Walker - 1967 - A geography of Italy

PART I I : SOME PHYSICAL GEOGRAPHY
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there is no true B-horizon. This type is widely represented in the Pre-Alps and
the higher calcareous massifs o f the peninsula where the climate favours the
beech.
The other type, terra rossa, has been the subject o f much controversy. Most
authorities regard it as having evolved on limestone under Mediterranean conditions,
but some question the monopoly o f both limestone and o f the Mediterranean
climate in its formation. Even the assumed residual character o f terra
rossa raises some difficulty; experiments with the soluble content o f the limestones
involved (most typically the Cretaceous) suggest that great thicknesses of
rock would have to be dissolved over an excessively long period to produce a
modest thickness o f soil. In consequence the suggestion has been made that
terra rossa did not originate wholly in situ and that it represents material transported
by water and wind imder tropical conditions in the past; in short, that it is
a fossil soil. In a given area the distribution o f terra rossa may be very patchy,
pockets o f soil, accumulated in depressions, alternating with rocky outcrops. It
has no B-horizon (cf. rendzinas), is clayey in texture and very poor in humus.
Generally it contains large quantities o f bases as well as sesquioxides o f silicon,
aluminium and iron, the last o f which accounts for the bright red colour. One
explanation o f the high concentration o f iron and aluminium in the topsoil is that
colloidal solutions o f silica and iron oxide formed in the subsoil in the wet season
are brought to the surface in summer by capillary action, and under the dry
conditions and high temperatures obtaining at that season the oxides are deposited
as a varnish around fine particles in the soil. In winter, in spite o f heavy leaching,
the temperatures are not high enough to free this varnish and so restore the
oxides to the subsoil, and so their contribution to the upper layers tends to increase.
Only the application of humus will free the varnish and check the process;
this would suggest that the development o f terra rossa is encouraged by deforestation.
In general soils derived from the coarser-grained igneous and metamorphic
rocks fit most conveniently into the zonal pattern (see below) but one or two
characteristics related to the parent rock may be mentioned. Granite, gneiss and
mica-schist provide a high proportion o f coarse inert material, especially quartz,
which helps to produce a loose gritty texture but contributes little in the way of
mineral plant food. In this latter respect the superiority o f the calcareous schists
has already been noted. Under Mediterranean conditions rocks o f the granite
family weather to much greater depths than in northern Europe so that, given a
suitable terrain, a thick layer o f detritus may accumulate. In hollows or on lowlying
ground this may become waterlogged but elsewhere it may serve as a
valuable subsoil reservoir which can be tapped for irrigation. On the whole the
granite soils are not the worst in the Mediterranean area.
T h e attention so far given to the parent rock should not be allowed to obscure
the paramount importance o f climate in soil formation. The relationship between
precipitation and evaporation, as revealed in soil profiles, is particularly signifi-
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