Skeletal humus floor

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Profile of a skeletal humus soil made of basalt rubble (Kassel Mountains)

The skeletal humus soil (FS) belongs to the division of terrestrial soils , more precisely to class F (O / C soils), like the rock humus soil . It consists of loose coarse material (skeleton), the spaces between which are filled with humus .

Origin and Distribution

This type of soil arises from a mineral subsoil horizon , which mainly consists of soil skeletons such as rubble or rubble . It has little or no fine earth and hardly any fine skeleton ( gravel and gravel ). Since this starting material is not massive, but has numerous gaps, humic material can accumulate in the crevices. For this reason, the coarse soil is mixed with more or less organic matter that is in the cavities. Most of the time, the rock itself is almost unweathered.

On the one hand, the humus is brought in by the wind from other locations (leaves, etc.). On the other hand, the biomass also comes from pioneer plants such as lichens and mosses that settle on the rock.

Due to the entry of dust and the weathering of the starting material, the O / C soils of the scree fields quickly develop into deeper soils. That is why skeletal humus soils occur naturally only in small areas in mountains in Germany . There they lie on slopes below steep walls, on which loose rock material is constantly being supplied due to the weathering of the rocks. In addition, constant erosion processes prevent the formation of thicker soil horizons from fine material. In addition, skeletal humus soils are also typical subsequent soils of human activities, as they form on debris deposits, slag heaps or track ballast .

All over the world, skeletal humus soils are found not only in mountains but also in cold deserts, where there is strong erosion processes and low biomass production.

Soil socialization

Rock humus soils are closely related to skeletal humus soils, with which they also form a common soil class. There is no coarse loose material there, but a massive subsoil with a layer of humus. As a rule, rock and skeletal humus soils are in a close spatial relationship. Typical is a steep slope (rock humus soil) that lies directly above a debris pile made of erosion material (skeletal humus soil).

Without the influence of erosion, skeletal humus soils develop rapidly and the actual soil development begins. Due to the dust and weathering, the proportion of organic matter in the filler material in the gaps begins to decrease. At the same time, the mineral component increases. As soon as there is less than 30 percent by weight of organic matter, a mineral soil horizon is reached. The soil created in this way is referred to as a Lockersyrosem . In most cases, loose syrosemes and skeletal humus soils are closely related.

When the topsoil has reached a greater thickness, the so-called Ah / C soils join in the development: on limestone (> 75% lime) this is the rendzina , on marly rocks (2–75% lime) the pararendzina and on lime-free rocks (<2% lime) of the Ranker . Soils of this class also occur in the vicinity of skeletal humus soils.

Skeleton-rich river valleys are often not skeletal humus bottoms. They are mostly assigned to the floodplain soils . The associated soil type of gravelly river banks is called Rambla .

Leveling

The horizon sequence in the German soil system is: (O) / xC + O / (xC) / lC, mC

  • O: An organic layer ('O') can rest on the floor skeleton. However, this is often missing because the humus falls into the spaces between the subsoil.
  • xC + O: In any case, the soil type has an interlocking horizon ('+'). The starting material ('C') consists of a rough skeleton, where the 'x' stands for 'stony'. In the gaps there is organic matter ('O'). There are two reasons why we speak of interlocking: On the one hand, the stones and humus are on the same horizon. On the other hand, there is no proper mixing, since humus and rock could be separated from one another without great effort. The depth of the horizon can vary widely and be several decimeters. It depends, among other things, on the width of the gaps, the amount of precipitation or the existing humus.
  • xC: It is possible that the humus has not (yet) penetrated into the deepest spaces. In this case there is a rock-rich starting horizon without any humus content.
  • lC, mC: Loose ('l') or solid ('m') material follows under the coarse soil at some point.

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

Properties and use

Top view of a skeletal humus floor made of basalt rubble

An often typical property of rock humus soil is the alternation of rock and humus deposit, so that the properties vary greatly in small areas. In the fissures, dozens of centimeters of humus can reach into the subsoil, while directly next to it there is only bare stone. In spite of the not massive material, this soil cannot be classified as digging.

The entry of nutrients occurs primarily through dust and precipitation . The material for the formation of an O-horizon consists for the most part of dead plant residues. The humus can be very acidic due to precipitation and decomposition processes.

Skeletal humus soils, like rock humus soils, have problems with changing moisture conditions, as there are hardly any bodies of soil to store water. The deeper the humus has penetrated into the spaces in between, the less extreme the drying out and the larger the root space and the nutrient storage. Deep skeletal humus soils can even occasionally support trees.

However, rock humus soils only offer a suitable location for a few plants such as some grasses and herbs. A forestry use is difficult; an agricultural impossible. Skeletal humus soils are important areas for nature conservation.

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