Many natural bodies, such as plants and animals, are discrete
entities which can be classified and guidelines for their identification
followed. Soils are much more difficult to identify and classify
than these discrete bodies for two main reasons: (i) soil is more
or less a continuum covering the land surface of the earth, not
a set of discrete entities; and (ii) most of the soil is below
ground and therefore not readily visible. Soils grade into one
another across the landscape usually without sharp boundaries between
one type of soil and another. Soil surveyors who make maps of soils
have to use their skills in reading changes in the landscape coupled
with auger borings in the soil to identify the nature of the soil.
There are several ways of classifying a soil, from the simple
to the complex. A soil type may be as simple as ‘a sandy
soil’ or ‘a clayey soil’ and this is often the
perception of many land users, such as farmers or civil engineers,
who see it as material they have to deal with to achieve an end
result, such as the growing of a crop of wheat, or the building
a road. Simple classifications tend to be of local and restricted
relevance only. At the other end of the spectrum is the soil scientist
who needs to understand how soils have formed, which types occur
where, and for what the different types of soil can be used. The
soil scientist seeks a much broader understanding, with the aim
of underpinning the use and preservation of this important natural
resource, and this has manifested itself in a number of detailed
soil classification systems worldwide.
There is an enormous diversity of soils across the world. This
is hardly surprising given the fact that soil formation and soil
type are influenced by several key factors: the parent material,
usually rock or sediment, but occasionally organic materials such
as peat; climate, particularly temperature and rainfall; vegetation
and other biota; topography/ relief; time; and, increasingly, the
influence of humans. All these factors will have an influence on
soil development and hence soil type. The potential for different
combinations of these factors across the world is immense and hence
it is not surprising that there are many thousands of different
types of soil in the world, with different properties and potential.
Just as Linnaeus produced a classification of plants that is in
regular use today, over the centuries attempts have been made to
develop classifications of soils on the basis of their properties,
their genesis and in some cases their use potential. Since the
late 19th century there have been several national classifications
developed and used. Most countries now have a system of nomenclature
for their soil types. These classifications have grouped together
similar soils and given names to the soils in the various classes.
The classifications are mostly hierarchical in the sense that they
consist of a number of levels, starting at the top with a number
of major divisions, which can be subdivided into further subclasses
and degrees of complexity.
There have also been several major attempts at a world classification
of soils, each of which have differed in the emphasis they have
given to different soil properties. Such classification systems
enable soil types to be classified and identified within a national
and international framework. The two most widely used international
classifications are those of Soil Taxonomy (developed by the US
Soil Conservation Service) and based on soil properties that can
be objectively measured and observed and the FAO/UNESCO legend/classification
which is along broadly similar lines but not as precisely defined.
It is regrettable that to date it has not been possible to produce
an international classification that has been adopted worldwide
but soil science is a young science with an important future and
a unified classification system should eventually be developed.
Until that is available, soil types should be named using both
national nomenclature and have this linked to one or more of the
Examples of some soil types from different parts of the world
are given in the section on Soil Profiles.