Agriculture Reference
In-Depth Information
and Robco map units, therefore, do not meet the deÝnition of a consociation type of map unit as
deÝned in the Soil Survey Manual (Soil Survey Division Staff, 1993), and the authors recommended
that a multitaxa unit be correlated. If done, the two map units would presumably have signiÝcantly
higher taxonomic and interpretive purities.
Other authors have conducted assessments of the taxonomic purity of map units, and have
reported results similar to the two studies described above. Edmonds et al. (1982) suggested that
taxonomic criteria that require laboratory methods to be determined often cannot be reliably inferred
in the Ýeld, and therefore are not easily mapped. They showed that for an area studied in Virginia,
variability that gives rise to mixed and oxidic mineralogy classes could be found within a distance
of 7 m. In this landscape, then, not only is it impractical to map this class difference at scales
commonly used in soil survey, soil volumes as small as individual pedons will likely contain
properties of both classes. Ransom et al. (1981), Edmonds et al. (1985), and Edmonds and Lentner
(1986) all point out that because soil series have property ranges limited by taxonomic boundaries,
and interpretations have generally been generated based on series properties, our ability to interpret
map units adequately is hampered. They suggest that we need to be able to record property ranges
for map units themselves, and then interpret the map units.
Today, by taking advantage of improved computer technology, we have begun to move from
interpreting soil series in our survey reports to interpreting components of map units. Soil series
are conceptual soil classes deÝned by limits of key diagnostic properties. Components of map units
are natural bodies of soils in a particular landscape. Whereas the soil series is the lowest class in
our taxonomy and is constrained by higher class limits, a soil component is that portion of a map
unit either Ýtting within the concept of the series or is close enough in its properties to interpret
in essentially the same way for most uses. In the past, interpretations were generated and stored
for each series nationally, and then used locally for the soil survey. While they could be adjusted,
this was a cumbersome process. Today we can store soil property values for individual map unit
components and generate interpretations based on the property ranges recorded for that speciÝc
soil survey area. While our OfÝcial Series Descriptions remain the lowest level of Soil Taxonomy,
and are therefore required to have ranges extending no further than higher taxa limits, the National
Soil Information System (NASIS) allows soil scientists to record soil property values for compo-
nents of map units that more closely reÞect ranges of properties in the Ýeld. Low and high values
depicting the range, along with a Ñrepresentative valueÒ (RV), are recorded. The RV is required to
be within the range of the taxonomic class (generally series), but the low and/or high value may
Ñextend beyond the established limits of the taxon from which the component gets its name, but
only to the extent that interpretations do not changeÒ
(Soil Survey Staff, 2001b). Thus in effect,
the soil survey has developed a procedure to effectively accomplish what was debated by the NCSS
leaders at the 1963 conference. Rather than allowing the series to Ñstretch family class limits,Ò we
have allowed the map unit component to stretch the series class limits. Interpretive criteria are then
applied to the component data.
REFERENCES
Alexander, E.B. 1983. Comment on ÑThe relationship between Soil Taxonomy and soil mapping.Ò
Soil Surv.
24:15Ï16.
Bailey, G.D. 1978. A brief history of the Miami soil series.
Horiz.
Soil Surv. Horiz.
19:9Ï14.
Borst, G. 1983. Taxonomy and soil mapping units: A reply to R.L. Guthrie.,
Soil Surv. Horiz.
23:16Ï19.
Cline, M.G. 1963. Logic of the new system of soil classiÝcation.
96:17Ï22.
Cline, M.G. 1977. Historical highlights in soil genesis, morphology, and classiÝcation.
Soil Sci.
Soil Sci. Soc. Am.
41:250Ï254.
Cline, M.G. 1980. Experience with soil taxonomy of the United States,
Proc.
33:193Ï226.
Edmonds, W.J., Iyengar, S.S., Zelazny, L.W., Lentner, M., and Peacock, C.D. 1982. Variability in Family
differentia of soils in a second-order soil survey mapping unit.
Adv. Agron.
Soil Sci. Soc. Am. J.
46:88Ï93.
