develop in the material gradually weathered from the
underlying rock. True sedentary soils are uncommon because most loose
rock is eroded, but the same process can be seen where great depths of
transported material have formed the parent material, as in the boulder
clays left behind after the Ice Ages. A hole dug in such a soil shows the
gradual transition from unweathered rock to organicmatter rich topsoil
(Figure 17.5). Under cultivation a distinctive topsoil develops in the
|Figure 17.5 The development from a young soil consisting of a few
fragments of rock
particles to a deep sedentary soil is shown alongside
transported soil. A subsoil, topsoil
and leaf litter layer can be
each soil. Simple plants such as lichens and mosses
establish on rocks or
fragments to be succeeded by higher plants as
soil depth and organic
matter levels increase.
Once rock fragments and soil particles are created they become
subject to erosion
. Transported soils
are those that form in eroded
material that has been carried from sites of weathering, sometimes many
hundreds of miles away from where deposition has occurred. They can
be recognized by the definite boundary between the eroded material
and the underlying rock and its associated rock fragments. Where more
than one soil material has been transported to the site, as in many river
valleys, several distinct layers can be seen. The right-hand part of
Figure 17.5 shows an example. How they are moved depends on where
the loose material lies:
- Gravity affects anything on a slope. On steep sides, e.g. cliffs,
particles fall and accumulate at the bottom to form heaps of rock
- On gentler slopes particles are helped downhill by rainsplash.
Raindrops striking soil dislodge loose particles that tend to move
downhill. As a result, surface soil is slowly removed from higher
ground and accumulates at the bottom of slopes. This means that soils
on slopes tend to be shallow, whereas at the bottom deep, transported
soils develop, known as colluvial soils.
- Glaciers carry vast quantities of rock downhill and deposit their
load at the ‘snout’ (terminal moraines). Of more significance is the
enormous load that was left behind when the glaciers retreated after
the last Ice Age (10 000 years ago). This is known as ‘till’ or ‘boulder
clay’ (it comprises boulders down to clay size particles).
- Material washed away in running water eventually settles out
according to particle size. The river valley bottoms become covered
with material (alluvium) in which alluvial soils develop.
- Wind removes dry sands and silts that are not ‘bound in’ to the soil.
The soils that develop from wind-blown deposits are known as ‘loess’
Many of these transported soils provide ideal rooting conditions for
horticultural crops because they tend to be deep, loose and open. Most
are easily cultivated. However, those that have a high silt or fine sand
content, notably the brick-earths, may be prone to compaction.
The nature of a new soil (regosols) is largely determined by the rock
minerals from which it is formed, but it continues to undergo changes
under the influence of climate, vegetation, topography
These interact over time
to give rise to characteristic soil profiles in
different parts of the world. The soils that develop can be described in
terms of the characteristics of the different horizons (layers) that make
up the soil profile.
The ‘O’ or ‘L’ horizon is the organic matter found on top of the mineral
soil and commonly referred to as the litter layer. The upper layer of the
soil, from which components are normally washed downwards, is the ‘A’
horizon. This is usually recognized by its darker colouring, which is a
result of the significant levels of humus present. The lighter layer below
it, where finer materials tend to accumulate, is the ‘B’ or illuvial horizon.
Under cultivation, the ‘A’ horizon broadly aligns with the ‘topsoil’ and
the ‘B’ with the ‘subsoil’. The parent material below these is the ‘C’
horizon and where there is an underlying unweathered rock layer it is
often known as bedrock.