Fjölrit RALA - 15.06.2004, Side 38

Andosols in the World Reference Base for Soil Resources and their
correlation within other classifícation systems
O.C. Spaargaren
ISRIC - World Soil Information, Wageningen, The Netherlands
WRB Andosols
In the World Reference Base for Soil Resources (WRB), Andosols are defined as: soils
having a vitric or andic horizon starting within 25 cm from the soil surface; and having no
diagnostic horizons (unless buried deeper than 50 cm) other than a histic, fulvic, melanic,
mollic, umbric, ochric, duric or cambic horizon (IUSS-ISRIC-FAO, 1998).
The salient characteristic for an Andosol is the presence of a vitric or andic horizon. These
horizons represent, respectively, slight and moderate weathering stages of, mainly, pyroclastic
deposits. To be diagnostic, these horizons must have a minimum thickness of 30 cm.
A vitric horizon has 10% or more volcanic glass and other primary minerals in its fine earth
fraction (fraction less than 2 mm), and has a bulk density of more than 0.9 kg dm’3, an Alox +
Vi Feox2 of more than 0.4%, or a phosphate retention of more than 25%. The requirements
have been modified with respect to the 1998 ones (Driessen et al. (eds), 2001), because the
original appeared to be unsatisfactory (properties of fresh volcanic ashes did also comply with
the 1998 definition).
An andic horizon, representing a more advanced stage of weathering than the vitric horizon,
requires a bulk density of less than 0.9 kg dm’3, 10% or more clay, an Alox + Vi Feox of 2% or
more, phosphate retention of 70% or more, and less than 10% volcanic glass in the fine earth
fraction. Two subtypes of andic horizons are recognized and used to differentiate amongst the
Andosols: the silandic and the aluandic horizon. Allophane and similar minerals predominate
in the silandic horizon, whereas aluminium, complexed by organic acids, prevails in the
aluandic horizon. The difference between the two is also reflected in the soil reaction: the
silandic horizon being acid to neutral, the aluandic horizon acid to extremely acid.
The initial division of Andosols, through its ranking of “quahfiers”, is based on the type of
diagnostic horizon present. Soils with a vitric horizon key out first - as Vitric Andosols. Soils
with a silandic horizon key out next, either as Eutrisilic Andosols (high content of
exchangeable bases) or as Silic Andosols. The implication of this sequencing is that Andosols
that do not meet the criteria of Vitric or (Eutri)Silic Andosols, automatically belong to the
aluandic type. Therefore reference to this type is not made in further subdividing Andosols.
As a second step for further division, typical horizons or properties associated with
Andosols are used: melanic or fulvic horizons, and hydric (high water content) and pachic
(thick surface horizons) properties. As a consequence of the first division, a Melanic Andosol
will have an aluandic horizon, otherwise the soil would be classified as Melani-Silic Andosol.
The next line of subdivision shows the intergrades to other reference soil groups, i.e. histic,
leptic and skeletic, gleyic, mollic, duric, umbric and arenic. In cases where buried horizons
occur, intergrades to Podzols (placic), Ferralsols (acroxic and vetic) and Luvisols (luvic) can
be used. If other buried horizons are present, the quahfier Thapto in combination with the
diagnostic name of the buried horizon is used, e.g. Thaptomelanic Vitric Andosol.
Chemical characteristics appear last in the sequence of qualifiers: calcaric (calcareous),
sodic (sodium-rich), dystric (low base saturation) and eutric (high base saturation).
Links with other WRB reference soil groups are made through andic qualifiers in Cryosols,
Gleysols, Ferralsols, Phaeozems, Alisols, Nitisols, Acrisols, Luvisols, Lixisols and
2 Alox and Feox are acid oxalate-extractable aluminium and iron, respectively (method of Blakemore et al., 1987)
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