Fragipan Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 156

Fig. 15.3 Distribution of soils with fragipans by order with the conterminous USA (From
Bockheim and Hartemink 2013 )
15.5 Factors Influencing the Formation of Fragipans
Parent material is important in the formation of fragipan soils, as 41 % of the soil
series with fragipans are derived from loess and about 20 % of the soil series with
fragipans are formed in low-lime tills. About 70 % of the fragipans have loam, silt
loam, or silty clay loam soil textures, suggesting that they are rarely formed in
coarse- or very fine-textured soils. In PA fragipans are primarily loams and silt
loams (Petersen et al. 1970 ; Ciolkosz et al. 1992 ). These data suggest that silt-rich
soils are especially susceptible to fragipan formation (Table 15.3 ).
In some regions fragipan development is enhanced by lithic or paralithic con-
tacts (James et al. 1995 ; Aide and Marshaus 2002 ) or lithological discontinuities
(Smeck et al. 1989 ; Habecker et al. 1990 ; Bockheim 2003 ; Wilson et al. 2010 ). For
example, 71 % of the soil series with fragipans in MO are derived from thin loess
covering residuum or old alluvium; and 80 % of the fragipan soils in MI are
bisequal. The bisequal nature of fragipan soils in IL became more expressed as
the loess became thinner from north to south (Grossman et al. 1959a ). However, in
the present study, lithologic discontinuities or lithic/paralithic contracts occurred
in only 36 % of the fragic soil series.
Fragipan soils appear to be most common in formerly or presently forested areas
(Fig. 15.3 ). In their early map of the distribution of fragipan soils, Grossman and
Carlisle ( 1969 ) showed that the western extent of the fragipan soil zone follows the