Terra fusca

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The Terra fusca (from Latin terra , land, earth 'and fuscus brown'), also called Kalksteinbraunlehm, is a soil type that refers to a bedrock of limestone or gypsum developed. It is a plastic, clay-rich and dense soil that emerges from the accumulation of solution residues from a rendzina created by a lime or gypsum solution .

Structure and properties

Brown earth Terra fusca in the Dinkelberg area (southern Black Forest)

The terra fusca has the soil horizons Ah / Tv / cC. The uppermost horizon is a mineral soil horizon (A) with a significant proportion of humus (h), which, however, is below 30 percent by mass. Below the A horizon, a mineral subsoil horizon at least 10 to 30 centimeters thick is formed from solution residue from carbonate rocks (Tv instead of the usual B horizon). This solution residue consists of more than 65 percent by mass of clay (residual clay), mostly with the clay minerals illite and kaolinite as the main components. The bright yellowish to reddish brown color of the T horizon is usually the result of a relictic or recent browning process (v). It also has a pronounced polyhedral structure . It is acidic, i.e. it has a low pH value , and passes over a transition zone of loosened solid rock into the C horizon of massive (m) carbonate-containing (c) unweathered solid rock (cmCn).

According to the pedological mapping instructions, the Terra fusca belongs to class C, the Terrae calcis, like the Terra Rossa . In the international soil classification World Reference Base for Soil Resources (WRB), the soil type of terra fusca is not specifically specified. Here the Terra fusca belongs to the reference floor assembly of the Cambisole .

Terra fusca is very dense and difficult to work due to its high clay content, especially since it often alternates with rocky, shallow soils. Their humus content varies. Particularly with poor ventilation and in a cool, humid climate, there may be a strong accumulation of humus. Terra fusca has a high water capacity , but the amount of water that can be used by plants is limited by the high clay content in the Tv horizon of the soil. Because of these properties, it is mainly used as forest or pasture land.

Emergence

The cause of the emergence of terra fusca is the solution of the underlying carbonate rock. The soluble carbonate in limestone (limestone, dolomite ) and gypsum rock (gypsum, anhydrite ) is removed by solution, what remains are the insoluble parts, i.e. mostly silicates such as clay minerals. The often bright color can be the original color of the solution residue, but in most cases it is due to browning, i.e. a release of the iron bound in the carbonates and silicates, which was then oxidized. The formation of terrae fuscae no longer takes place under today's climatic conditions in Central Europe, so that such soils are regarded as soil formations from the period of the Tertiary to the Early Pleistocene .

In Central Europe, terra fusca only occurs on old land surfaces far from active erosion , for example from rivers. Such conditions can be found, for example, on the Mesozoic limestone of the southern German and Swiss Jura . So-called floor ores appear differently there in the soil formations . These are small, spherical concretions made of iron, the origin of which is not yet fully understood. It is also unclear whether the formation of these concretions can only be attributed to the long period of weathering or whether the tropical climate of the Tertiary is the main cause.

See also

literature

Web links

Individual evidence

  1. ↑ Explanations on Terra fusca and Terra rossa, Lecture Soil Science, Geographical Institute University of Zurich