Physical Resources / 109Figure 3.9. Where drainage is excessive, the soil will be generally dry, favouring trees that can root
to considerable depth. As the distance narrows between the surface and water table, the region
hospitable to plant roots becomes shallower, and the plants smaller. Most of the organisms engaged
in the decomposition of plant material require oxygen for respiration, so the decreasing depth of the
aerated zone is accompanied by a slowing of the rate of decomposition until, where the soil is
waterlogged, partly decomposed material may form acid peat.
The stage of this process that involves purely physical and chemical mechanisms constitutes
weathering; as living organisms become the predominant agents it is called ‘pedogenesis’. Plants
growing at the surface penetrate the soil with their roots and supply a topmost layer of dead organic
material, called ‘litter’. This provides sustenance for a diverse population of animals, complete with
their predators and parasites, fungi, and bacteria. These break down the material, which enters the
soil proper, much of it carried below by earthworms, where it feeds another population. Compounds
released by decomposition dissolve in water draining through the soil and are carried to a lower
level, where they accumulate. At the base of this layer, the ‘subsoil’, rocks and mineral particles,
detached from the underlying rock, are being weathered, and below this layer lies the bedrock itself.
If a vertical section, called a ‘profile’, is cut
through the soil from surface to bedrock, it may
reveal this structure as layers, called ‘horizons’,
clearly differentiated by their colour and texture.
Figure 3.10 shows the principal horizons, but
in any particular soil there may be more or fewer
and in some soils horizons are not easily
distinguished at all. Conventionally, the
horizons are identified by letters: O for the
surface layer of organic matter; A for the surface
horizons; B for the accumulation layer; C for
the weathering layer; and R for the bedrock.
The horizons are classified further by the
addition of numbers: A2 is a mineral horizon
somewhat darkened by the presence of organic
matter; A3 is a transition zone between the A
and B horizons. Letters are then added in subscript to denote particular characteristics: ca means the soil
contains calcium and magnesium carbonates; g (for ‘gleying’) means the soil is poorly aerated and frequently
waterlogged; m means the soil is strongly cemented together, like a soft rock.
Soils vary according to the rock from which they are derived. This affects the size of their mineral
grains, ranging from coarse sand (600–2000 μm) to silt (2–60 μm) and clay (less than 2 μm), and
their chemical characteristics. Soils derived from granites, for example, develop slowly, are usually
sandy, and contain relatively few plant nutrients; those developed from limestones are usually fine-
grained and relatively rich in plant nutrients.
Once formed, soils begin to age. The rate at which they do so depends principally on the climate and
vegetation. Desert soils age slowly, and so do those in polar regions, but in the humid tropics soils age
much more quickly as luxuriant plant growth extracts nutrients and returns them for decomposition
into soluble forms, which are leached rapidly by the abundant water. It is possible, therefore, to describe
as ‘young’, ‘mature’, or ‘ancient’ soils that may have been in existence for the same length of time.
We obtain our food from soil, we erect buildings of varying weight upon it, and we use clay taken
from it as construction material that may or may not be fired to make bricks. Clearly it is of great
Figure 3.10 Profile of a typical fertile soil