Tuesday, August 7, 2012

My Favourite Soil: Terra Rosa

This months MFS comes from Antonio Jordán at the University of Sevilla, Spain. This post was originally published at EGUSSSD and has be reposted with minor edits. You can get to know Antonio at Sunday Soil Scientist

Red soil at São Brás de Alportel (Portugal)

The term "terra rossa" comes from the Italian for "red soil" or "red earth". Although terra rossa exists in other places in the world, these soils are common in areas with Mediterranean-type climates: alternation of a rainy and cool-to warm-dry season.

The terra rossa soil is heavy and clay-rich (silty-clay to clayey) soil, strongly reddish, developed on limestone or dolomite. It is a colloquial way to refer to land included within the Rhodustalfs (but also other sub-orders included in Alfisols, Inceptisols, Mollisols and Ultisols of the Soil Taxonomy), Chromic Luvisols (but also other soil types inside Cambisols, Luvisols and Phaeozems of the WRB) or modal fersiallitic red soils (French classification).

Lorena, while sampling a red soil in São Brás de Alportel, Faro, Portugal

There are several theories about the formation of terra rossa. The first one, traditionally accepted, states that it derives from the insoluble residue of the underlying limestone. Following dissolution of calcium carbonate by rain, clay contained in limestone sediments with other insoluble substances or rock fragments, forming discontinuous residual layers variable in depth. Under oxidizing conditions iron oxides appear, which produces the characteristic red color. According to this theory, terra rossa is a polygenetic relict soil, formed during the Tertiary and subjected to hot and humid periods during the Quaternary.

Karstic landscape (Cerro del Hierro, Sevilla, Spain)

A more recent theory is based on the geochemical composition of the soil, and suggests that these soils would have formed about 12.000 to 25.000 years from wind transported sediments over long distances.

Prof. Nicolás Bellinfante (Univ. of Sevilla), talking about the genesis of red calcareous soils

However, although in this case soil material is considered to be allochthonous (eg, aeolian dust from the Sahara), formation of the Mediterranean terra rossa is closely related to the properties of the limestone substrate.

Despite their clayey, red soils are usually well drained, due to the strong development of its structure, which allows agricultural use.

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