Agriculture Reference
In-Depth Information
this causes. It is conceivable that soil families could become the lowest category of taxonomy, but
some ingenious person may Ýnd a better solution.
Soil Individuals
The concepts of the pedon and polypedon were introduced by Johnson (1963) to relate map
units depicted in soil surveys to the new classiÝcation system. These were intended to bridge the
gap between the conceptual taxonomic categories of soil series with the soil bodies delineated on
maps. The pedon was conceived to be the basic soil unit consisting of a volume just large enough
to depict the horizons present and their relationships to one another. The pedon is likened to the
unit cell of a crystal (Soil Survey Staff, 1975). It is generally considered to be the entity that we
describe and sample in the Ýeld. Inherent weaknesses of the pedon concept are that while its
relatively small size is convenient for study, it is too small to exhibit the full range of properties
for a series, and it cannot show the nature of the boundary with adjacent soils in the landscape,
the surface shape, or other site characteristics of the soil. Additionally, rarely if ever have soil
scientists truly identiÝed, described, and sampled the three-dimensional pedon in the Ýeld. It has
been argued that we more commonly describe and sample soil proÝles rather than pedons
(Holmgren, 1988).
The concept of the polypedon was introduced to overcome some of the weaknesses inherent
in the pedon concept. Johnson (1963, p. 215) described it as a real soil body consisting of contiguous
pedons Ñall falling within the range of a single soil series.Ò It was conceived to provide a link
between pedons and the taxonomy on one hand, and to relate taxonomic units to map units on the
other hand. The polypedon was considered to be the individual we classify, and, in JohnsonÔs words,
Ñcomparable to individual pine trees, individual Ýsh, and individual men.Ò
These concepts presented practical difÝculties. First, the relationship between real soil bodies
(i.e., polypedons) and the conceptual taxonomic class of the soil series presented a serious dilemma.
Soil properties within a three-dimensional soil body are not mindful of arbitrary class limits of our
taxonomy. The report of the Committee on the Application of the New ClassiÝcation System
recorded in the 1965 NCSS Conference Proceedings (Soil Survey Division, 1999) includes a
discussion of Ñguidelines for allowable tolerances in the stretching of family class limits by series
class limits.Ò The debate was whether the range of characteristics for a series must be within the
limits of the family. Alternatively, only the typical pedon itself would be required to Ýt within the
family, thus allowing the range of characteristics to stretch beyond the family range. It was agreed
that series, as the lowest level of the taxonomy, must have ranges no wider than the family to which
they belong. Having made this decision, however, the NCSS leaders wanted to avoid the possible
proliferation of new series simply to cover pedons and polypedons slightly beyond the rigid family
class limits. They decided to study the problem further. Two years later at the 1967 NCSS conference
(Soil Survey Division, 1999) there was discussion of
taxonomic inclusions, plesioseries,
and
taxal
deviants
as devices to classify pedons close to, but outside the range of, a given series. These
concepts evolved into the
. While we no longer correlate variants (we establish
new series for these), we frequently use the taxadjunct to this day when the pedon used to typify
a series in a survey area is outside the limits of the family to which the series belongs.
The difÝculties presented by the concept of pedons and polypedons, and the constraints on
allowable ranges for soil series, have been debated ever since that time. Soon after JohnsonÔs
introduction of the pedon and polypedon, Knox (1965, p. 83) pointed out that polypedons have no
real existence apart from series concepts. He stated that Ñ
taxadjunct
and
variant
their signiÝcance as individuals seems
less than the signiÝcance of individual pine trees, individual Ýsh, and individual men
.Ò Webster
(1968) suggested that in order to deÝne and maintain effective series concepts, there is a need for
an ÑunconformityÒ between the series and higher taxonomic classes. Guthrie (1982) recognized
that the use of the correlation devices of taxadjuncts and variants are a manifestation of the problem.
He suggested two alternatives when a pedon is chosen to typify the central concept of a series that
