Geoscience Reference
In-Depth Information
When necessary, it is also possible to break up the ironstones. But many
of these soils belong to marginal areas given over to forest.
Coniferous trees are well adapted and competitive in podzolic soils.
On better soils, when they are introduced by man, it is often necessary
to suppress for several years the many wild plants that enter into
competition with them for soil and space.
A study conducted on the Pilat massif (Legros 1975) enabled mapping
of the hazards of soil podzolization, i.e., risk of excessive acidification.
This happens whenever a plant cover of coniferous trees is the only
environmental factor that is lacking to turn a soil of the Entic Podzol
type into the 'Podzol' type. In France, this corresponds to small areas
where planting of conifers should be avoided. Elsewhere, conifers can
be planted in place of broad-leaved trees without noticeably degrading
the soil (Nys and Ranger 1985). The debate that once put foresters and
ecologists in opposition should be defused.
Forests are not fertilized because the operation will, at best, return
a profit only when the trees are cut down, that is to say, many decades
or centuries after the application of fertilizer. In these circumstances,
it is more worthwhile if more remunerative placements are found. We
continue to utilize forest through an ancestral system of harvesting
without returning to their soils the elements (P, K…) that were extracted
from them. Then, there is progressive impoverishment through loss of
bases by removal of timber and, therefore, acidification. The process is
faster under conifers because they are more productive than broad-leaved
trees. This can trigger deficiencies (Cu) and aluminium toxicity (Juste
1965). Yellowing of fir and spruce, which has been proved to have
multiple causes, affects chiefly poor, shallow and acid soils (Bonneau
1990). The environment corresponding to Podzols is, therefore, fragile.
Acidification is reinforced by atmospheric pollution such as acid rain
(Party
et al
. 1997; Party 1999).
The case of temperate forests
The infertility of giant Podzols of Amazonia is severe. When the forest
is cut, it struggles to grow again (Chap. 5, § 5.6.1 and § 5.6.2). In other
cases, it disappears naturally and is replaced by a bushy vegetation
called
campinas.
Podzols of other regions
