Environmental Engineering Reference
In-Depth Information
weatherable chemicals (magnesium, manganese) reaching the lower slope, while less
mobile elements (iron, titanium) are retained at midslope. If rainfall is very high, and
parent materials acid, many of the bases are lost from the catena entirely, and the soils
may have similar low pHs on all parts of the slope.
CONCLUSION
Soils are made from rocks and sediments by a two-stage sequence of development.
Weathering produces the soil parent material, which is then acted upon to produce the
soil profile. The particular profile formed at any point on Earth depends upon the
prevailing climate, vegetation, rock type and topography, together with the length of time
of soil formation. Generally soils in mid and low latitudes are deeper, more weathered,
redder and less fertile than soils of higher latitudes.
The fundamental unit for studying soils is the soil profile, the vertical section from the
soil surface down to underlying rock. The profile is formed by vertical movements of
water and materials, both downwards and upwards. These processes are leaching,
decalcification, clay translocation, podzolization and laterization in a predominantly
downward direction. Processes of salinization, alkalization and calcification involve
precipitation of chemicals
in situ
, or by upward movement. The processes of rubefaction
and gleying give distinctive colours to soil.
The main departure from the central concept of the soil profile comes with the concept
of the soil catena. In temperate climates soil changes along slopes are largely conditioned
by hydrology. In subtropical and tropical climates soil changes along a catena are also
greatly influenced by the movement of mineral particles by rainwash erosion. In all
natural regions it is necessary to be aware of geomorphology and landscape history, as
polycyclic soils are a common feature of polycyclic landscapes.
FURTHER READING
Avery, B. W. (1990)
Soils of the British Isles
, Wallingford: CAB International. The definitive
account of the distribution, formation and properties of British soils. A good source of examples
and illustrations.
Ellis, S., and Mellor, A. (1995)
Soils and environment
, London: Routledge. Up-to-date coverage of
soil formation, soil profiles and soil properties.
Soil Survey of England and Wales (1983)
Soil Map of England and Wales
, Harpenden: Soil Survey
of England and Wales. Six regional bulletins of England and Wales, together with
accompanying maps at the scale of 1:250 000. Many photographs and block diagrams to explain
soils in the landscape.
KEY POINTS
1
Soils are formed by a series of soil-forming processes occurring within the top one or
two metres of the soil surface. Many processes will be going on simultaneously, but
the soil formed will mostly reflect the dominant one. High rainfall plus a permeable
parent material favour leaching Acid vegetation and parent material promote
