The purpose of this technical
note is to provide guidance in populating the data elements taxonomic
moisture class and taxonomic moisture subclass in NASIS. The
moisture class is shown in the Component Taxonomic Moisture Class table, and
the subclass is shown in the Component table. It is useful to be able to query
the database to select soils with particular moisture regimes. The taxonomic
classification alone, however, is not always sufficient for this purpose
because the soil moisture regime is not always described completely by the
taxonomic classification. The two data elements are used not only in general
queries that require information about soil moisture regimes but also in some
NASIS forestry interpretations, such as seedling mortality.
Taxonomic moisture class reflects
the soil’s moisture regime. In most instances, this information is contained
within the taxonomic classification. For example, a Typic Haplustalf has an
ustic moisture regime. A Typic Hapludalf is generally considered to have a udic
moisture regime, although it could have a perudic regime. In these examples,
the proper entries for taxonomic moisture class are ustic and udic (or possibly
perudic), respectively. The entry for taxonomic moisture subclass for each
would be “typic.”
A taxonomic moisture subclass
that is not “typic” reflects a taxonomic intergrade or extragrade. Some soils
having a particular moisture regime are transitional to another regime
(intergrades) or are grading away from the regime to which they are assigned
(extragrades). An example of an intergrade is an Aquic Haplustalf. The
taxonomic moisture subclass is aquic. An example of an extragrade is an Aeric
Albaqualf. The taxonomic moisture subclass is aeric.
For many soils, the proper
assignment of the taxonomic moisture subclass is less obvious. Consider Mollic
Endoaqualfs and Udollic Endoaqualfs. The taxonomic moisture class is aquic for
both. For the Mollic subgroup, the taxonomic moisture subclass is understood to
be typic because the Aeric subgroup (the name of which indicates the only other
moisture subclass to be used with aquic) precedes the Mollic subgroup in the
key, leaving typic as the only remaining choice. For the Udollic subgroup, the
appropriate taxonomic moisture subclass is aeric because the Udollic subgroup criteria
include the same moisture criteria as the moisture criteria for the Aeric
subgroup (Keys to Soil Taxonomy, 9th ed., page 44).
In some instances, knowledge of
the series concept is required. Consider Vertic Palexeralfs (Keys to Soil
Taxonomy, 9th ed., page 80). Although it is clear that the taxonomic
moisture class is xeric, the proper subclass is not apparent from taxonomy
alone. The Vertic subgroup is first in the keying order, so members of this
subgroup could be appropriately placed in a typic, aquic, oxyaquic, or other
taxonomic moisture subclass. An example in which the taxonomic moisture class
is not apparent is Oxyaquic Quartzipsamments (Keys to Soil Taxonomy,
9th ed., page 145). The subclass is oxyaquic, but you can determine if the soil
has a xeric, ustic, or udic moisture regime only with knowledge of the series
concept.
Some Mollisols require an
understanding of the taxonomic criteria and the series concept when the
taxonomic moisture class and subclass data elements are populated. Consider
that Ustolls and Xerolls allow either an ustic moisture regime or an aridic
regime bordering on ustic. Calciustolls can serve as an example (Keys to
Soil Taxonomy, 9th ed., page 215). On the basis of the keying order, we
can say that Typic Calciustolls have an ustic taxonomic moisture class and a
typic subclass and that Udic Calciustolls have an ustic taxonomic moisture
class and a udic subclass. Aridic Calciustolls have either an aridic taxonomic
moisture class and an ustic subclass or an ustic taxonomic moisture class and
an aridic subclass. You must consider the series concept to determine for
sure. Also, although it may seem backwards at first glance, the Aridic
subgroups of Xerolls, by definition, have an aridic taxonomic moisture class and
a xeric subclass.
The NASIS data elements taxonomic
moisture class and taxonomic moisture subclass are used in
database queries that require information about soil moisture regimes. These
data elements also are critical for generating some NASIS forestry
interpretations. In many instances, the proper placement is apparent from the
taxonomic formative elements. In some cases, the moisture regime, including
intergrade or extragrade criteria, is not explicitly included in the taxonomic
classification. In these cases, knowledge of the criteria for specific taxa,
the keying order of great groups within the suborder or subgroups within the
great group, or soil series concepts can help in determining the correct
taxonomic moisture class and/or subclass.
The contact for this technical
note is the National Leader for Soil Classification and Standards, National
Soil Survey Center, Lincoln, NE.
Soil Survey Staff, 2003. Keys to
Soil Taxonomy, 9th edition. United States Department of Agriculture, Natural
Resources Conservation Service. Available online at:
http://soils.usda.gov/technical/classification/tax
keys/