Lessivation

Lessivation is a soil-forming process. In soil science it is also known under the terms clay silting up or clay shifting . This involves the displacement of the finest clay mineral particles (<0.002 mm) into deeper soil horizons .

The process of learning comprises several sub-processes. In the topsoil , the clay particles go into suspension due to peptization in the seepage water . This is possible if the proportion of polyvalent, positively charged ions in the seepage water is low, because clay minerals have a negative surface charge and flocculate through polyvalent cations . As a rule, this will only happen in the case of heavy precipitation to a recognizable extent. The suspension reaches deeper horizons with the seepage water flow. The flocculation is caused here by higher nutrient and therefore higher levels of divalent calcium or divalent magnesium ions . Another sub-process involved in the dry subsoil is the penetration of the transport medium water into the dry soil aggregate. Here, the clay particles are deposited on the surfaces of the soil aggregates or the pore walls, and over time a clay enrichment , tonilluviation or Bt horizon forms. Lessivization can only start in the course of soil development when the lime and the Ca ²⁺ and Mg ²⁺ ions from it have been washed out, that is, the first stage of soil acidification has passed. The de-activation remains active until the increasing soil acidification leads to the presence of Al ³⁺ ions in the soil solution in the topsoil. Correspondingly, the de-activation can take place in a pH range between approximately 6.5 and 5. Soils in which this pH range is passed through quickly due to lack of lime, low nutrient content, high water permeability or location in a very precipitation-rich climate, do not form morphologically perceptible clay-enriched B horizons .

Lessivation is found primarily in soils made from loess or clay till . Lessivized soils are differentiated in the German soil systematics in the Lessivés class into the soil types parabrown earth and pale earth . In the World Reference Base for Soil Resources - depending on the current nutrient content and their cation exchange capacity - it is a question of Luvisole , Alisole or Acrisole or, in the case of a tongue-shaped intervention of the, in this case, very light clay washout horizon in the Bt horizon, Albeluvisole .

literature

• Hans-Peter Blume et al .: Scheffer / Schachtschabel. Textbook of soil science. 16th edition. Spektrum Akademischer Verlag, Heidelberg / Berlin 2010, ISBN 978-3-8274-1444-1 .
• W. Ziechmann, U. Müller-Wegener: Soil chemistry. BI Wissenschaftsverlag, Mannheim / Vienna / Zurich 1990, ISBN 3-411-03205-7 .