Geoscience Reference
In-Depth Information
Many mineral species are involved! Also, the transformation of one to
another is often rapid and reversible. But in some way, it is very simple in
principle: according to the characteristic of the soils, the iron compounds
are found in more or less pure, more or less well-crystallized, variably
hydrated and variably oxidized forms (Trolard
et al
. 1993).
Field observations
In the field it is possible to use certain coloured indicators to
determine the oxidation state of the iron in the oxides:
a solution (1%) of potassium ferricyanide K
3
Fe
3+
(CN)
6
. The treated
face of the test aggregate becomes dark blue if the medium is
reducing for iron,
a solution (2%) of dipyridyl in 10 per cent acetic acid. A dark red
colour indicates presence of Fe
2+
, therefore a reduced medium.
Field diagnoses regarding soils with excess water pertain to:
use of the land (kind of crop, vegetation),
organic matter content because many soils saturated with water
permanently or for part of the year have lower biological activity,
leading to accumulation of carbon (see § 12.3),
duration of saturation with water if data is available to
estimate it,
depth to the water table,
pH (see § 12.4.2).
12.3
EVOLUTION OF ORGANIC MATTER WHEN EXCESS
WATER IS PRESENT
12.3.1 The Broad Groups of Soil Microorganisms
Microorganisms are distinguished (Gobat
et al.
1998) by:
their energy source, which may be solar (
phototrophy
) or
closely related to oxidation of mineral or organic compounds
(
chemotrophy
),
their carbon source, which can be the CO
2
of the air (
autotrophy
)
or organic matter (
heterotrophy
),
their tolerance to oxygen: aerobes, anaerobes, facultative aerobes,
oxygen-tolerant anaerobes,
the transformations they engineer.
Some scientists talk of
lithotrophy
when the electron donor is
mineral and
organotrophy
when it is organic; but other investigators
