Agriculture Reference
In-Depth Information
everything from boulders to finely ground material.
The soils that develop in this 'till', also known as
' boulder-clay ', are very variable and much is not easy
to cultivate.
Soil formation
Initially there is only a shallow layer of rock fragments
that remains subject to erosion until something starts
to grow there. Usually it is the lower plants such as
mosses, algae and lichens (see p. 52) that are able
to colonize such inhospitable areas and in doing so
they modify the effect of water and wind erosion. The
vegetation that develops tends to protect the loose
material by covering it and by reducing wind speeds.
The reduced wind speed also leads to particles
being dropped from the air. Along with this additional
material, the soil deepens as the parent rock continues
to be broken up below. Furthermore, organic matter
is added as plants die, which decomposes to release
plant nutrients. A hole dug in such a 'sedentary' soil
reveals the characteristic horizons that gradually
emerge: typically an organic-rich litter layer at
the surface with different horizons down to the
unweathered parent rock at the bottom (Figure 12.6).
Initially this may be only a few centimetres deep but
over time, as the original plants are succeeded by
higher plants with roots, eventually shrubs and trees,
the soil profile can become more than a metre deep.
Their roots bind the soil and generally reduce further
erosion. However, the soils that form on the steeper
slopes tend to remain very shallow.
12
Figure 12.5 Soil creep on hillsides. On the gentler
slopes, gravity causes the loose rock debris and soil
particles disturbed by rainsplash to move gradually
downhill. This process slows as the particles are
covered with grasses but continues creating the
characteristic striations on the hillside
(Figure 12.5) leading to deep layers of debris at the
base of slopes.
Water
As the water with its load of rock fragments slows
down, it is unable to carry as much. Initially, it is the
larger particles that drop to the bottom first. Typically,
it is in the streams that boulders, pebbles and coarse
sands can be seen, whereas the fine sands are not
dropped until the water is in the slower-moving rivers.
Rivers in flood spread out over a valley bottom or
across a plain beyond their banks so when the water
level goes down, layers of fine sand and silt are left
behind along with much organic material. This gives
rise to the fertile ' alluvial ' soil (see Figure 12.2). Much
of the silt is still in the very slow-moving river until
approaching the discharge point and is dropped around
estuaries. The clay fraction is made up of such small
particles that it stays suspended in the water to be
carried out into a lake or out to sea where it gradually
settles in still waters.
A soil horizon is a specific layer in the soil seen
by digging a soil pit.
'Transported' soils form in essentially the same
way, but in material that comes from another area.
This means they are not made up of the rock found
underneath the site but rather from particles that have
been eroded, that is, transported, often from many
miles away (Figure 12.6). Alluvial soils form in the
material brought by rivers and loess in the wind-blown
deposits. Average agricultural soils develop in much of
the variable ' boulder clay ' left behind by the glaciers;
very little of which makes good horticultural soil unless
substantially improved or if used to grow crops that
need very little cultivation, such as orchards.
Organic matter is added at the surface - for example,
leaves, dead annuals and perennial 'tops' giving rise to
the 'organic layer' (also known as the 'litter layer'). This
is food for many small animals such as earthworms
that start the decomposition process and incorporate
Wind
The wind also carries particles that are small enough,
typically sands in hot, dry conditions. Again the larger
particles are dropped first as the wind slows down or
when the particles touch water. This leads to large
areas of sandy material in which form the ' loess ' soils.
Glaciers
Large areas of Britain and Ireland were covered by
glaciers in, geologically, recent times. When the
glaciers melt, they drop their load, which comprises
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