Agriculture Reference
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potential. To overcome the gap between the interpretative value of soil classes and actual production
potential, a comprehensive characterization of topsoils (Purnell et al., 1994) has been proposed for
the practical purpose of assessing and monitoring fertility status and guiding management practices.
Topsoil characterization should, at an appropriate scale, become an adjunct to current soil classi-
Ýcation and provide a way to link soil classes with soil management. The proposal identiÝes about
60 main topsoil properties related to organic matter status, biological activity, and physical, chem-
ical, and moisture conditions, allowing for the characterization of topsoil over and above the
diagnostic horizons and features required for deÝning a soil class. This approach could help
overcome criticisms by other disciplines and by land users regarding shortcomings in the application
value of taxonomic systems. Recently, a proposal was even made (Krogh and Greve, 1999) to adopt
a two-tier soil classiÝcation system, the Ýrst tier consisting of largely undisturbed soils, while a
second tier would accommodate signiÝcantly changed cultivated soils with anthric properties.
Soil classiÝcation has traditionally been considered a means to optimize land use and to transfer
technology between comparable soils in different areas. This approach is based on the assumption
that soil classiÝcation stratiÝes the environment in sufÝcient detail that transfer by analogy can
take place. This assumption may be valid when the emphasis is on broad assessments, but it should
be fully realized that technology transfer should rely not only on soil attributes, but on other factors
as well, such as climate, relief, hydrology, level of inputs, and socioeconomic conditions. If any
of these factors vary between locations, similarity of soil classes will not result in successful
transfers. Hence soil survey interpretation has progressively evolved into land evaluation (FAO,
1976), encompassing soil, atmosphere, geology, hydrology, plant and animal populations, and the
results of past and present human activity.
Special attention needs to be given to the climatic conditions that determine the upper limits
of crop production. Since soils with similar morphology and chemical characteristics can occur
under different climates, some soil classiÝcation systems have introduced climatic data at various
levels of generalization. The number of climatic subdivisions that can be made must of necessity
be limited if the number of taxa in the classiÝcation system is to remain manageable. However,
annual means of soil moisture and temperature do not provide sufÝcient information for the transfer
of site-speciÝc experience. An overlay of more detailed climatic data, independent of soil classiÝ-
cation, may be preferable. An attempt in this direction was made in a study of the land use potential
of major agro-ecological zones (FAO, 1978) in which the climatic inventory was expressed in terms
of the length of the growing period, based on moisture availability, temperature, and radiation at
intervals of 30 days (e.g., 90Ï119 days, 120Ï149 days, 150Ï179 days, etc.). Combining climatic
and soil requirements made it possible to establish and map suitability ratings for speciÝc types of
land use. The adoption of climatic phases, rather than broad temperature and moisture regimes,
could enhance the application of soil surveys for development purposes.
APPROACHES FOR THE FUTURE
The continuum of the soil cover at the earthÔs surface can hardly be apprehended in its entirety.
Hence a subdivision is required in order to recognize and remember the different components of
this continuum and to understand the relationships with the factors of their formation. Classes are
constructions of the mind, and differentiae are selected and weighted according to their signiÝcance
in soil formation and land use.
Opinions regarding the rationale of establishing and ordering classes, and regarding the impor-
tance of differentiating characteristics, vary widely. The assumption that a soil classiÝcation should
provide for a taxonomy, for making soil surveys, and for interpreting them led to the development
of elaborate hierarchies to capture and oversee the great number of differentiae involved. It appears,
however, that the three purposes can hardly be served equally well. Concerns in this respect have
been voiced previously, but no major adjustments have been made so far (Dudal, 1986).
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