Geoscience Reference
In-Depth Information
Chapter 1
Introduction
Diagnostic epipedons and subsurface horizons are soil horizons at specific locations
in the soil profile that have key characteristics that allow them to be used in
classifying soils; hence, they play a key role in modern soil classification systems
such as
Soil Taxonomy
(ST; Soil Survey Staff
1999
,
2010
). They are also used in the
World Reference Base for Soil Resources (IUSS Working Group WRB
2007
) and
the Australian, Chinese, and Russian classification systems. In ST, diagnostic
horizons were employed at the time of the Seventh Approximation (Soil Survey
Staff
1960
) because of the absence of general agreement on soil horizon designa-
tions. This means that diagnostic horizons do not necessarily correlate with soil
horizons. For example, the mollic epipedon may include not only the A horizons
but also some transitional and even B horizons.
At the present time, there are eight epipedons, or diagnostic surface horizons,
and 19 diagnostic subsurface horizons in ST. Additionally, there are 21 character-
istics diagnostic of mineral soils, several characteristics diagnostic of organic soils,
and 14 characteristics diagnostic for both mineral and organic soils. This topic will
address primarily the diagnostic epipedons and subsurface horizons, which are
listed in Table
1.1
.
It is of interest that some diagnostic horizons are used at the order level, others at
the suborder or great-group level, and others at several levels, including the
subgroup level (Tables
1.2
and
1.3
). The only diagnostic horizons required at the
order level are the mollic for Mollisols, the histic for Histosols, the argillic for
Alfisols, the argillic or kandic for Ultisols, the oxic or kandic for Oxisols, and the
spodic for Spodosols. Most diagnostic horizons are
used
initially at the suborder
level, including the agric (no soil series with an agric horizon are recognized
presently in ST), albic (Mollisols and Spodosols), calcic (Aridisols), cambic
(Aridisols), duripan (Aridisols), gypsic (Aridisols), salic (Aridisols), anthropic
(Inceptisols), folistic (Histosols), and plaggen (Inceptisols). Diagnostic horizons
also may be used at the great-group level, including the fragipan (Alfisols,
Inceptisols, Spodosols, Ultisols), glossic (Alfisols), natric (Alfisols, Aridisols,
Mollisols, Vertisols), petrocalcic (Aridisols), petrogypsic (Aridisols), placic
(Andisols, Spodosols), sombric (Oxisols, Ultisols), melanic (Andisols, Inceptisols),
