Fragipan Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 152

Table 15.1 (continued)

Bulk
density
(g/cm 3 )

CEC
(cmol c /
kg)

Base
sat.
(%)

Depth
(cm)

Sand
(%)

Silt
(%)

Clay
(%)

OC
(%) pH

Fe d
(%)

Al d
(%) Horizon

Dickson: fine-silty, siliceous, semiactive, thermic Glossic Fragiudults (Pedon No. 98P0362)
Ap 0-23 21.2 68.1 10.7 1.52 1.00 6.4 5.2 87 1.5 0.3
Bw 23-51 17.7 64.3 18.0 1.61 0.16 6.6 6.2 66 1.5 0.4
E 51-58 15.4 64.9 19.7 1.54 0.04 4.7 6.8 34 1.7 0.3
Btx1 58-79 18.7 58.8 22.5 1.75 0.07 4.7 7.3 26 1.9 0.4
Btx2 79-102 30.3 27.1 42.6 1.67 0.06 4.2 14.5 21 3.4 0.6
2Bt1 102-140 17.2 52.9 29.9 1.56 - 4.4 9.7 16 2.8 0.5
3Bt2 140-183 47.1 31.8 21.1 1.57 - 4.3 6.2 11 1.5 0.2
3C 183-203 30.4 57.0 32.6 1.49 - 4.3 13.8 20 3.6 0.5
Savannah: fine-loamy, siliceous, semiactive, thermic Typic Fragiudults (Pedon No. 60AR12500)
A 0-5 42.7 52.4 4.9 - 2.98 4.4 8.7 31 0.3 -

E 5-15 44.6 49.9 5.5 - 0.34 4.7 3.2 13 0.3 -

Bw 15-23 40.9 49.1 10.0 - 0.34 4.6 5.1 22 0.5 -

Bt1 23-48 34.1 47.3 18.6 1.66 0.18 4.5 8.3 14 0.8 -

Bt2 48-66 32.3 48.7 19.0 1.68 0.14 4.5 8.4 13 0.8 -

Btx1 66-91 29.9 49.5 20.6 1.65 0.11 4.4 8.9 13 0.8 -

Btx2 91-122 33.1 48.0 18.9 1.66 0.08 4.7 9.1 12 1 -

2C1 122-137 32.8 38.5 28.7 1.77 0.10 4.5 16.1 27 1.2 -

total porosity. In Quebec Spodosols the mean pore diameter was 25 to 120 min

μ

non-fragic soil material but only 0.5 m in the fragipan (DeKimpe et al. 1983 ). The

smaller pore size reduced the hydraulic conductivity from 20 to 0.2 cm hr 1 and had
a major effect on plant rooting and growth. In fact, plant rooting may be limited to
the polygonal ped interfaces (Miller et al. 1971a , b ). According to Grossman
and Carlisle ( 1969 ), the defining property of fragipans is the failure exhibited by
moist material when a pressure is applied. Fragipans in Kentucky (KY) had a triple-
beam-balance strength of

μ

60 Mg m 2 , whereas non-fragic argillic horizons had
values of < 40 Mg m 2 (Norfleet and Karathanasis 1996 ). Rupture resistance was
strongly correlated with the abundance of coarse clay (Grossman and Cline 1957 ;
McCormack and Wilding 1979 ), especially hydrous mica (Knox 1957 ).
There are some key differences in chemical and mineralogical properties
between fragic and non-fragic soil materials. Fragipans often have a lower amount
of organic C and a higher base saturation than non-fragic materials (Table 15.1 ).
Some fragipans have illuviated clay. Micromorphological investigations include
argillans in-between prisms (seams) (Nikiforoff et al. 1948 ; Miller et al. 1971b ;
Habecker et al. 1990 ; Lindbo et al. 1997 , 2000 ), on ped faces and Fe-Mn nodules, in
channels or pedotubules, and in vesicles (Ransom et al. 1987 ; Smith and Callahan
1987 ; Lindbo et al. 1997 ), and zones of silt accumulation (siltans) (Habecker
et al. 1990 ; Weisenborn and Schaetzl 2005a ). Weisenborn and Schaetzl ( 2005a )
showed closely packed fabric, packing voids, and inter-grain bridging by silt,
whereas Lindbo et al. ( 1994 ) observed “proto-prisms” in thin sections of an
Oxyaquic Fraglossudalfs.