Agriculture Reference
In-Depth Information
Soil classification has its roots in antiquity. Systems of classifying soils on the basis
of their suitabilities for growing food plants existed in China and Rome approximately
4000 and 2000 years ago, respectively (Simonson, 1984). This has remained a common
influence on many soil classifications up to the present. However, there are many
reasons for wishing to classify soils, commensurate with the diversity of uses to which
they are put. Classifications exist in which soils are rated in terms of their suitabilities
for various categories of land use within agriculture or forestry, although there is
a developing need for classifications that are more environmentally oriented. Soils are
also commonly classified (and mapped) on their capabilities for particular engineering
uses, including road making and architecture. Indeed, Kellogg (1974) reported that
ca.
50 % of the benefits of soil survey in the U.S.A. were derived from urban planning,
ca.
25 % from planning the locations of engineering structures and the remaining 25 %
of the benefits were derived from planning in relation to agriculture, forestry and recreation.
The above are known as technical classifications and aim to characterise soils in terms
of their capabilities for single or restricted purposes. They may therefore have little
utility beyond the purposes for which they were created or beyond a narrow range of
soils. Technical classifications contrast with the more general pedological classifications
which do not aim at classifying soils for any specific end land use or other function but
treat them as natural bodies. However, the pedological classifications are usually
effective in broadly predicting many soil functional and technical properties.
In contrast with the technical classifications, pedological classifications are relatively
recent in conception. Initially, these were based largely on geological characteristics.
However, the Russian pedologist Dokuchaev is credited with creating, during the latter
part of the nineteenth century, the first system of soil classification based on an
appreciation of soil formation as the product of the five soil forming factors discussed
above (Simonson, 1984).
As discussed below, a range of modern pedological soil classification systems exist
and many countries have developed their own national classifications. Some of these
include, either implicitly or explicitly, concepts related to soil genesis although most
recent systems rely more on directly-observable soil characteristics.
4.6.2
PROBLEMS OF SOIL CLASSIFICATION
In classifying soils, decisions need to be made as to the size and properties of the smallest
unit needed for adequate description and accurate representation. In contrast to biological
classification where the limits of non-colonial individuals are generally clear, the lateral
boundaries of soil units are often relatively diffuse and the problem of subdividing a
continuum emerges in delineating units.
The profile is the basic unit of study in many systems and is defined as a two
dimensional section extending through the vertical depth of the soil and into the C horizon,
A three dimensional extension of this, the pedon, has been defined as the equivalent unit
of primary study in Soil Taxonomy (Soil Survey Staff, 1999). Whatever the basic unit
used, it must be deep enough to cover the full depth of the soil and of sufficient lateral
extent to include small-scale cyclic variation. In practice and depending on the intensity
of the sampling program, mapping individual soils is often carried out from a knowledge
