Agriculture Reference
In-Depth Information
is commonly called the soil Ñstructure,Ò one of the parameters essential to its Ñmorphological
descriptionÒ.
have volumes that are more or less parallel to the surface of the
terrain, the lateral dimensions being far larger than the vertical dimension. They show one or more
types of structural aggregates, Ýtted together for the most part, and a certain number of associated
Ñpedological featuresÒ such as color staining on ped faces, clay skins, iron, and manganese con-
cretions in the mass, concentrations of organic products, etc. The upper horizon is characterized
by the presence or even accumulation of organic matter.
The genetic conditions of soils result in a double differentiation: in their vertical arrangement
and in their spatial distribution. The former corresponds to the common notion of soil proÝle,
Pedological
or
Soil horizons
or
solum, or pedon: succession of the soil horizons viewed in vertical cross-section. The latter
corresponds with the lateral arrangement of different types of horizon within the landscape, thus
allowing for the deÝnition of Soil-systems.
are often large fragments of the soil cover within which
we can distinguish different soil horizons organized in vertical superposition and lateral successions,
at the scale of a geomorphologic relief unit. Therefore a Soil-system is described in terms of
horizons and relationships between horizons, both horizontal and vertical. This constitutes the origin
of our current approach to typology.
Soil-systems
or
Pedological-systems
Subdivision or Partitioning of the Soil Cover
Analysis of soil mantle arrangements proceeds from an approach whose primary aim is to
establish and formulate the laws of spatial distribution of soils at different levels of organization
in a geographic area. Historically, this analysis has been expressed as a soil map. However, new
technology has provided this expression in the form of
Spatial Soil Databases
and
Geographical
Information Systems (GIS).
These constitute spatial analytical tools and bring considerable Þex-
ibility of choice in representative modes, thus facilitating the management and processing of data.
The usual job of soil surveyors, however, remains to divide the soil mantle into approximately
similar conceptual volumes to describe the contents and to locate them in space (Simonson, 1989;
Wilding, 1989; Girard, 1984; Buol et al., 1997). The continuum constituted by the soil cover may
be subdivided into two types of compartments (Figure 13.1):
¤
Homogenous
compartments: the
horizons
, enabling a better characterization, and corresponding
to vertical subdivisions within a proÝle
¤
Heterogeneous
compartments, constituting the territorial groups enabling the mapping: the
typo-
logical
and
mapping units
of soils, corresponding to a horizontal sequence of combinations of
horizons
For the Ýrst of those divisions, we Ýnd here the well-known concept of diagnostic horizons,
introduced in many taxonomies and reference systems (Soil Survey Staff, 1975; FAO, 1998). For
the second, we can distinguish several types of Soil Units as shown below.
¤
Soil Typological Units (STU)
These are made up of what we have deÝned as ÑProÝle,Ò ÑSolum,Ò or ÑPedonÒÐ that is to say a
volume of soil mantle showing the same superposition of horizons. These units have been the
basis of the conventional soil classiÝcations.
¤
Soil Mapping Units (SMU)
By drawing their boundaries onto a map, each STU should, in principle, correspond to one SMU.
However, the correspondence between STUs and SMUs is more complex than it would appear.
Actually, due to graphic limitations, it is scarcely possible to delimit the so-called simple or pure
SMUs, each corresponding to a given STU, except on large-scale topographic bases (1/5,000,
1/10,000). In most cases, a SMU has to regroup several STUs, so as to give the Ýnal document a
