Geoscience Reference
In-Depth Information
or less occasional submergence) plays an important role in the evolution
of such systems. The same is true of the regime of fresh water, which can
periodically wash the soils in estuaries. In addition, when the mangrove
swamps are located at the mouths of estuaries where large amounts of
sediments are brought in by streams, the process of sulphate reduction
is not active. The typical example is the western coast of Madagascar
(Lebigre
et al
. 1989).
12.6.4 Typology and Classification
Dutch farmers used to call the soils of these environments
Katteklei
(
cat
clay
), perhaps because of their pale, spotted colour.
Generally the soils of active mangrove swamps can be described as
'potentially acid sulphate' soils, while the emerged and acidified soils
are true acid sulphate soils.
In Soil Taxonomy, the soils that exhibit a diagnostic horizon with
sulphates (sulfuric horizon) or with sulphides (sulfidic materials) are
mostly undeveloped soils (
Entisols
), immature soils (
Inceptisols
) and peats
(
Histosols
).
In the Entisols, we find Sulf-aqu-ents. These soils with saturated
regime (aqu) contain sulphates. Strictly speaking, the prefix 'Sulf' used
here is not enough to distinguish sulphates and sulphides, although
Sulfaquent is correctly interpreted in French and in English. This has
allowed the creators of Soil Taxonomy to avoid doubling of vowels. On
the other hand, soils containing sulphides are distinguished only at the
subgroup level. Thus we have Sulfic Endo-aqu-ents, Sulfic Fluv-aqu-ents
and Sulfic Hydr-aqu-ents.
In Histosols, the distinction is easier, for example Sulfo-saprists
(sulphates) and Sulfi-hemists (fibres and sulphides).
In Inceptisols of saturated regime (aqu) or moist regime (ud), it is
simpler because soils with sulphides are not expected to occur. Soils
with sulphates are, for example, Sulf-aqu-epts or Sulf-ud-epts.
The WRB has proposed a 'Thionic' diagnostic horizon that contains
sulphides or sulphates. More precisely, it is characterized by:
pH < 4
presence of jarosite KFe
3
(SO
4
)
2
(OH)
6
or schwertmannite
Fe
16
O
16
(SO
4
)
3
(OH)
10
,10H
2
O or even soluble sulphates or
sulphides.
In these conditions, a suffix 'Thionic' can be used for characterizing
a 'Histosol', and also a Fluvisol, a Gleysol, a Stagnosol and others.
