Agriculture Reference
In-Depth Information
of a soil proÝle, which we have become so accustomed to seek and appreciate, may not, in the
Ýnal analysis, be as important as their spatial or temporal variance. Alternative approaches advocate
a more comprehensive representation of the soil pattern. Soil sociology (Schlichting, 1970) rather
than soil taxonomy has been proposed as a basis for mapping soil scapes (Buol et al., 1975), soil
landscape (Hudson, 1995), soil cover (Fridland, 1980), genons (Boulaine, 1978), or Ñensembles
pdologiquesÒ (Ruellan, 1984). The approach is a dynamic rather than a static characterization of
soils. The leaching and accumulation processes in the solum have their equivalent in the discharge
and recharge Þows in the landscape (Richardson et al., 1992). The emphasis on the use of taxa in
soil surveys, carrying precise criteria for separating them, may have reduced awareness of the lateral
and gradual links in the soil landscape. Boundaries between taxa are sometimes difÝcult to establish
in the Ýeld, as real soils may not abide by the rules that govern a taxonomic hierarchy. Boundaries
based on the visible landscape might better reÞect the three-dimensional nature of the soil cover.
ÑThere is a need for devices that will adapt quantitative limits of criteria more realistically to soil
variation in the Ýeld in order to reconcile the conceptual framework of taxonomy with the reality
of soils in natureÒ (Cline, 1980).
INTERPRETATION
Interpretation of soil surveys and predictions of the effect of management practices require that
soils be grouped according to their qualities and limitations for different types of use. It is assumed
that basic taxonomic classes can be grouped, or subdivided and regrouped, to permit speciÝc
predictions. The shift from a qualitative genetic approach in soil classiÝcation to a quantitative
expression of properties of the soil itself has led to a more effective assessment of soil-use
relationships. Most soil classiÝcation systems use a limited number of class differentiae, preferably
those that are stable, and can be measured or observed. Transient properties of the surface layers,
which vary with soil management practices, are generally avoided to accommodate both cultivated
and undisturbed soils. In the early stage of the development of the USDA Soil Taxonomy, it was
decided that soil management should not change the classiÝcation.
The emphasis on stable subsoil characteristics and the genetically inspired selection of taxa
differentiae do not necessarily provide the criteria relevant to soil use. It is striking that parameters
of soil hydraulics are given little consideration in soil classiÝcation, even though they are of prime
importance to plant growth and to the assessment of water Þows, which strongly affect environ-
mental issues. Soil texture, as a distinctive characteristic, appears only in the lower categories, even
though it is a main indicator of moisture retention, workability, and trafÝcability. It is signiÝcant
that a number of technical classiÝcations have been designed in which only a part of the data
gathered in a soil survey is combined with added information collected for the speciÝc purpose.
The Ñsoil fertility capability classiÝcationÒ (Buol et al., 1975) is an example of a classiÝcation
based on characteristics that have been speciÝcally selected for their relevance to soil fertility
management. Similar groupings have been made to accommodate land capability, suitability for
irrigation, erosion control, engineering, wildlife habitats, drainage design, land remediation, and
others. Boundaries of these utilitarian groupings may not correspond to those of the mapping units
of a soil survey, and are often more accessible to users than the taxonomic nomenclature. However,
taxonomy may provide a scientiÝc base for the recognition and characterization of the soil pattern.
In many instances, phases rather than taxonomic differentiae are the factors that determine land
use choices, such as slope, landform, stoniness, Þooding, and erosion. Special attention is called
to the properties of the surface layer, of paramount importance to plant growth. Soil surface layers
inÞuence germination, are the seat of biological activity, store a large part of plant nutrients, contain
a major portion of the roots of annual crops, and are determining factors for tillage practices. Yet
these properties are not reÞected in the deÝnition of taxa because they are transient. The human
factor of soil formation is underemphasized, though it is a major component of soil production
