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matter. Considering the absence of clay and the very low contribution
to the CEC by oxides and hydroxides, this is not surprising.
At the same time, a close correlation is found between the anion
exchange capacity, AEC, and the contents of Fe
2
O
3
and Al
2
O
3
in the
profile; this is also logical.
Content of exchangeable aluminium is often greater than 4 cmol (+)
kg
-1
, surpassing the amount of exchangeable bases present.
If sodium fluoride solution is added to a pinch of Andosol and a
pH-indicator paper is dipped in the mixture, after two minutes delay a
pH higher than 9.5 is obtained, because of the release of
OH
ions. This
is characteristic of the presence of reactive aluminium. Actually:
Al
(
OH
)
3
+
6(
Na
+
F
-
)
Æ
Na
3
AlF
6
+
3(
Na
+
OH
-
)
This use of sodium fluoride is a test for classification of Andosols
(more precisely Andisols) in Soil Taxonomy. But it is not sufficient
because podzolic soils also react similarly!
NaF pH
10.5
TYPOLOGY AND CLASSIFICATION
10.5.1 Basis of the Typologies
Since there are many kinds of short-range-ordered minerals it is not
abnormal to find Andosols that do not contain allophane (see Fig. 10.7).
These are 'Non-allophanic Andosols' (Dümig
et al
. 2008b). On the other
hand it is more astonishing to find soils that contain allophanes but do
not satisfy all the criteria that will classify them in Andosols. There
are, for example, Non-Andic Allophanic Inceptisols (Vacca
et al
. 2009).
Current classifications do not emphasize these peculiarities, in contrast
to many publications.
For the revision of the WRB, a research group released in 1996 the
proposal reproduced in Figure 10.11 (Shoji
et al
. 1996) for the classification
of Andosols.
Knowing that oxalate extracts organo-mineral complexes,
oxyhydroxides and also amorphous materials (Table 10.1), we can
classify the soils according to degree of evolution on the basis of the
factor 'Al
o
+ ½Fe
o
'. We should note in passing that this quantity also
takes into account phosphate retention, which is one of the criteria of
andosolization (Dümig
et al
. 2008b). Iron is taken into consideration to
include the Andosols on basalt, those of Hawaii in particular (P. Quantin,
