Fjölrit RALA - 15.06.2004, Page 89

Andosols criteria and Classifícation of European Volcanic Soils : up to date
proposals. Genesis, key factors and distribution
Paul Quantin
5 rue Boileau, Dijon, France
1. Classification
A new proposal is presented for the classification of COST 622 reference profiles of
European volcanic soils. That is a tentative up to date classification conceming 24 soil
profiles from : Italy, Azores, Iceland, Tenerife (Canary Is.), Greece (Santorini Is.), France
(Massif Central), Hungary and Slovakia.
However many problems are still remaining, due to : either lack of diagnostic criteria, like
soil moisture and temperature regime, water retention capacity, volcanic glass content, exch.
Al, bulk density; or a doubt for the credibility or reliability of data, like clay content or exch.
bases saturation ratio; or discrepancy of soil description or data comparing new with former
reference data, for example profile 12 in Tenerife; or about limits of diagnostic criteria, like
for andic/vitric horizons, eutric or eutri-silic qualifiers, colour of melanic/fulvic or umbric
epipedon, or the priority andic/histic in H horizons of profile 9 in Iceland. There is a lack of
information on the trae parent material of soils over old volcanic bed rock reworked by glacial
or periglacial processes. Another problem arises from the buried paleosols, in order to express
better the whole soil profile and its genesis.
About andic/vitric diagnostic criteria limits some little changes are proposed, namely :
- Alo + Vi Feo , to be lowered to 1.7% , only for eutrisilic and aluandic horizons, according to
Pret. > 70%
- glass content not useful; to be deleted and replaced by other chemical (CEC) or physical
(water retention capacity) properties, but taking into account org. C content.
- Z Bases to be lowered to 15 cmol Kg"1 for eutric or eutrisilic qualifiers.
2. Genesis
The soil genesis and distribution are schematically related to 3 key factors : time of
weathering, climate and topo-climatic sequences.
2.1. Time : At first on recent pyroclastics deposits, Andosols, namely silic Andosols, are
predominating, although there is a gradual evolution of weathering from vitric to silic
Andosols. While on old volcanic bed rock, aluandic Andosols predominate under a mesic -
udic climate regime, although the time of present soil formation is holocenic. In this case the
nature formerly preweathered of periglacial parent material must be taken into account.
2.2. Climate : - soil moisture regime : On recent pyroclastics, according to the moisture
regime, Andosols are fulvic or melanic and dystric under udic regime, or eutrisilic under ustic
regime, or vitric under xeric regime. But in Greece, due to rather aridic conditions,
pyroclastics soils remain weakly weathered, as tephric Arenosols. - soil temperature regime :
On recent pyroclastics the variation from thermic or mesic to frigid or cryic, does not seem
affecting greatly the nature of the weathering products; although in Iceland the cryic
temperature regime affects the soil structure.
2.3. Topo-climatic sequences : According to climate distribution, from top ( frigid or mesic,
udic ) to downwards ( mesic or thermic, ustic or xeric ) of a toposequence, there is a gradual
evolution of soil properties. For example in Massif Central ( France ), we observe : on recent
pyroclastics an evolution from fulvi-silic to eutri-silic Andosols and even to eutric Cambisols;
while on older lava flows : from fulvi-alic Andosols to andic or humic Umbrisols and eutric
Cambisols. In Azores from top to downswards, there is a change from hydri-alic Andosols to
fulvi-silic Andosols.
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