Saturday, 15 July 2006

Icelandic Andosols and Vitrisols.

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

Icelandic soils, recently reviewed by Arnalds (2004), are predominantly Andisols according to Soil Taxonomy.  They make up a substantial proportion of the world's Andisols (up to 5%).  The majority of the soils can be divided into two broad groups (Icelandic classification scheme): soils under vegetation (Brown, Gleyic, and Histic Andosols plus Histosols) and various types of Vitrisols found mostly on desert areas.  Icelandic soils form under climatic conditions that range from cold frigid areas in lowlands to arctic highlands, and they are extensively influenced by cryogenic processes.  The weathering environment is characterized by relatively steady state of eolian deposition of basaltic glass materials originating from sandy desert areas, and frequent but small tephra additions.  The eolian and tephra deposition and accumulation of organic materials have built up a 0.3 - >2 m thick mantle that covers most of the vegetated part of the country: the parent materials of Icelandic Andosols and Histosols.

     The basaltic tephra weathers readily, in spite of cold climate, forming allophane, imogolite and ferrihydrite, releasing cations that maintain the pH of the soils. Icelandic soil databases show clear relationships between pH, allophane formation, and organic content, which is partly independent of climate.  These relationships are largely controlled by the rate of eolian and tephra deposition (vitric input) which varies depending on distance from source of eolian materials (sandy deserts) and volcanoes.  The eolian and tephra input, together with the position of ground water table explains the variability in properties of Icelandic Andosols to a great extent.  Rapid vitric input (often > 1 mm yr-1) results in pH favorable for allophane formation (5.5-6.5), while soils far from eolian sources of vitric materials tend to have lower pH (< 5.5) and become dominated by aluminum-humus complexes and organic materials, but andic influences as expressed by (Al+˝Fe)ox, P-retention, and low bulk density, are always present. 

     The poster presents a soil map of Iceland, based an Icelandic classification scheme.  It is expressed using the gradient caused by increased vitric material input near volcanoes and deserts that are sources for eolian redistribution of vitric materials.  The following soil classes form the highest level of the scheme.

     Histosols with OC>20% occur in poorly drained areas far from vitric sources.  They cover only about 1% of Iceland.  They do exhibit pronounced andic influences.

     Histic Andosols have 12-20%C in surface horizons but meet criteria for Andosols.  They occur far from sources of vitric materials at poorly drained landscape positions.  They have a unique combination of andic and histic properties, often with poorly decomposed organic materials, raising interesting classification arguments regarding to which class they should belong (Andosol vs Histosol or Gleysol).

     Gleyic Andosols  have <12%C and are Andosols of poorly drained areas near vitric sources; thus they are typical Andosols, often allophanic in nature, but also have properties of Gleysols.

     Brown Andosols are typical freely drained Andosols, with variable amount of allophane depending on how much tephra and eolian vitric materials are deposited (allophanic far from vitric sources; vitric in nature near volcanoes and deserts). 

     Vitrisols, the sandy soils of Icelandic deserts (400-2000 mm rainfall) cover up to 40 000 km2 or 40% of Iceland.  They meet criteria for Andisols according to Soil Taxonomy, although consisting mostly of volcanic glass, partly because of the basaltic nature of the tephra (e.g. (Al+˝Fe)ox >0.4%).  When vegetation is established on these soils, they can develop to become Brown Andosols within few decades.

     Other types of soils occur in Iceland, including Cryosols (andic in nature), Regosols, Fluvisols, Arenosols and Leptosols, but these are either of limited extent or have not been researched.  


Arnalds O 2004.  Volcanic soils of Iceland. Catena 56:3-20.


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