Approach - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 11

Chapter 2
Approach
A list of the possible soil taxa containing a diagnostic horizon was prepared from the
Keys to Soil Taxonomy (Soil Survey Staff 2010 ). From this list, a list of soil series
containing the diagnostic horizon was obtained from the “Soil Classification Data-
base” (SC; Soil Survey Division 2013a )( http://soils.usda.gov/technical/classification )
and “Official Soil Descriptions” (OSD; Soil Survey Division 2013b )( http://soils.usda.
gov/OSD ) functions of the National Resource and Conservation Service (NRCS) of
the US Department of Agriculture (USDA). Case studies were also examined from the
literature, which were used in conjunction with OSDs to prepare tables summarizing
the role of soil-forming factors and the pedogenic processes involved in development
of diagnostic horizons. Primary soil characterization data were obtained from http://
ncsslabdatamart.sc.egov.usda.gov/ (Soil Survey Division 2013c ).
Maps of soils containing some of the diagnostic horizons were prepared using
the July 5, 2006 version of the Digital General Soil Map of the USA published by
the NRCS. This dataset consists of general soil association units created by gener-
alizing more detailed soil survey maps. Since the taxonomic nomenclature for a
map unit is recorded at the component level and a map unit is typically composed of
one or more components, aggregation is needed to reduce a set of component
attribute values to a single value that will represent the map unit as a whole. For
taxonomic order, suborder, and great group distribution maps, data were aggregated
to the map-unit level using the “dominant-component-aggregation” approach. This
approach returns the attribute value associated with the component with the highest
percent composition in the map unit, which may or may not represent the dominant
condition throughout the map unit. For taxonomic subgroup distribution maps, data
were aggregated to the map-unit level using the “presence method”; that is, if any
component attribute matched the taxonomic subgroup of interest, then that map unit
would be shown on the map regardless of its map-unit composition.
Other maps of diagnostic horizons were prepared by the author from “geo-
graphic extent” maps of OSDs by plotting soil series containing the diagnostic
horizon by county on a US map showing soil counties. Finally, maps of broad-based
diagnostic horizons, such as the ochric and cambic, were generalized from the soil
order and suborder maps of the USA prepared by the NRCS.