Geoscience Reference
In-Depth Information
The retention of anions is high at acid pH, as we have seen. The
following is the order of retentivity:
PO
4
> SO
4
> Cl > NO
3
Added phosphate is held and blocked in the anionic form as well
as in the form of aluminium phosphate (
fixation
of agronomists).
The ratio
available P/total P
is generally close to 5 to 20 per cent but
could drop to a low of 1 per cent in soils rich in aluminium oxides
(Colmet-Daage
et al
. 1965).
Nitrogen can be retained in the form of nitrate, which is beneficial.
Still it is necessary that it be present. Lack of nitrogen is a limiting factor
in these soils where wild nitrogen-fixing legumes establish. They are
natural colonizers after eruptions.
In Aluandosols, there sometimes is aluminium toxicity. However,
as the pH is not extremely acid, the amounts of Al
3+
in solution are
limited (Chap. 3, Fig. 3.8). Furthermore, formation of aluminium
phosphates removes the metal from the soil solution. At one time
it was thought that aluminium polymerizes in the form of the Al
13
polymer [AlO
4
Al
12
(OH)
24
(H
2
O)
12
]
7+
, which will be more toxic. But this
hypothesis is half-abandoned. Toxicity is correlated with the amount of
KCl-extractable aluminium (exchangeable Al
3+
).
Silandosols are less affected except when they are acidified through
intensive cropping with unbalanced fertilization (nitrogen) not restoring
the bases (calcium, magnesium…) used up by the crop.
Acid Andosols can be limed. But the best is to grow tolerant species
(e.g. tea).
Aluminium toxicity
CONCLUSION
The coincidence of rapid weathering of a vitreous and pyroclastic
volcanic rock and blocking of argillization by organic matter leads to
what we could describe as a 'freeze frame' in the film of the evolution
of Andosols. We see large quantities of the precursors of clay minerals
seen nowhere else. The study is instructive beyond the particular case
of the soil in question.
But volcanic rocks can also yield all sorts of other soils when aided
by time or as soon as the climate is less humid and less cold. Thus, on
the flanks of the volcanic islands of the intertropical zone, where the
altitudinal climatic gradient is one of the strongest in the world, we
