Retisol

Retisol (RT) is a reference soil assembly from the World Reference Base for Soil Resources (WRB). Retisols are soils characterized by the shifting of clay (lessivization), in which the clay-depleted and the clay-enriched horizon penetrate like a network ( Latin rete = network). The diagnostic horizon is the argic horizon (Bt) characterized by tone enrichment . The penetrations are called retic properties.

The retisols were newly introduced in the third edition of the WRB 2014. They include the more narrowly defined Albeluvisols, which were defined in the first two editions of the WRB (1998 and 2006) and which were abolished in 2014. In the legend of the world soil map (1974) and in the revised legend (1988) there was the podzoluvisole, which were defined similarly to the retisole.

The typical horizon sequence according to the FAO Guidelines for Soil Description is:

• A - topsoil with mostly medium humus contents
• E - tone depletion horizon ( eluvial , mostly relatively bright)
• Bt / E - penetration horizon with retic properties
• Bt - clay enrichment horizon ( illuvial ) in the subsoil
• C - parent rock

description

Retisols can be dominated by both three-layer clay minerals and kaolinite , but three-layer clay minerals are much more common. The base saturation can be high or low, but more often low. There are two theories about how it came about.

One theory assumes that the clay shift is old and possibly already took place in the Eem warm period . During the Glacial Vistula there were frost crevices in the dry periglacial area under permafrost conditions in the clay enrichment horizon, into which the lighter, less clayey material fell. A subsequent cryoturbation created the net-like pattern.

The other theory assumes a purely Holocene formation. When the snow suddenly thaws after snowy winters, large amounts of water poor in electrolytes arise, which detach clay minerals and oxides from the aggregates along their preferred flow paths and lead them into the depths.

Both theories characterize the distribution areas of the retisole, namely the periglacial areas of the last glacial period and the cooler continental median latitudes. Accordingly, in Europe they can be found from Belgium via northeast Germany to Russia. Due to their very special way of origin, they usually do not take up any large contiguous areas.

Retisols are widely used in forestry. Grazing is also common. The physical properties of retisols in particular cause problems for arable crops. The topsoil is often extremely poor in clay minerals and iron oxides. In the subsoil, the water flows preferentially along the clay-poor areas, while the clay-rich and thus nutrient-rich areas are often insufficiently moisturized. In addition, these clayey areas are difficult to root through. These properties make many retisols also suitable for soil animals, e.g. B. for earthworms, not very attractive. The topsoils of the retisols are very silty or sandy in many places and therefore at risk of erosion.

Related soil types

The WRB knows five reference soil groups with an obligatory argic horizon . Only the retisols have additional retic properties. The other four do not have any and are differentiated according to potential cation exchange capacity per kg clay (KAK / kg clay) in the argic horizon and effective base saturation (BSeff) in the subsoil. With the Luvisoles , both are high. In the case of the alisols , the KAK is high and the BS is low. The KAK is low and the BS high for the Lixisols , and both are low for the Acrisols .

At first glance, some retisols look like stagnosols . In the case of stagnosols, however, the color pattern is the result of redox processes , and the different contents of iron oxides in the typical stagnosol are not coupled with differences in clay content within the same horizon. Retisols always have distinctive differences in clay content between the lightened and the intensely colored areas within the same horizon, and the color differences are due to the fact that oxides form complexes with clay minerals and are carried along during the lessivation. Soils that have both characteristics are Retic Stagnosols.

In the German soil systematics , they correspond most closely to the pale earths . However, only those pale earths are retisols that actually have an Ael + Bt horizon.

Web links

• Profile photos (with classification) WRB homepage
• Profile photos (with classification) IUSS World of Soils

literature

• IUSS Working Group WRB: World Reference Base for Soil Resources 2014, Update 2015. World Soil Resources Reports 106, FAO, Rome 2015. ISBN 978-92-5-108369-7 ( PDF 2.3 MB).
• W. Zech, P. Schad, G. Hintermaier-Erhard: Soils of the world. 2nd Edition. Springer Spectrum, Heidelberg 2014. ISBN 978-3-642-36574-4 .
• W. Amelung, H.-P. Blume , H. Fleige, R. Horn, E. Kandeler , I. Kögel-Knabner , R. Kretschmar, K. Stahr , B.-M. Wilke: Scheffer / Schachtschabel textbook of soil science. 17th edition. Heidelberg 2018. ISBN 978-3-662-55870-6 .
• FAO: Guidelines for Soil Description. Prepared by R. Jahn, V. Asio, H.-P. Blume , O. Spaargaren and P. Schad. Rome 2006. ISBN 92-5-105521-1 .