Environmental Engineering Reference
In-Depth Information
Chapter 19
Soil formation
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.
SOIL DEVELOPMENT, SOIL PROFILES AND SOIL HORIZONS
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 zone of soil formation . Figure
19.1 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 19.2
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.
 
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