|Figure 17.6 Soil types: (a) Podsol; (b) Gley;
(c) Brown earth;
In the British Isles four main types of mineral soil are found; brown
earths, gleys, rendzinas
(see Figure 17.6). Peats
in waterlogged conditions (see organic soils).
Brown earth soils
develop in the well-drained medium to heavy soils
in the lowlands of the British Isles. They are associated with a climax
vegetation of broad-leaved woodland especially oak, ash and
sycamore, the roots of which have ensured that nutrients moving down
the soil profile are captured and returned to the soil via the leaf fall.
Surplus water does not accumulate and the soil remains aerobic for most
of the year. The plentiful earthworms incorporate the deep litter layers.
The resultant dark A horizon (‘topsoil’) rich in organic matter merges
gradually into a bright brown and deep B horizon (‘subsoil’). The soil
structures that develop in the surface layers are granular and rounded
fine blocky in which there is an excellent balance of air and water and
into which roots can readily penetrate.
Brown earths are usually mildly acid (pH 5.5 to 6.5), but acid brown
earths (pH 5.5 to 4.5) can develop on lighter textured soils in wetter areas (800–1000 mm rain per year) especially under beech
or birch woodland. There is less earthworm activity,
with a resultant reduced incorporation of organic
matter down the profile. The soil structure is usually
less satisfactory and clay particles that work their
way down can form clay pans in the B
horizon. These can be productive soils if ameliorated
with lime and fertilizer (see Plant nutrition
|Figure 17.7 Soil colours.
occur in poorly drained soils. Surface water gleys
are found where the percolation
of water is restricted by the poor structure in the
A or B horizon to produce a perched water table
. This is typically where the subsoil is
heavy and impervious, especially in wetter regions.
Oxygen in the waterlogged soil is depleted and, in
these anaerobic conditions below the water table,
the iron oxides that colour the soil become dull grey
or bluey (in aerobic conditions the iron oxides are
rust coloured). The extent of the waterlogging that
the soil has been subjected to as the water table
fluctuates can be judged from the degree to which it
has become completely grey; usually there is a rusty
mottle present, indicating that aerobic conditions
exist in the soil for part of the year (see Figure 17.7).
Plants growing in them are often shallow rooted and
suffer from drought in dry periods. These soils are only productive after
they have been drained, limed and fertilized.
Ground water gleys
develop where there is a permanent water table
that is very near the surface of the soil, so that to lower the water table
drainage has to be undertaken on a regional basis, e.g. Romney Marsh.
Drainage pipes can only be used when the water can be run to a ditch
with a water level below that desired in the field; for some
areas this can only be achieved by maintaining an artificially low level
by the use of pumps (powered in former times by windmills).
(from the Russian ‘under-ash’) are strongly leached, very acid
soils that develop on freely draining soils, such as coarse sands and
gravels, commonly under heather or pine or spruce forest in high rainfall
areas. Because of the high acidity levels, earthworms are absent so there
is a build-up of the litter layer. Poorly decomposed organic matter that is
not incorporated (a ‘mor’ humus) is characteristic of this soil type. Some
of the organic matter combines with the iron in the top layers to form
soluble compounds which are leached (‘podsolization’) to leave a grey
(‘ash-like’) A horizon (all that remains are bleached sand grains). These
compounds become insoluble again in the conditions that prevail in the
B horizon, where organic matter accumulates to create a dark or black
which is an iron rich red layer. The iron compounds that
accumulate can form a strongly cemented ‘iron pan’. As a continuous
pan that water (and roots) cannot penetrate is formed, a waterlogged area develops and peat can form at the surface. Podsols are prone to
drought and are ‘hungry’ soils that require considerable ongoing inputs
of lime and manure to make them productive. They are of little use in
horticulture except for the growing of acid loving trees and shrubs.
are very thin dark-brown, sometimes black, soils with a strong
granular structure sitting directly on chalk or limestone. They are typical
of the soils on the steeper slopes of chalk or limestone hills under grass.
Shallow soils develop because of the continued erosion on the slopes,
which also keeps these soils heavily charged with lime. Where the soils
become deeper on the less severe slopes it is common for the A horizons
to become acid, as the lime is leached downwards. They are well drained
because of the slope and because of the porous nature of the underlying
rock. Rendzinas are not suitable for most horticultural purposes because the
high lime content causes induced nutrient deficiencies. Roots
are severely restricted by the shallow soils and vulnerable to drought.