Dwogmarsch

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The Dwogmarsch is a marshland that occurs mainly in the Altmarschen . In the profile of this soil there is at least one so-called dwog , i.e. a former terrain surface that was covered with further sediments by the sea . In the German Soil Systematics, the soil type is assigned to the department of semi-terrestrial soils of class M (marshes). Its abbreviation is MD.

Origin and Distribution

Dwogmarschen are only found in the marshes and occur mainly in the Altmarschen. They are Holocene sedimentation areas of the seas. Marshlands are relatively rare worldwide. By far the largest contiguous area is on the North Sea between Belgium and Denmark .

If the former seabed falls permanently dry, marshy soils with an overgrown, humus-rich surface form in a temperate climate. During the last millennia of the Holocene there was a more or less continuous rise in sea ​​levels , which continues to this day. But although the coastlines vary constantly due to storm surges and erosion , old marshes do not necessarily have to be inundated by the rising sea.

Enormous loads of sediment are moved in the sea and especially in the tidal currents . During storm surges, the waves move further than average. Where they run out, the abruptly decreasing water movement leads to deposits, which is why the area directly on the coastline is usually slightly elevated. Unless the erosion of storm surges led to sea advances, as in the case of the Dollart or Jade Bay , the old coasts rose in this way in parallel with sea level. Since marshland also sags slightly over time, old marshland are now often well below the current sea level. They are also known as Sietland .

In particularly strong storm surges, the waves overflow the higher coast. Large amounts of sediment are transported inland with the seawater and are deposited there. This process was repeated regularly before the coast was dyed . If several storm surges occurred in close succession or if there was particularly heavy sedimentation, then already developed marshes were covered by new, powerful sedimentation packages. The overlying topsoils are clearly visible as dark (humus) bands in the soil profile and are referred to as dwog . They represent a disturbance of the soil profile caused by sedimentation.

Embankment completely prevents sedimentation inland, so that, except in the event of a dike breach, no new Dwogmarschen can form.

Leveling

According to KA5, a march is a dwog march if it fulfills two conditions: On the one hand, the thickness of the tidal sediments must be at least 70 cm. On the other hand, the profile must consist of two different layers (two-layer profile), with the shift change in the top 40 cm. The disturbance is usually the eponymous dwog horizon , which must have a thickness of over 5 cm. The two conditions determine that not only a former surface was covered with marine sediments. The old surface must have been a march too.

In the international soil classification World Reference Base for Soil Resources (WRB), marshes are not specifically distinguished. There they belong to other water-affected soil groups. The Dwogmarschen are all assigned to the reference floor pan of the Gleysole . The principal qualifiers Stagnic, Dystric and Eutric are possible.

Because of the profile disturbance, Dwogmarschen have a comparatively large number of horizons (six). For the sake of simplicity, it should be noted that marshes generally have the following horizon order: Ah / Go / Gr; i.e. topsoil / aerated groundwater area / unaerated groundwater area. If the disturbance is disregarded, this is also the case with Dwogmarschen.

Leveling : Ah / Sw / IIfAh ° SD / FGO ° SD / Go / Gr

The first two horizons are younger sediments that were deposited on the old surface. They form the first layer in the profile.

  • Ah - the topsoil ('A') is humic ('h') and therefore brown to black.
  • Sw - Because the dwog has a damming effect, the area immediately above the third horizon is affected by backwater ('S'). However, there is still ventilation so that the horizon is water-conducting ('w'). It shows signs of pseudo-gazing .

The second layer of the older, overlaid sediments follows these two horizons. The horizons fAh and fGo are the dwoves.

  • IIfAh ° Sd - The third horizon represents the layer change ('II'). It is the fossil ('f') topsoil horizon ('Ah') covered by younger sediments. The fAh is also called Humusdwog. The existing humus colors it dark, which is why it appears like a black band in profile. Because of the shift change it is a profile disturbance and hinders the infiltration of rainwater. It is therefore dense ('d') and causes waterlogging ('S'). The '°' indicates that the fAh is the cause of the Sd.
  • fGo ° Sd - The fossil topsoil is followed by the fossil subsoil (fGo). 'G' stands for the influence of the groundwater and 'o' for oxidizing (air supplied). The fGo is also called Eisendwog, which indicates an increased iron content. It is also dense ('d') and causes waterlogging ('S'). The '°' indicates that the fGo is the cause of the Sd.
  • Go - Below the horizons influenced by backwater there is the pure groundwater influence area ('G') present in all marshes. The upper areas are still supplied with air (oxidizing; 'o'). Red-brown spots of rust indicate processes of fermentation .
  • Gr - A further horizon influenced by groundwater ('G') follows up to the Pleistocene subsurface, in which reductive processes ('r') dominate. The dark, almost black color is caused by the large quantities of iron sulfide here. Due to the old age of the Dwogmarschen, the unventilated area can often only be found at great depths.

Notes :

Dwöge are diagnostic horizons that inevitably lead to dwogmarsch. In addition to the actual dwog, condensed horizons ( kf <10 cm / d) can sometimes also lead to the address as dwog march. If there is no dwog, but only a compacted horizon, a classification with the soil type of the Knickmarsch should first be considered.

In marshes, groundwater is widespread up to the surface. In these cases, the first layer is affected not only by waterlogging, but also by groundwater. The second horizon is then referred to as Sw-Go or Go-Sw.

Not every Dwogmarsch contains a Humusdwog (fAh) and an Eisendwog ​​(fGo), because it is also possible that fAh or fGo are present alone. In the first case, the fGo ° Sd is not applicable. In the second case the IIfAh ° Sd is omitted and the fGo ° Sd has to be renamed to IIfGo ° Sd.

properties

Dwogmarschen are predominantly in the area of ​​the Altmarschen. That is why they are already heavily decalcified and washed out. Their nutrient availability is therefore well below that of the Jungmarschen . The water conditions must be mentioned as an additional difficulty. The deep location - often below sea level - requires constant drainage. The dwog ensures that in addition to the groundwater problem, there is also waterlogging.

use

Due to the water conditions, Dwogmarschen are almost exclusively used as grassland. For cultivation, a functioning, narrow drainage must always be operated, which at least penetrates the dwog. Since this horizon is in the first 40 cm of the ground, drainage ditches are rather shallow. The excavation of the trenches at a distance of about 7 m is often thrown between them in order to improve the drainage of some meadow areas. This makes the surface wavy. This management provided for the humpback grassland areas characteristic of many marshes in northwest Germany.

literature

  • Streif, H. (1990): The East Frisian coastal area - North Sea, islands, mud flats and marshes , Geological Guide Collection 57, 2nd completely revised edition, Brothers Borntraeger, Berlin, Stuttgart, ISBN 978-3443150518
  • Kuntze, H. (1965): The marshes - heavy soils in agricultural evolution; Opportunities for use and improvement of difficult locations ; Paul Parey publishing house, Hamburg and Berlin
  • Ad hoc working group on soil; Federal Institute for Geosciences and Raw Materials in cooperation with the State Geological Services of the Federal Republic of Germany (Ed.) (2005): Soil-scientific mapping guide . 5. revised u. exp. Edition. Hanover, ISBN 3-510-95920-5 .
  • Mückenhausen, E. (1993): Soil science and its geological, geomorphological, mineralogical and petrological basis . DLG-Verlag, Frankfurt am Main, ISBN 978-3-7690-0511-0 .
  • 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 .