Geoscience Reference
In-Depth Information
horizon has features intermediate between two typical horizons, it is
identified by the two letters. Thus, there may be an AB horizon or
even a BA if it is desired to indicate that it resembles a B horizon more
than an A. A horizon that locally exhibits very marked characteristics
of an A horizon in some places and distinct features of a B horizon in
others will be designated A/B. If the soil profile is differentiated in two
superimposed geological materials, the corresponding discontinuity is
brought out by a Roman two (II) preceding the symbols of the horizons
pertaining to the second geological material. For example, A, IIB, IIC.
French scientists of the IRD (formerly ORSTOM), while studying the
ancient soils of the intertropical zone, noticed that, in a trench cut
horizontally through the soil on a slope, 'soil volumes' not always
parallel to the surface were found instead of horizons. In shape, they
resembled lenses that tapered off laterally (Fig. 1.1).
Limitations of the horizon concept
Soil volumes
Soil
surface
200 m
Fig. 1.1
Example of the lateral organization of the soil seen in section on a slope in Guyana
(Veillon 1990).
In such a case, the concept of representative profile is therefore not
effective. It would be better if the morphology, the nature and dynamics
of each soil volume are directly studied to understand the weathering
phenomena.
1.1.2 The Idea of Pedogenesis
Vassili Vassiliyevitch Dokuchaev (1846-1903), a geologist at the University
of St. Petersburg, had been involved in the mapping of Russia in Europe
in various situations between 1875 and 1879 (Legros 2006). To do his
work properly as a practical man he apparently traversed 10,000 km on
foot. Dokuchaev thus gained considerable experience and was the first
to assert, with supporting examples, that soils depend on the following
five factors of formation:
