Saturday, 15 July 2006

Vitrisols: a proposed WRB soil group.

Olafur Arnalds, Agricultural University of Iceland, Keldnaholt, Reykjavik, IS-112, Iceland

The concept of Andosols is tied to soils that develop in volcanic ejecta, a notation that links the soil class with parent material.  Short-range order minerals and/or metal-humus complexes provide the soils with properties that characterize Andosols and a measure of these constituents are used for their identification (e.g. > 2% (Al+˝Fe)ox).  The presence of vitric materials is also used as a diagnostic criterion in Soil Taxonomy (ST) and WRB with relaxed requirements (e.g. 0.4 instead of 2%).  Otherwise, poorly developed vitric soils that do not meet the lowered Andosol criteria are registered with a diversity of other poorly developed soils.  This causes problems in classifying soils in volcanic areas, and raises questions about how these soils are classified.

     Iceland has large areas (>40 000 km2, 40% of Iceland) of poorly developed soils that constitute mostly basaltic vitric materials. Much of the materials meet criteria of >0.4% (Al+˝Fe)ox and are therefore Andisols according to ST.  However, their classification alternates between Andosols and Regosols over few m distances according to the WRB, depending mostly on thickness, which in many cases is difficult and even awkward to define.  They are termed Vitrisols in Iceland, using OC% and Siox (allophane) contents to differentiate between Vitrisols and Andosols. The Icelandic Vitrisols include vitric subgroups of many other soils according to WRB.  


The concept of vitric soil materials has generally been approached in relation to the definition and investigation of andic soils; Andosols.  It sounds more reasonable to define vitric soils as such, rather than in relation to characterization of other type of soils (Andosols). 

     Oxalate extraction may dissolve the basaltic vitric materials and the presence of magnetite and other factors may also interfere with this method in volcanic areas.  These reasons make oxalate values questionable for classification purposes of vitric materials. 

     Vitric materials have unique soil characteristics, both physical and chemical, making them very different from inert parent materials such as quartz of other poorly developed soils.  Tephra can have up to 30 m2 g-1 surface area, and grain size is often chiefly a result of how much energy is used to fractionate the materials.

     Separating vitric soils into two components, partly with Andosols (based on questionable criteria), and a range poorly developed soils (such as Regosols and Arenosols), in spite of unique soil properties is worth re-consideration.  Such separation gives a poor representation of Icelandic soils for scientific purposes, land use considerations, and regional (e.g. European) representation.  Mapping becomes difficult and the maps convey less information.

     I perceive this as a wide ranging problem in volcanic areas, where information about vitric soils, with their unique properties are lost unless a detailed levels of mapping are used.  


A primary goal of soil classification is to separate soils in a clear and meaningful manner. A soil should be recognized for what it is; the notion of pedogenesis should not restrict our ability to classify soils.  Parent materials can have overriding influence on properties of soils. 

     Therefore, the use of the soil group Vitrisols is suggested, as is done in some other classification systems.  The idea fits well within the general purpose of the WRB which states:at the higher categoric level classes are differentiated mainly according to the primary pedogenetic process that has produced the characteristic soil features, except where ‘special' soil parent materials are of overriding importance”.  Vitric materials, together with organic residues (histic) and athropogenic influences are of “overriding” importance for soil properties.  Further, much of the vitric materials have unique soil behaviour, both physical and chemical, which separates them from other rock materials.

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