Agriculture Reference
In-Depth Information
Recently developed soil classiÝcation systems have greatly contributed to the taxonomy of soils,
i.e., their identiÝcation, recognition, and relationships to the factors of their formation. Further
consultations will, however, be required in order to ensure that the weighting and hierarchy of
differentiae be harmonized, and that taxonomy concurrently becomes a means of communication.
MAKING SOIL SURVEYS
Soil surveys started well before the creation of soil taxonomies. In the United States, the phrase
Ñsoil typesÒ was adopted as a label for mapping units set apart in the Ýrst surveys made in 1899
(Simonson, 1997). A few years later, the soil series was introduced in order to group types.
Distinguishing kinds of soils was based on features that appeared to inÞuence their relations to
crops. Primary emphasis was given to texture. A secondary purpose of the surveys was to distinguish
kinds of soils that differed in their formation, such as those derived from different kinds of rocks.
The impact of the DokuchaevÔs concepts of soil formation and distribution (Glinka, 1914) led to
the creation of a more comprehensive system of soil classiÝcation, including great soil groups as
a higher category (Marbut, 1928). The lowest categories, the soil series and types, created on the
basis of the work of the soil survey, were considered to be the base upon which the whole structure
rested. It was decided that all the criteria for classes in higher categories were to be series criteria
as well. The justiÝcation for the higher categories was that grouping the very large number of series
into a smaller number of classes would allow concepts and relationships to be more easily under-
stood, and soils in different areas could be compared. Early efforts to place the series into great
soil groups on the basis of existing descriptions were not successful. Improvements in the categories
above the soil series were needed to complete the scheme. It was realized that higher categories,
although helpful in organizing knowledge, had a decreasing value for interpretation and for trans-
ferring experience. Hence the series remained the principal units of mapping, not only on a large
and intermediale scale, but also on a smaller scale. In the latter case, mapping units were expressed
in terms of associations of series, rather than on the basis of higher categories. One of the reasons
for this approach was the realization that the use of taxa, higher in the hierarchy, sacriÝces
information because of the greater heterogeneity of the classes.
The question arises as to whether the higher categories have been designed to group the lower
categories, or if the higher categories have been developed separately, imposing on the lower
categories differentiations with no clearly known signiÝcance in utility. The development of the
taxonomy actually proceeded at both ends of the level of generalization, resulting in a difÝcult
matching of differentiae and in a hiatus between the higher, more genetically inspired categories
and the more pragmatic lower ones, geared toward responses to management and manipulation for
use. Cline (1980), in his assessment of the use of Soil Taxonomy in the United States, felt that the
precision and detail of Ýeld description had improved, but that standards of mapping had not
markedly changed. With the emphasis on lower categories as mapping units, the role of the higher
categories of a soil taxonomy for making soil surveys requires clariÝcation.
Current soil surveys record soil classes that are characterized by the description and analysis
of sampling units. The soil cover is represented in terms of one class, in homogenous areas, or of
an association of classes when contrasting kinds of soils occur. However, associations as presently
conceived do not necessarily reÞect the relationships between the soils they contain, the sequence
in which they occur, or the proportion of the area that they occupy. They seldom reÞect the structure
and the dynamics of the soil-landscape. Soils are characterized not only by a vertical succession
of horizons, but also by spatial variations in their properties, related to lateral movements at the
surface and within the soil. The soil moisture regime along a slope, the lateral Þow of nutrients
and weathering products, runoff and erosion, and the development of salinity and waterlogging are
functions of the dynamics of soil formation in the landscape. The three-dimensional nature of soil
is barely reÞected by a 10-m
pedon. Its fourth dimension, time, is mostly overlooked. The properties
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