Rock humus soil

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The rock humus soil (FF) belongs to the division of terrestrial soils , more precisely to class F. As the name suggests, it is humus that rests on unweathered rock.

Origin and Distribution

First moss colonization of the rock - the initial phase of the rock humus soil

A location is generally referred to as soil if it can permanently serve as a location for higher plants.

The humus is created by an accumulation of dead plants. The most common pioneer plants in the colonization of bare rocks are lichens and mosses that adhere to the surface of the rock. Their biomass collects over time as humus in the hollows of the rock and in the crevices created by physical weathering . The lichens and mosses also promote chemical weathering of the rock through their root excretions .

The forces of erosion counteract the increasing thickness of the soil . This is the case on steep mountain slopes where no material can find a hold. In addition, this type of soil is often found in very cold regions such as tundras and high mountains , in which, in addition to the strong erosion processes, there is also very little biomass production.

Soil socialization

The skeletal humus soil is very closely related to the rock humus soil. But this has no massive rock in the subsurface, but loosely stored skeletal material; So rubble , gravel or rubble . Both soil types form the class of O / C soils . They are often close together and flow into one another over the course of a slope.

Without sufficient erosion, the rootable layer on the rock becomes thicker and thicker due to the deposition of dust and the increasing weathering of the rock. The initially purely organic layer develops into a mineral material. This is when the actual soil development of the site begins. The initial stage of soil development is usually exceeded quickly. This is the case almost everywhere in Germany, so that rock humus soils only occur in extreme locations.

As soon as the organic content of the overlay falls below 30 percent by weight , a mineral soil horizon is present. The next soil type in the development series, the Syrosem , has thus been reached. If the proportion of mineral substance in individual cracks is already very high, rock humus soils flow over into syrosemes, so that both soil types are right next to each other.

The so-called Ah / C soils , in which a mighty humus topsoil has formed, are connected to this and also closely associated with the rock humus soils . On limestones (> 75% lime) this is the rendzina , on marly rocks (2–75% lime) the pararendzina and on lime-free rocks (<2% lime) the rankers .

Leveling

The horizon sequence in the German Soil Systematics is: O / mC

  • O: The overlay is an organic horizon ('O') with a proportion of more than 30% by weight of organic matter. A minimum thickness of the O-horizon has not been specified, since even the thinnest humus layer, provided it is able to sustain higher plants over the long term, is considered to be soil formation. The depth of the horizon can be several decimeters under certain circumstances.
  • mC: Underneath is massive ('m') starting material ('C'), i.e. bare rock that shows almost no signs of weathering. There is no connection between the support and the substrate.

When it comes to organic requirements, a distinction must be made between the bog and bog : A bog horizon results from peat formation . However, this does not occur due to the fluctuating humidity of the location.

In the international soil classification World Reference Base for Soil Resources (WRB), the rock humus soils are classified as histosols if they have an organic layer over 10 cm thick. If the circulation is lower, they fall into the group of Leptosols .

Usage and properties

Rock humus soil on basalt ( Kassel Mountains )

A typical characteristic of the rock humus soil is the uneven thickness of the surface. This can reach dozens of centimeters into the rock in isolated crevices. Directly next to it, however, completely absent on more exposed areas.

The nutrients are primarily brought in via dust and precipitation , as the rock is almost unweathered and the soil is hardly formed. This means that hardly any nutrients or root space are available. In addition, there is extreme alternating dryness, as the small amount of substrate cannot store any water. The humus layers are usually very acidic due to precipitation and decomposition processes.

Rock humus soils are therefore only suitable for a few plants that are adapted to extreme conditions. This can be some grasses and herbs. Agricultural or forestry use is impossible. Rock humus soils are important areas for nature conservation.

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