Environmental Engineering Reference
In-Depth Information
permeable materials water penetrates to the subsoil, leaving higher ground well drained.
On lower land soils are affected by ground water, as shown in Figure 19.4(b).
The length of time a soil remains undisturbed by erosion or deposition is important in
its evolution. The glaciations of the Pleistocene era removed the old soils from much of
Britain and other countries of mid to high latitudes and deposited thick drift in other
regions. Soil formation began again on new surfaces after the final retreat of the ice some
10,000 years ago. Old soils are more common in low latitudes where the soil cover was
not eroded or buried during the Pleistocene.
Human activities have many effects on soils. Indirectly, the native vegetation can be
modified or removed. Directly, soils are changed by agricultural practices. For example,
pollen analysis (
palynology
) of upland Britain shows that the clearance of the upland
deciduous forest by Neolithic people led ultimately to the development of heathland.
Deforestation broke the nutrient cycles of the brown earths under deciduous trees and led
to the acidification of soils and the invasion of heather. This caused acid humus and thin
peat on wetter sites, both leading to podzolization. Soils used for arable agriculture have
their relation to soil-forming factors changed by ploughing, draining and the use of lime
and fertilizers.
The five factors of soil formation do not operate as single independent factors, of
course. Climate influences vegetation and human activities and is itself affected by
altitude and topography. Figure 19.5 shows the relationship of climate, altitude, slope and
soils for northern England. Precipitation increases with altitude, giving intense leaching
and podzolization on well drained sites, chiefly steep slopes. Waterlogging occurs on
high ground owing to higher rainfall, lower evaporation and lower transpiration. Organic
matter accumulates as peat on summits, or as peaty surface horizons (stagno-podzols and
stagno-humic gleys) for the same reasons. On lower ground, where rainfall is lower and
temperatures are higher, leaching is weaker, weathering is stronger and brown earths tend
to form. Podzols are restricted to coarse-textured base-deficient parent materials.
The combined influence of the five factors of soil formation is to produce a set of
soil
-
forming processes
which produce the world's distinctive soil profiles and their
constituent horizons. The processes are as follows and they will be discussed in the
remainder of this chapter, except for
permafrost soil processes
(
cryopedology
), which
are covered in Chapter 24, and
rubefaction
, which is analysed in Chapter 26.
1 Permafrost soil processes (cryopedology)
2 Leaching
3 Clay translocation
4 Podzolization
5 Decalcification
6 Calcification
7 Gleying
8 Rubefaction
9 Salinization
10 Alkalization
11 Solodization
12 Laterization
