Geoscience Reference
In-Depth Information
18
CHAPTER EIGHTEEN
Soils in their
environment
Soils are derived from the rocks and minerals which make
up the surface of Earth. They may be developed on parent
materials which have not been involved in any erosion
cycle; thus hard or soft bedrocks weather
in situ
to give
residual soils
. Such country rocks are residual parent
materials and will consist of igneous, sedimentary or
metamorphic rocks. Alternatively the soil-forming parent
material may have already passed through one or more
cycles of erosion and soil formation; these are
transported
soils
on parent materials and consist of sediments that
have been moved by ice (moraines, till, fluvioglacial
deposits), wind (aeolian sands, loess), water (alluvial,
marine, lacustrine) and gravity (colluvium). In Britain
these deposits form the majority of parent materials,
many of which date from Pleistocene times.
zone of soil formation
.
Figure 18.1
illustrates two soil
profiles, one on a hard country rock, e.g. granite, and one
on a glacial deposit. In the latter case the C parent material
has been much altered from the glacial deposit which was
originally laid down by the ice. The zones of soil formation
and weathering are not always close and juxtaposed. In
tropical regions the weathered material can be as deep as
60 m. In that case soil-forming processes will be going on
in the soil profile at the surface, whilst rock breakdown
and weathering will be operating at the junction of the
weathered residue and fresh rock many metres below
the solum.
Soil development is a complex of many processes acting
over many years.
Figure 18.2
shows the connections
between the main processes. Soil development is viewed
as two processes, namely
weathering
and
morphogenesis
.
Atmosphere and hydrosphere provide gases (oxygen,
carbon dioxide, nitrogen) and water which support plants
and organisms (soil fauna and soil organisms) which
provide the soil with its organic matter and organisms.
Parent material weathers under the influence of the
atmosphere and hydrosphere (carbon dioxide, oxygen,
water) to produce four components in soil: a relatively
resistant residue
consisting of quartz, feldspars and heavy
minerals (e.g. zircon, iron minerals); secondary minerals
or
alteration compounds
synthesized by weathering
processes and consisting of clay minerals and hydrous
oxides of iron and aluminium; a component of organic
matter derived from plant and animal residues; and finally
a
weathering solution
containing cations, anions and silica.
Of these four components, only the resistant residue
is relatively stable and changes only slowly. The organic
SOIL FORMATION
When considering soil formation it is important to
distinguish two related but fundamentally different
processes which are occurring simultaneously. The first is
the
formation of soil parent materials
by the weathering of
rocks, rock fragments and sediments. This set of processes
is carried out in the
zone of rock decomposition
or
zone of
weathering
. The end point is to produce parent material
for the soil to develop in. This material is referred to as C
horizon material. This applies essentially in the same way
for glacial deposits as for rocks. The second set of processes
is the
formation of the soil profile
or
solum
by
soil-forming
processes
which change the C horizon material into A, E
and B horizons. This is carried out near the surface in the
