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pers. comm.). Then we get the horizontal lines dividing the triangle in
Fig. 10.11. At the first level are the
Regosols
, immature sandy soils that
scarcely or do not contain the constituents characteristic of Andosols.
Below that are the
Vitric Andosols
, rich in primary minerals particularly
glasses, but which also contain paracrystalline minerals. At the bottom
are shown the mature Andosols, rich in all the secondary compounds
extracted by oxalate. Within them, the Al
p
/A l
o
ratio allows the separation
of two kinds of Andosols.
Regosols
0.4%
25%
Vitric Andosols
(immature)
2.0%
85%
Aluandic
Andosols
(Al-humus
complex
es)
Silandic
Andosols
(allophanes)
Retention of
phosphate
Al + ½Fe
o
o
0.5
0
1
Al /A
po
Fig. 10.11
Proposed classifi cation of the Andosols (Shoji
et al
. 1996). With: Al
p
extracted
with pyrophosphate; Al
o
and Fe
o
extracted with oxalate. Numbers have been modifi ed to
match the standards fi nally retained by the WRB.
At the left of the triangle are placed the
Silandic Andosols
(WRB), rich
in allophane, imogolite and ferrihydrite. Their pH is higher than 4.9 and
goes up to 7 or 8. Their Si
o
content is, as a rule, 0.6 per cent or greater.
Their Al
p
/A l
o
ratio is less than 0.5 as indicated in the diagram.
At the right of the triangle are grouped the
Aluandic Andosols
, in
which aluminium is present chiefly in the form of organo-mineral
complexes. These soils correspond to what many authors call
Non-
allophanic Andosols
. They are seen in the most humid and coldest climates.
They are also the most acid (pH < 4.9) and are often the richest in organic
matter. The Si
o
content is 0.6 per cent or less. No longer combined, silica
is eliminated or remains as opal. These soils correspond generally to
the oldest volcanic materials.
Also, in the same profile, Al-humus complexes can dominate in
the A horizon (very acid medium) and allophanes in the less acid B
(Ndayiragije and Delvaux 2003).