Conclusions - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 301

Chapter 26
Conclusions

26.1 Relative Abundance of Diagnostic Horizons

This analysis is based primarily on the numbers of soil series rather than the soil
series areas, which are more difficult to estimate. However, there is a significant
( R 2 0.89) correlation between number of soil series and area of soil orders

(Fig. 26.1 ).
The ochric epipedon is the dominant diagnostic surface horizon in soils of the
USA; it is common in six soil orders - the Alfisols, Aridisols, Entisols, Inceptisols,
Ultisols, and Oxisols - and is present in about 56 % of the more than 21,000 soil
series identified to date (Table 26.1 ). The next dominant epipedon is the mollic,
which is present in 39 % of the soil series, primarily in the Mollisols but also in
Alfisols and Vertisols. The umbric epipedon is present in a surprisingly low number
of soil series, 587, particularly in Inceptisols and Andisols. However, this analysis
may not have identified all of the soils with umbric epipedons, since they are not
readily detectable from soil taxonomic formative elements. The histic epipedon is
present in 288 soil series, mostly in Histosols but also in Gelisols and other soil
orders. The melanic and folistic epipedons have been identified in 152 and 31 soil
series, respectively. The human-affected epipedons, the anthropic and plaggen, are
recognized in ST but have not been used in establishing soil series at the present time.
The argillic horizon is the predominant diagnostic subsurface horizon and occurs
in 42 % of the soil series recognized in the USA (Table 26.1 ). This horizon occurs
primarily in Alfisols, Mollisols, and Argids. The cambic horizon is the next most
common subsurface horizon identified in US soils, especially in Inceptisols,
Mollisols, and Aridisols but also in Andisols and Vertisols. Because it is impossible
to detect the presence of the cambic horizon from the Keys to Soil Taxonomy (Soil
Survey Staff 2010 ), the number of soil series with a cambic horizon was estimated
by difference. This assumes that there is only one dominant diagnostic subsurface
horizon present in each soil, which certainly is not the case for many soils. Other
common subsurface horizons were calcic (10 % of all soil series) and kandic (6 %).
The albic horizon was present in 4 % of the soil series. Although the albic horizon is

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