The Future of Soil Taxonomy - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 307

Chapter 27
The Future of Soil Taxonomy

The press for a universal soil classification system and the increased popularity of the
WRB has put pressure on Soil Taxonomy as a future global soil classification system.
The increased complexity of the system has been criticized not only by international
soil taxonomists but also by ecologists and geologists using the system in the USA.
For ST to survive in the next several decades, the following changes may be
necessary: (1) simplification and expansion of the number of diagnostic epipedons,
(2) reconsideration of the use of soil climate at the suborder level or the definition of
soil-temperature and soil-moisture classes for more widespread application outside
the USA, and (3) addition of new orders based on new or existing diagnostic
subsurface horizons.
This analysis suggests that there are an insufficient number of epipedons for
distinguishing among soils of the USA. Four of the eight epipedons, the ochric,
mollic, umbric, and histic, are used in 99 % of the soil series described and mapped
in the USA. The definitions of the mollic, and to a lesser extent the umbric horizon,
have become unnecessarily complex. In view of the interest in computing soil
stores of organic C in response to global warming, changes will need to be made in
the number and ranges in properties of diagnostic surface horizons. Perhaps it is
time to subdivide the mollic and umbric epipedons into two epipedons each based
on thickness and/or organic C content. For example, they could be subdivided into
10-15 and 15-20 cm depth intervals. Alternatively, the 0.6 % organic C require-

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ment could be changed to 0.6-1.5 and 1.5 %, e.g., voronic horizon in WRB. The

pachic, cumulic, and andic horizons could be redefined and used as epipedons. The
NRCS could cooperate with archaeologists to utilize the plaggen and anthropic
epipedon at the nearly 100 cultural sites identified in the USA.
Some of the subsurface horizons are over-utilized and others are underutilized.
The argillic horizon, which occurs in 42 % of the soil series, could be subdivided
into other clay-enriched horizons as is done in the WRB, such as the nitric horizon.
The agric horizon should be used on a trial basis for heavily tilled soils. The WRB
differentiates human-affected diagnostic horizons to a greater extent than ST. For
example, the agric horizon in ST is divided into the anthraquic, hydragric, irragic,

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