Geoscience Reference
In-Depth Information
Plate 25.1
Red Mediterranean soil or terra rossa on limestone in northern Cyrenaica, Libya. The pedogenic process of
rubefaction produces haematite iron oxide, whose intense red colours mask other properties of the soil.
Photo: Ken Atkinson
(FAO: Chromic Luvisol;
terra rossa
) is favoured in drier
situations (lower rainfall, hotter and longer summer
drought, low elevation, permeable parent material, greater
age). Alluvial soils is found in the alluvial plains and deltas
around the Mediterranean. These
vega
soils have built up
by the fluvial accretion of a mixture of clays, silts and
sands. The profiles are typically black/dark brown in
colour, with high organic matter content. Like the other
two soils, they display good water-holding properties,
owing to their heavy texture. Not surprisingly, they are
mostly irrigated for high-value crops such as rice,
vegetables and citrus orchards.
The deep profiles of the alluvial soils are clearly of
depositional origin, in contrast to the 'sedentary' nature
of Brown and Red Mediterranean soils. However, even
with the latter two types it is recognized that airborne salts,
lime and dust can make an important additions to the
profiles as aerosols. Wherever leaching and accumula-
tion processes in the soil profile are finely balanced, the
degree to which soils are non-calcareous or calcareous, or
non-saline or saline, can be due to the amount of atmos-
pheric loading. Aerosol deposition of salts can be sufficient
to give saline soils, especially in coastal locations. Aerosol
input of calcium and magnesium carbonates can be
sufficient to make the profiles calcareous, especially near
calcareous source rocks. Normally Mediterranean soils
are non-calcareous, and in fact the absence of lime is a
necessary condition for both argillation and iron oxide
production. Calcareous soils must therefore indicate a
secondary impregnation, either aerially, following a
change in climate or surface cover, or by the burrowing
and mixing of soil fauna.
Soil fertility
The fertility status of Mediterranean soils shows both
positive and negative features. Apart from the saline cases
noted above, salt levels are low, and exchangeable sodium
forms a minor proportion of cations on the exchange
complex. pH values are close to neutrality and at the
