Environmental Engineering Reference
In-Depth Information
contrast the Red Mediterranean soil (FAO Chromic Luvisol;
terra rossa
) is favoured in
drier situations (lower rainfall, hotter and longer summer drought, low elevation,
permeable parent material, greater age). The alluvial soil is characteristic of many of 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 addition to the profiles. Where leaching and accumulation processes 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 atmospheric loading. Aerosol deposition of salts can
be sufficient to yield saline soils, especially in coastal locations. Aerosol input of calcium
and magnesium carbonates ('lime') can be sufficient to make the profiles calcareous,
especially near a calcareous source rock (limestone, marl). 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.
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 optimum range for many plant nutrients. The soils have high
contents of calcium and magnesium, and a cation exchange complex that is base-
saturated. However, the low organic matter content (typically between 1 per cent and 2
per cent at the soil surface) is a result of the high rates of mineralization and
decomposition during the summer months. Unfortunately this leads to deficiencies of
nitrogen and phosphorus, both major plant nutrients. Native vegetation has to adapt to
these deficiencies, whilst agricultural management depends upon the application of
suitable chemical fertilizers or the use of legumes in the crop rotation. From the point of
view of physical properties, the soils show high water-holding capacities, owing to their
clayey textures, if rain is able to infiltrate. Summer desiccation of the soil surface usually
forms a cracking pattern, which can be especially deep in the case of alluvial soils. Such
soils have high content of expanding lattice, montmorillonitic clay minerals. However,
rehydration after the first autumn rain causes the clays to swell, thus closing the cracks
and shutting off infiltration.
DEVELOPMENT AND ADAPTATION OF VEGETATION
The vegetation of the Mediterranean region has experienced many changes during the
past 15,000 years, i.e. roughly during the time since ice sheets in Europe were at their
Würm (Devensian) maximum. Vegetation is rarely static, and plant communities change
rapidly in response to factors which control their structure and productivity (see Figure
