washing in and relocation of the organic matter would therefore be of great importance for the formation of the black earth in addition to the mixing.
The close connection of the black earths in Central Europe to Neolithic settlement areas could underline this development. The importance of the human influence on black earth formation in Central Europe is underlined by studies on the black earths on the Baltic Sea (Fehmarn, Großenbrode, Poel, Funen). Here, too, the dates seem to lie after the settlement phases and, with C14 ages from 1000 to 2000 years ago, are significantly younger than those of the loess black earths. A formation of the Central European black earth before the Neolithic would therefore not be certain.
The classical doctrine can, however, adequately explain numerous questions about the formation of black earths. The strongest argument can be found in the prevalence of the soil type: black soils in their pure form only occur on areas that have all five formation factors. Locations outside this zone (e.g. in Germany) also show black earth, but always with a tendency to transform into later stages of development. Settlement of the areas by early arable farmers applies to Central Europe, but not to the main distribution areas of the black earth in Central Asia and North America. The close connection between Neolithic settlements and black earths can be explained by the fact that the early farmers certainly preferred good locations to poorer ones. In addition, fires are a completely natural phenomenon in steppes, causing pyrogenic carbon.
The black earth in its pure form is the typical soil of the steppe areas with warm summer and cold winter. The largest black earth areas are therefore in the steppes of Hungary ( Puszta ), Romania ( Bărăgan ), Ukraine , Russia , Kazakhstan , Mongolia and China ( Manchuria ) and in the steppes of North America ( prairies ).
In addition, black soils are also found spotty in Central Europe and the taiga , among others .
Black earth in Central Europe
The geographical distribution of black earths and parabroun earths in Germany speaks in favor of the classic doctrine of black earth formation: the further west a loess deposit is, the more maritime and humid the climate. At the same time, towards the west, the thickness of the humus topsoil decreases and the black earth character of the locations decreases.
The loess deposits in the Cologne Bay and near Osnabrück (as well as in Belgium and France ) are located furthest to the west . The soils there are parabrown earths and show almost no black earth properties. To the east joins the Hildesheimer Börde, where Parabraunerden occur with a tendency to black earth. Again further to the east is the Magdeburg Börde. Black earths dominate there with a tendency towards parabrown earth. Black earth in its pure form can be found in the very east of Germany and in the Thuringian Basin in the rain shadow of the Harz Mountains . This is where the lowest rainfall occurs, coupled with the most continental climate.
The black earth is a soil with two soil horizons (Axh / Cl). The characteristic Krotowinen usually results in an interlocking (Axh / Axh + Cl / Cl) between the top soil (Axh) and the starting material (Cl).
- Axh: The topsoil horizon (A) is calcareous and at least 40 cm thick (mostly significantly more; sometimes more than 1 m). It is humic (h) and strongly mixed with animals (biogenic) (x).
- Cl: The starting material (C) is loose (l) and also contains lime (mostly loess).
Since black soils are very fertile, they are often used for agriculture. In this case another horizon (Ap / Axh / Cl) is created on the surface.
- Ap: plowed (p) topsoil horizon (A) about 30 cm thick.
Black earth class soil types
In the international soil classification World Reference Base for Soil Resources (WRB) the black earths are divided into Chernozem (typical steppe climate: long grass steppe), Kastanozem (drier steppe climate: short grass steppe) and Phaeozem (humid steppe climate: steppe with groups of trees). In addition to the deep dark topsoil, chernozemes and Kastanozemes have secondary carbonate as a further diagnostic feature. The minimum thicknesses for the humus A-horizons are smaller in comparison to the black earths of the German soil systematics.
Properties, use and soil functions
Black soils generally have good conditions for arable farming:
- The soil type is silty with a relatively high clay content
- Easy to heat up
- Loose and advantageous structure ( crumb structure )
- High water conductivity
- Optimal distribution of the total pore volume (45% by volume) with one third each of coarse pores (seepage rate), medium pores (plant-available water) and fine pores (nutrient exchange)
- High nutrient retention capacity ( KAK )
- Very high natural nutrient content (raw material, hardly any leaching)
- High base saturation and thus high pH values around pH 5 (hardly any decalcification)
- Rich soil life
- Lots of humus (in Germany around 6%, in Siberia over 12%) of optimal quality ( gauze )
- Tight C / N ratio around 12
As a result, they offer plants good growth conditions and are also easy to work with. The number of arable land in Germany is often well over 90, with the most productive soils in the country being found on the black earths of the Magdeburger Börde (100 out of 100 possible points). They are also high-yielding and fertile in a global comparison, which is why black soils, provided the distribution of rainfall allows it, are almost always under agricultural use and make a significant contribution to feeding the world population (American Corn Belt and Grain Belt , wheat-growing areas between Ukraine , Russia and Kazakhstan ).
