The soil profile is one of the most important concepts in soil
science. It is a key to understanding the processes that have taken
in soil development and is the means of determining the types of
soil that occur and is the basis for their classification. The
soil profile is defined as a vertical section of the soil from
the ground surface downwards to where the soil meets the underlying
rock. The soil profile can be as little as 10 cm thick in immature
soils and as deep as several metres in tropical areas where the
climate is conducive to rapid alteration of the underlying rock
to form soil. In temperate areas, the soil profile is often around
a metre deep and in arid areas somewhat shallower than this.
Virtually all soil profiles are composed of a number of distinctive
layers, termed horizons, interpretation of which is the key to
understanding how the soil has formed. Most soils will have three
or more horizons. Soils that have not been cultivated will normally
have L, F and H layers at the surface. These layers largely represent
different degrees of decomposition of organic matter, the L layer
representing the litter layer formed of recognisable plant
and soil animal remains, the F layer below, the fermentation
layer, usually consisting of a mixture of organic matter in
different stages of decomposition, and the H layer, the humose
layer, consisting largely of humified material with little
or no plant structure visible. Below these, and in cultivated soils
occupying the surface layer, is the A horizon composed of a more
or less intimate mixture of mineral and organic matter. The A horizon
is often referred to as the ‘ploughed layer’ in cultivated
soils. It is an important part of the soil because it is a source
of plant nutrients and contains the majority of plant roots. The
A horizon may lie directly on the B horizon or, in well developed
soils, there may be an intermediate leached horizon, termed E or
A2, depending on the nomenclature system used. The E/A2 horizon
is usually paler in colour than the horizons above and below because
it is a horizon that has been subject to leaching and loss of components
compared to the A and B horizons.
The B horizon is the horizon most widely used to identify soil
types. Its morphology is important in supporting the classification
of soils. In some soils the B horizon results purely from the weathering
of the underlying rock whereas in other soils this weathering is
supplemented by the translocation of materials from overlying horizons.
Thus the B horizon needs to be inspected carefully in order to
understand the genesis of the soils. B horizons may have a number
of different subscripts indicative of the nature of the materials
that have moved into the horizon, e.g. Bh indicates the translocation
of humus into the horizon, Bs, the translocation of sesquioxides.
These subscripts will vary according to the nature of the soil
component that has accumulated but also with the system of nomenclature
of soil types (See Section on Soil Types).
Below the B horizon is the C horizon. This latter horizon is often
consistent with the parent material and may have been little altered
from the material in which the soil originally formed.
Most soils have A, B and C horizons. Some, generally weakly developed,
soils may have A horizons lying directly on C horizons. When next
you see a profile down through the soil, perhaps in an excavated
pit or in a roadside cutting, take time to look at the profile
and see if you can identify some of the different soil layers that
make up the profile. (For some examples of soil profiles and soil
types, see section of Soil Types).