Agriculture Reference
In-Depth Information
Refinement of Soil Series Concepts
Another consequence of the shift to a quantitative system was a change in the range of
characteristics, geographic distribution, and precision of interpretation for some series. The soil
series evolved from an early concept based on parent rock and geographic province (USDA, 1903),
to one that included the recognition of common genetic horizons and morphology (Kellogg, 1937),
and Ýnally to becoming the lowest level of the classiÝcation system, thus sharing limits with each
of the higher taxa to which it belongs (Soil Survey Staff, 1975). With the use of class limits for
particle size, mineralogy, temperature, and other family criteria, along with soil property limits in
the higher categories (including moisture class), some series ranges had to be trimmed and geo-
graphic distributions limited. As a result, some new series were established. Bailey (1978) described
the impact of the implementation of Soil Taxonomy on the Miami series. Before the adoption of
Soil Taxonomy in 1962, the concept of the Miami series included pedons with subsoil clay contents
straddling the family particle-size class limit of 35%. Since a series could not range beyond the
limit of the family, the Miamian series (Ýne, mixed, mesic, Typic Hapludalfs) was split out of the
Miami series (Ýne-loamy, mixed, mesic, Typic Hapludalfs) in 1969. As Soil Taxonomy continues
to be amended by revising or adding new differentiating criteria (Soil Survey Staff, 1999), series
concepts continue to be reÝned. For example, in 1992 the International Committee on Aquic Soils
introduced the Oxyaquic subgroup within several great groups to recognize soils with seasonally
high water tables within 100 cm, but not meeting the criteria for higher aquic taxa. In 1996, the
International Committee on Families introduced additional family classes for cation-exchange
activity classes (CEC). Today, the family to which the Miami soils were originally classiÝed has
been subdivided into three (semiactive, active, and superactive) of the four CEC classes, and the
new Oxyaquic subgroup. These subdivisions resulted in further narrowing the range of the series
and recognition of six families, where previously there had been one. Today, the Miami series is
a member of the family of Ýne-loamy, mixed, active, mesic Oxyaquic Hapludalfs.
The use of quantitative criteria deÝning class limits, with the resultant reÝnement of series
ranges, has allowed for the development of increasingly precise interpretations. For example, the
introduction of the Oxyaquic subgroup to separate soils with water tables of short duration within
100 cm of the soil surface (such as Miami) from otherwise similar soils in the typic subgroup of
Hapludalfs (such as Amanda) allows us to better describe the suitability of these classes of soils
for homes with basements or on-site sewage disposal. We must remember, however, that providing
interpretations for soil series (a conceptual taxonomic class) is not the same as interpreting a soil
map unit. Map units contain not only the soil(s) for which they are named, but also similar and
dissimilar inclusions (Soil Survey Division Staff, 1993).
ReÝnements to the concept and distribution of individual soil series, while providing the beneÝt
of improved interpretations, also present some problems. Coordination between survey areas
mapped at different times requires careful correlation to achieve adequate joining of yesterdayÔs
and todayÔs series concepts on each side of the survey boundary. This has been especially evident
in recent efforts to accelerate the digitizing of soil surveys that were mapped over a period of 30
years or more in the United States. Unless adequately coordinated and joined, GIS users cannot
easily achieve effective analysis across survey area boundaries. Changing series concepts are also
a burden for others who use soil survey information, because they must periodically learn new
names for the soils they have become familiar with. They donÔt understand why the soils Ñchange.Ò
Improvement in Soil Correlation
Soil correlation is the process the NCSS uses to deÝne, map, name, classify, and interpret soils
and to join soil map units consistently within and among soil survey areas (Simonson, 1963; Soil
Survey Staff, 2001a). It is our most important quality control process. The need to provide com-
prehensive Ýeld descriptions and associated laboratory data to adequately classify the soils tended
