JRC Soils Atlas
24/6/05
1:41 pm
Page 17
Introduction
Soil forming processes
Soil processes on well drained sediment in relation to
time
an Alisol. At this stage the soil is so acid that the clay in the
illuviated horizon will disintegrate or be redistributed to
other parts of the profile and tongues of silt and sand will
cut into the clay illuviated horizon. This is referred to as an
Albiluvisol. Finally, the leaching will enable an iron pan to
develop and the soil turns into a Podzol.
The processes described in this section form the most
common types of soil found in Europe.
In some sediments only a few of the processes described in
the previous pages will be active while in other soil types or
locations, several processes could be running simultaneously
or following each other. The figure below illustrates how
different soil processes can produce a succession of soil
types in a loamy calcareous sediment.
Soil in relation to man
The natural soil reflects the history of the soil forming
factors. Therefore, in a natural state each soil type has some
distinctive characteristics. For example, a Podzol is an acid
soils with a mor layer. This will normally be true in forests
but ploughing and liming on arable land will remove the
mor layer and turn it in to a mull layer about 30cm thick
and with a pH of around 6. A wet soil at the base of a slope
with gleyic characteristics can be drained. Rivers can be
straightened thus lowering the water table in the adjacent
soil. Gley features in such areas express a former state and
as such behave as a natural well drained soil.
Initially, we have the newly deposited sediment. Only a
shallow humus layer has developed and free calcium
carbonate is still to be found close to the surface. This soil
is known as a Regosol. After some time, the calcium
carbonate will be leached from the top part of the soil, a
deeper mull layer has developed and the weathering of the
primary silicates and the formation of iron oxides has
created a strong coloured B-horizon just below the mull
layer. The soil is now called a Cambisol. The continuous
leaching moves the calcium carbonate front further
downwards, the pH drops to about 6 and clay illuviation
starts. We have now a nutrient rich Luvisol but the leaching
will continuously remove the base elements from the soil.
This will make the profile so acid that it will be classified as
Histosols will often disappear when drained because the
peat layer will physically shrink due to water loss followed
by the decomposition of the peat due to oxidation.
Other soil types with poor agricultural value due to low
nutrient levels can be improved through fertilizing and
manuring to become a high yielding soil. In Denmark, liming
of the strongly leached, loamy Saale till has raised the pH by
about 1.5 units in the plough layer but at a depth of 1.2 m
the effect of the liming has gone. On the calcareous, loamy
Weichsel till, the leaching of the topsoil has lowered the pH
to about 5.0 but at a depth of 1.2 m, the pH is about 8. On
arable land the liming has increased the pH to about 7 and
at the depth of about 1 m while the pH on arable land and
forest are the same (about 8).
This changing of the chemical characteristics of the soil can
stop, reverse or reactivate some of the pedological
processes. Podzolization stops immediately when the mor
layer has been removed and the pH raised to about 6 while
clay migration, which mainly happens in slightly acid soil,
can revitalize when lime is applied.
In the drier parts of Europe inadequate irrigation of the
fields can lead to salinisation while agriculture in
mountainous areas can lead to severe soil erosion. This
latter aspect has been demonstrated in historical time in the
Mediterranean area. Mature soil with well developed
horizons can been washed away completely and replaced by
bare rock.
Mans activity has also created new soil types such as urban
soil, soil in huge waste deposits and, in former times, the
famous plaggen soil (see section on Soil and Archaeology
later in the atlas).
Due to the creation of gardens and parks, soil in urban areas can
be modified significantly compared to its original natural state
(EM).
Imperfectly and poorly drained soils in relation to topography.
The series of photographs below illustrates a typical topographically related sequence of soils known as a catena (a term derived from the Latin meaning a chain); soil profiles are linked together
when traced down a slope from the ridgeline to the valley floor. Soils change character according to slope angle and drainage conditions.
On the crest of the landscape we find thin soils with underlying parent materials very close to the surface. These are the Regosols or Leptosols. On the slope towards the valley the precipitation
surplus can leach nutrients laterally or vertically and illuviated profiles can be found. In the valley bottom we can find groundwater gley at shallow depths. In the centre of the valley, peat
formation has taken place and thick layers of fen peat have developed. The nature of the parent material can be important
A shallow soil. (HBM)
Leaching. (HBM)
Waterlogging leading to
gleyic conditions. (HBM)
A fen peat. (EM)
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