Fragipan Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 149

along inter-ped faces and streaks” (p. 20). The key properties include evidence of
alteration, separation of structural units by a defined spacing, a SOC concentration
of 0.5 %, a defined proportion of air-dry clods slaking in water for a particular

length of time, the lack of cementation upon repeated wetting and drying,
a penetration resistance at a field capacity

<

50 kPa, the lack of effervescence,

and a thickness of 15 cm.

The earliest case study in the USA of what was defined as a “fragipan” may be
that of Winters ( 1942 ) who recognized a “silica hardpan” in Ultisols near
Washington, D.C. Smith and Browning ( 1946 ) described a “hardpan” or “siltpan”
in subsoils of West Virginia (WV), emphasizing the reversibility of cementing and
the brittleness. Nikiforoff et al. ( 1948 ) recognized a similar hardpan in the
Mid-Atlantic coastal plain but attributed its origin to geologic rather than to
pedologic processes. Grossman et al. ( 1959a ) used the term “fragizone” to refer
to the portion of the soil profile where fragipans occurred. The term “fragipan” was
coined by Guy Smith and was featured in the Seventh Approximation (Soil Survey
Staff 1960 ). The term comes from the Latin fragilis , meaning brittle, and pan,
i.e., “brittle pan.” It seems that the publication of the Seventh Approximation
spurred an interest in soils with fragipans.
Since that time, there have been several reviews of fragipans, including the
worldwide distribution of fragipans (Witty and Knox 1989 ) and those of soils with
fragipans in the eastern USA (Grossman and Carlisle 1969 ), midwestern USA
(Franzmeier et al. 1989 ), northeastern USA (Lindbo and Veneman 1989 ), the
lower Mississippi Valley (Lindbo et al. 1997 ), and Pennsylvania (Petersen
et al. 1970 ; Ciolkosz et al. 1992 ). These reviews have clearly shown the many
pathways that lead to the formation of fragipans in different parts of the USA.
We have used the information from those reviews and case studies and combined it
with an analysis of fragipan soil information in the extensive databases of the
NRCS. These databases hold spatial and tabulated information on more than
20,000 soil series of the USA and have been used in elucidating the nature and
properties of soils with fragipans.

15.2 Characteristics of Soils with Fragipans

Figure 15.1 is a photograph of a soil with a fragipan. In the NRCS database fragipan
horizons are designed as “Btx,” as “Btgx” for Aqualfs and Aquults, as “Bx” for
Inceptisols with fragipans, and as “(E/B)x or (B/E)x for Fragiorthods and other
bisequal soils. Fragipans commonly occur in soils with kandic, cambic, spodic, and
argillic horizons. The argillic horizon normally is above or coextensive to the
fragipan, but some pedons have the argillic horizon below the fragipan.
The mean depths to the surface and base of the fragipan are 63 cm (range ¼ 13-

114 cm) and 130 cm (range 56-241 cm), respectively, yielding a mean thickness

of 67 cm (data not shown). The range in fragipan thickness is 15-218 cm; this
is a much wider range than has been reported in the literature. For example,

¼