Geoscience Reference
In-Depth Information
ensure drainage in case of a heavy shower and also retain
groundwater that can be utilized in case of drought.
Still farther from the river (3), the soils become loams or clay
loams. On the one hand, as we have said, the alluviation is
finer. On the other hand, the altitude is a little greater than in
the sands, a few decimetres or a metre or two. This is already a
very young terrace, traditionally called
upper level of recent alluvia
.
The environment is thus scarcely affected by floods. The miner-
als begin to weather to clay minerals. The corresponding soils
often suffer from excess water and thus exhibit rather strong
hydromorphic features. The texture is fine and does not permit
good natural drainage. In many countries, these environments
are perfectly suited to livestock farming.
Lastly, special soils are found in three characteristic positions:
excessively wet soils of dead branches of the stream (4), soils of
all kinds laid down by lateral tributaries (5), and very clayey
soils of troughs or basins (6). The last-mentioned, before being
cleaned up and reclaimed by cropping, were actually marshes
(
backswamps
). For example, they are found in the Indus valley in
Pakistan and in the Authion valley, a lateral depression in the
Loire in France (Servant 1968).
2.5
VEGETATION AND HUMUS
2.5.1 Action of Vegetation on the Soil
We have seen above and also in Chapter 1 that soils have been linked
at the global scale to the vegetation-climate pair. It is the same on
mountainsides (concept of
climatosequence
already mentioned; see
Fig. 2.19 below).
The vegetation plays a role in the differentiation of soils directly by
its living roots and indirectly by the wide variety of residues it supplies
every year. In return, the soil plays a part in the selection of plants that it
bears. The climate acts on the soil directly, as we have seen (temperature,
rainfall, etc.). It also influences the nature of the vegetation. We are thus
facing a complex interactive system.
The roots, whose primary functions are to anchor the plant in the soil
and to ensure its nutrition, has the following functions in regard to the
soil:
Role of the roots
