Geoscience Reference
In-Depth Information
Fluvisols are young soils developed on alluvial river sediments. Their name has
roots in the Latin
fl uvius
which means river, but they also occur on lake and sea sedi-
ments. Many of them are fl ooded periodically up to the recent time, and in such
cases their fertility is usually reestablished. Their profi le frequently manifests signs
of stratifi cation, especially in old alluvia where conditions for sedimentation were
historically changing. With the accumulation of humus originating from plant rem-
nants remaining in place, the top A horizon is more or less distinct. In the bottom
part of the profi le, signs of temporal chemical reduction combined with periods of
aeration could be found if there is a correspondence between the water level in the
riverbed and the groundwater level. In such cases, the existence of a gley horizon,
easily recognized by its greenish to blue patches and low pH value, is attributed to
the acidifi cation caused by waterlogging within such a deep layer. Fluvisols gener-
ally belong to fertile soils. This favorable condition for plant production provides a
logical explanation of why archeologists found the fi rst steps of agricultural revolu-
tion in regions distinguished by Fluvisols. Only tidal soils are the exception in
Fluvisols' fertility since they are usually very saline. Fluvisols appear in national
classifi cation schemes under the name Alluvial Soils, Auenböden, Sols Minéraux
Brut, or Sols Peu Évolué. The US Taxonomy classifi es them as suborder Fluvents in
Entisols.
Gleysols have been developed on wetlands with high elevation of groundwater
level that can reach up to the root zone. Occasionally, they evolved from Luvisols
when the groundwater level rose during most of the year either due to changed natu-
ral conditions or sometimes due to human activity. The bottom part of the profi le is
fully saturated by the groundwater during the whole year. Due to the weathering of
minerals and the transformation of organic matter in the absence of oxygen and
under permanent anaerobic conditions, this horizon has a greenish-blue color with
a gray hue. Above this G horizon is the mottled gleyish horizon where the anaerobic
waterlogged conditions are regularly interchanged with aerobic conditions when
water is drained from a portion of big pores and reddish or orange mottles indicate
localized reoxidation of ferrous compounds in the soil matrix. These small, irregu-
lar patches are often associated with root channels, animal burrows, or cracks of the
soil material when waterlogging occurs at greater depths or is even absent. On top
is the humus horizon, which may sometimes be interchanged with peat or swamp.
We have already mentioned that the folk word for glue in Ukraine is
gley
that was
transferred to soils being overwet and sticky. If Gleysols are regularly tilled and
cultivated when they are too wet, their soil structure is destroyed with their physical
conditions no longer being favorable for cultivated plants. Permanent grasses or
swamp forests then become preferred alternatives. When Gleysols are drained and
the groundwater table lowered, the majority of soils start to belong to fertile soils,
provided that the pH is kept at optimal values. Usually liming is recommended. In
Southeast Asia rice is frequently grown on Gleysols. They appear in national tax-
onomies also under the name Gleyzems, Meadow Soils, Hydromorphic Soils, or
simply Gleys. The US Taxonomy ranks them as suborders of Inceptisols (Aquepts),
Entisols (Aquents), and Mollisols (Aquolls).
