Browning
The Verbraunung is an essential process of soil formation ( Pedogenese ), in which in the soil by weathering form intensely colored iron compounds, the ground color effect.
There are numerous iron-containing primary minerals such as olivine , biotite , amphibole and pyroxene in the soil . As a result of chemical weathering , the colorless iron (II) formerly built into the mineral framework is released. Then it oxidizes in the presence of water and oxygen to different and colored iron (III) compounds. These cover the mineral grains as a fine membrane and, together with humus (black), have a significant influence on the color of the soil.
Which compounds are formed and in which quantity depends on the climatic conditions (temperature and water availability).
- In the cooler climates of the temperate zone , brownish iron hydroxides such as lepidocrocite (γ-Fe 3+ O (OH)) or goethite (α-Fe 3+ O (OH)) tend to form . For this reason, almost all soils in Central Europe have brownish tones. The weathered subsoil horizon ( B horizon ) is also known as the browning horizon and is a characteristic of the widespread soil type of brown earth .
- Under warmer, subtropical to tropical conditions, reddish iron (III) oxide ( hematite / Fe 2 O 3 ) in particular forms . That is why the soils of these climates are often intensely red or orange (e.g. Ferralsol or Terra rossa ). The term browning does not apply in the actual sense. Rather, it is a matter of rotting ( rubefication ).
The formation of the coverings around the original minerals prevents iron from shifting from the soil horizon. The initially unterbundene podsolisation reached after lowering of the concentration of lime (decalcification) and the adjusting of pH values below 7 (acidification) a higher extent.
Browning is inextricably linked with the process of corruption . Here, new secondary minerals are formed in the soil from the released components of the weathered primary minerals. However, these have a smaller grain size , which makes the texture finer towards silt and clay .
literature
- Fritz Scheffer: Textbook of soil science / Scheffer / Schachtschabel . rework. and exp. by Hans-Peter Blume u. a.,. 15th edition. Spectrum Akademischer Verlag, Heidelberg - Berlin 2002, ISBN 3-8274-1324-9 , pp. 448 f .
