Geoscience Reference
In-Depth Information
These soils form a 'coastal' strip parallel to the shores of the Atlantic
Ocean, the North Sea and the Baltic Sea. They are associated with
humid climates and mild temperatures that favour weathering. In the
Northwest, they are replaced by Podzolic soils (as in Scotland and
Scandinavia—in white in Fig. 8.11). These soils, associated with very
cold climates and acidifying vegetation, will be studied in Chapter
11. On the other hand, that is, in the South, in dry climate, Luvisols
also disappear.
8.4.2
Ecology of Planosols
The Planosol, characterized by great depletion of clay from the surface
layer, is formed more easily in environments with strong seasonal
contrasts favouring alternate wetting and drying, with, in consequence,
ferrolysis or lateral translocation of the fine particles (Morras 1979).
Thus, it is not merely the ultimate stage of development of Luvisols in
temperate environment. On the contrary, it is often differentiated at the
expense of Vertisols (Chap. 6, § 6.6) or is seen associated with Solonetz
(Chap. 14). For a Planosol to form, it is not enough that clay disappear
from the E, it is also necessary that the clay content in the B is greater
so that the contrast between the A and the B is high. Obviously this is
easier if the original material is clayey or can weather to clay.
To summarize, the Planosols are differentiated mainly in clayey
parent materials, in tropical climate with contrasting seasons.
8.4.3
Duration of pedogenesis
Telling the age of the soils studied here is difficult because two
difficulties are encountered.
Firstly, we face a continuum. We pass gradually from Cambisols to
more and more distinct Luvisols and then to the increasingly typical
Planosols. If the age is a quantified value, the soil that it refers to is
sometimes less well defined. The concept of Luvisol is, in particular,
very broad: it comprises young, poorly differentiated soils with only a
few per cent more clay in the B compared to the E, and older soils that
have three times more clay in B than in E.
Secondly, the soils observed are likely to be more or less at stable or
steady-state equilibrium (Chap. 3); their maturity, such as is observed
today, may have been acquired a very long time ago.
Difficulties in estimation
