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investigators also have implicated Si as the cementing agent in fragipans (DeKimpe
et al.
1983
; Steinhardt et al.
1982
; Karathanasis
1989
; Norfleet and Karathanasis
1996
). Early investigators referred to what is now mapped as fragipan soils as
“siltpans” or “silica hardpans” (Winters
1942
; Smith and Browning
1946
;
Nikiforoff et al.
1948
). The silica may originate from the weathering of feldspars
(Karathanasis
1989
). However,
Soil Taxonomy
(Soil Survey Staff
2010
) does not
recognize the silica bonding hypothesis. Not only is silica not mentioned in the
definitions of fragipans and fragic soil materials, but also fragic soils are not
classified in duri- great groups as are Spodosols containing ortstein and soils with
duripans and durinodes.
15.7 Summary
Soil mapping units with soils containing fragipans occupy 970,000 km
2
in the
USA, primarily in the southern Mississippi River valley and the central and
southern Appalachian Mountains. Fragipans are firm or massive and have high
bulk densities, high base saturation, and low soil organic C. There are 362 pedons
with fragipans in the NRCS database. Fragipan soils are classified primarily at
the great group but also at the subgroup level. Fragipans occur in four orders
(Alfisols, Ultisols, Inceptisols, and Spodosols), 10 suborders, 23 great groups, and
57 subgroups. From the NRCS database, fragipan soils have silty or loamy particle-
size classes (95 % of total soil series), an udic or aquic soil-moisture regime (98 %),
a mesic soil-temperature regime (60 %), and a mixed mineralogy class (75 % of
total soil series).
Soils with fragipans are derived from silt-rich materials, especially loess and
low-lime tills (61 % of total soil series) and occur primarily in lower, moderately
well-drained topographic positions. The genesis of fragipans involves wetting and
drying, possibly influenced by the presence of lithologic discontinuities, which lead
to hydro-consolidation that causes close-packing of grains and reduces porosity.
Although silica may bind clays and Fe oxides and bridge skeletal grains, this
property is not recognized in the definition of the fragipan.
The genesis of fragipans is complex and involves physical and chemical pro-
cesses. Research on fragipans in the USA increased dramatically beginning in the
1960s and peaked in the 1980s and early 1990s. Based on an examination of the
number of publications in the Thomson-Reuters Web of Science, it seems that the
research interest in fragipans in the USA has declined in the last decade, despite
their extent, their pedologic and economic importance, as well as the need for
understanding their exact genesis. This review has identified that the time required
for fragipan formation and the mechanisms for fragipan degradation are poorly
understood and could form the base for renewed systematic research into this
important soil feature of many soils in the USA and elsewhere.
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