However, there are a few restrictions:
- The areas with climatic conservation conditions are problematic as droughts can occur regularly ( dust bowl ).
- The above-mentioned increased clay content has a negative effect on intensive use due to the tendency towards soil compaction . For example, the black earths of the Hildesheimer Börde are often highly compacted.
- Loess soils are also at risk of erosion when exposed to relief energy , as silt is less stable than clay and sand. In this way, near Magdeburg through the extensive erosion ( erosion of the black earth already many sites in the preliminary stage of soil development) has been reset (Pararendzina A horizon <40 cm).
- The black soils of the Hildesheimer Börde showed a widespread potassium deficiency in the 1960s to 1990s despite fertilization.
Because of the above-average fertility of black earth, the natural vegetation of the areas (steppes, mixed forests) has been destroyed almost worldwide today. The function of soils from the human perspective is primarily the production of food.
Experts see it as extremely critical to designate these fertile and profitable locations as building areas. These valuable areas are permanently lost for the production of food through development. For this reason, black earth is considered a soil type in Germany that is endangered by overbuilding (loss of cultivated land).
Due to its old age, the black earth is an archive of natural and cultural history. Due to the only island occurrence in Central Europe, the black earth is a rare soil here.
Floor of 2005
The black earth is not the only soil that is endangered in its function due to misuse, overbuilding or erosion. However, due to its fertility and particular importance for world nutrition, a reduction in its occurrence is to be viewed particularly critically. In order to point out the finiteness of good arable soil - also in Germany - the black earth was proclaimed soil of the year 2005 on the occasion of World Soil Day on December 5, 2004 .
- P. Kossowitsch: The black earth ( Chernosiom ). In: International communications for soil science. 1, 3/4, 1911, pp. 199-354.
- G. Roeschmann: Pseudogley-Tschernoseme and their transition formations to parabroun earths in the loess area of the Hildesheimer Börde. In: Geological Yearbook. 85, Hannover 1968, pp. 841-860.
- MWI Schmidt, JO Skjemstad, E. Gehre, I. Kögel-Knabner: Charred organic carbon in German chernozemic soils. In: European Journal of Soil Science. 50, 1999, pp. 351-365.
- E. Gehre, Michael Geschwinde , MWI Schmidt: Neolithic, fire and Chernosem - or: What do the linear ceramicists have to do with black earth? In: Archaeological correspondence sheet. 32, 2002, pp. 21-30.
- Eileen Eckmeier: Are there black earths in the Rhineland? A search for clues in the laboratory. In: Landschaftsverband Rheinland (Hrsg.): Archeology in the Rhineland . Theiss-Verlag, Stuttgart 2002, pp. 204-206.
- Thomas Saile, Carsten Lorz: Anthropogenic black earth degeneration in Central Europe. A contribution to the current discussion? in: Praehistorische Zeitschrift 78, 2003, pp. 121–139. ( Online )
- Manfred Altermann, Jörg Rinklebe, Ines Merbach, Martin Körschens, Uwe Langer, Bodo Hofmann: Chernozem - Soil of the Year 2005. In: Journal of Plant Nutrition and Soil Science. 168, 2005, pp. 725-740.
- S. Brodowski, B. John, H. Flessa, W. Amelung: Aggregate-occluded black carbon in soil. In: European Journal of Soil Science. 57, 2006, pp. 539-546.
- Renate Gerlach , Eileen Eckmeier: The problem of the "black earth " in the Rhineland in an archaeological context - a summary. In: Astrid Stobbe, Ursula Tegtmeier (Ed.): Branches. An appreciation for Arie J. Kalis and Jutta Meurers-Balke (= Frankfurter Archäologische Schriften . Volume 18). Habelt, Bonn 2012, pp. 105–124.