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Figure 1
- Soil in its natural condition is able, thanks to its porosity, permeability and humidity, to retain
a large quantity of water from rainfall, contributing to regulate the surface runoff. In an anthropised
environment, on the other hand, the presence of impervious surfaces, the reduction of vegetation, the
removal of the surface layer, which is rich in organic matter, and the onset of compacting result in a
serious deterioration of the soil’s function. The decrease in evapotranspiration, and in the soil’s capacity
to absorb water, result in an increase in surface runoff and in the transport of large quantities of sediment
in natural collectors. The values shown in the figure are only indicative and can change significantly,
depending on a variety of parameters (the physical and chemical characteristics of the soil, the
topography and geology, as well as the duration and intensity of rainfall etc.)
Features of Urban soils
Whether in a natural or disturbed condition, soils vary widely in their properties. Disturbed soils
especially differ from soils in natural areas because their horizons have been mixed, destroyed,
removed or compacted mainly the superficial horizons. Natural soils may be buried under fill;
chemical and waste materials may have been added. All these activities can result in large changes
in the physical, chemical, biological and engineering properties of the urban soils that can range
from natural to completely man-made conditions.
WRB-World Reference Base (IUSS, 2014) classifies strongly anthropized soils in two groups:
−
Anthrosols
: soils that have been formed or profoundly modified through long-term human
activities, such as peri-urban land, garden or park soil.
−
Technosols
: soils containing many artefacts (material deriving from human activity). These
soils are dominated by a profound human influence such us in roads, industrial, mine or
military sites.