Geoscience Reference
In-Depth Information
which appellation it occurs in older Soil Survey reports
Several different development pathways are likely lead
to this soil profile. The Scottish pedologist FitzPatrick
suggests an alternative evolutionary sequence after observ-
ing that iron pans frequently occur above dense subsoils,
with the iron pan picking out the physical interface in the
subsoil. The interface has been shown to mark the upper
limit of soil permafrost, i.e. the permafrost table, in
Pleistocene times. FitzPatrick called these compact
subsoils of high density and low porosity 'indurated
horizons', but internationally they are termed
fragipans
(
Plate 18.10
).
Another development pathway is depend-
ent on vegetation change, with the placic podzol being a
polycyclic
soil, reflecting more than one cycle of soil
formation. Professor Dimbleby studied evidence of palyn-
ology, archaeology and radiocarbon dating in prehistoric
earthworks on the North York Moors to discover that acid
brown soils under forest preceded the placic podzols.
Thick Bs horizons below the pan suggest that the Eag and
Bf horizons of the present soils result from the formation
of surface peat between 2000 and 1000 years
BP
, following
the replacement of a deciduous forest cover by moorland
and heathland plants. Whether this vegetation change
resulted from a climatic deterioration or human influence
is not always clear. Similar soil histories have been studied
in Scotland and North Wales.
Pedogenic processes of clay formation
and clay translocation
The formation of clay-sized particles is a fundamental
feature of soil formation. The colloids consist of clay
minerals and hydrated oxides of iron and aluminium. In
many soils clay content increases from the A horizon
down to the B horizon and then decreases in the C
horizon. The B horizon may acquire its higher clay content
in two ways. First, percolating waters carry chemical
elements in solution which are precipitated in the B
horizon to form new clay minerals. Second, percolating
waters carry clay minerals from the A horizon in
suspension, which are deposited in the B.
The process of clay mineral formation
in situ
in the B
horizon gives cambic B or weathered B horizons desig-
nated as Bw. As the name suggests, the process is charac-
teristic of cambisols or brown earths. The type of clay
mineral formed in this way depends upon the parent
material and the degree of leaching in the soil profile.
In freely drained soils with intense leaching, silica tends
to be removed, thus tending to produce low silica 1 : 1
clay minerals such as kaolinite. Soil age may also be a
Plate 18.11
Humus-Ironpan Stagnopodzol (FAO: Placic
Podzol) developed in acidic glacial till in Glen Fiddich, Scotland.
Peaty surface (O) overlies grey, podzolized and gley layer
(Bf) which follows a wavy course through the soil. Traditional
name is peaty gley podzol.
Photo: Ken Atkinson
factor here, as kaolinite is the most resistant clay mineral
and tends to accumulate over prolonged periods of
weathering, as in tropical soils or Ferralsols. In poorly
drained soils with a moderate content of calcium and
magnesium, there is usually enough silica and bases to
form montmorillonite and vermiculite, as for example in
vertisols and chernozems. Under moderate leaching and
a moderate supply of potassium, illite and chlorite are
usually the dominant clay minerals. These are the
commonest clay minerals in British soils. Illite has a mica-
type structure, and is also formed directly from mica
minerals in parent materials. In acid soils such as podzols,
the increase in clay-size material in the B horizon is
due to the formation of hydrated oxides of iron and
