Fragipan Degradation and Nodule Formation in Glossic Fragiudalfs of the Lower Mississippi River Valley
D.L. Lindboa,
F.E. Rhotonb,
W.H. Hudnallc,
N.E. Smeckd,
J.M. Bighamd and
D.D. Tylere
a Dep. of Soil Science, North Carolina State Univ., Vernon G. James Research and Extension Center, 207 Research Station Road, Plymouth, NC 27962 USA
b USDA-ARS, National Sedimentation Lab., P.O. Box 1157, Oxford, MS 38655 USA
c Agronomy Dep., Louisiana Agricultural Experiment Station, Louisiana State Univ. Agricultural Center, Baton Rouge, LA 70803 USA
d School of Natural Resources, The Ohio State Univ., Columbus, OH 43210 USA
e Dep. of Plant and Soil Science, Univ. of Tennessee, Jackson, TN USA
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Plate. 1 Plan view of Btx1 horizon, Tate County, Mississippi, illustrating fragipan prism fragmentation. Dominant seam is 3 cm wide with numerous dendritic offshoots. Redox depletions associated with secondary structure are coalescing, thus isolating peds. Several peds appear darker due to Fe–Mn accumulations. Note 10-cm scale at right
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Plate. 2 Nodules are all that remain of the fragipan in the E'c horizon, West Feliciana Parish, Louisiana. Note the vertical orientation of nodule clusters. Tape is divided into 10-cm increments
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Plate. 3 Photomicrograph of nodules in the lower to upper B horizons. Nodule 1 appears to be fragipan matrix material and contains an argillan (A). Nodule 2 is somewhat concentric with a core (C) containing less Fe–Mn, based on its color. Nodule 3 is composed primarily of Fe–Mn rich material. Frame length is 4 mm, cross-polarized light, Bw1 horizon, West Feliciana Parish, Louisiana
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Plate. 4 Photomicrograph of a weathering rind (R) occurs on a nodule. The rind is lighter in color than the nodule, suggesting some removal of Fe and/or clay. Frame length is 4 mm, plain polarized light, E'/Bt2 horizon, Fayette County, Tennessee
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Plate. 5 Photomicrograph of the E'/Btx or Btx/E' nodules in situ indicating that they are related to each other as features such as Fe accumulations (F) can be traced from nodule to nodule (brown with yellow specks) across the eluviated matrix (yellow with white specks). Also, note that the borders of the nodules are sharp, yet they are formed in place. Frame length is 4 mm, cross-polarized light, E'/Bt1 horizon, Fayette County, Tennessee
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Plate. 6 Photomicrograph of nodules (N) in seam–E' horizon material showing silt (S) and/or argillans (A) draping or coating their borders. Frame length is 4 mm, plain polarized light, E'/Bx1 horizon, Obion County, Tennessee
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Plate. 7 Photomicrograph illustrating nodules (N) in albic material. Argillans (A) outline secondary structure indicating how the prisms have been fragmented by redox depletions (D) extending into peds, thus concentrating Fe–Mn as nodules. Frame length is 4 mm, cross-polarized light, Btx1 horizon, Yazoo County, Mississippi
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Plate. 8 Photomicrograph depicting a seam (E' or albic material) surrounded by Fe accumulations (F) that are dissected by clay coated planes or argillans (A). Frame length is 4 mm, cross-polarized light, Btx1 horizon, Tate County, Mississippi
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Plate. 9 Photomicrograph of seam (E), adjacent Fe accumulation (F), and argillans (A). The distribution of these features indicates that clay is accumulating on secondary planes, channels, and ped faces. Few Fe–Mn accumulations are directly associated with the argillans or secondary structure. Discrete nodules are rare, although some redox concentrations are barely visible within the matrix. Frame length is 4 mm, cross-polarized light, Btx1 horizon, Crockett County, Tennessee
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Fig. 2 Schematic of Stage 0 (a) macromorphological, (b) micromorphological (Fig. 7 is the key to schematics): Loess deposition and initial fragipan formation (2a). Leaching of carbonates and silicate weathering produce a brittle subsoil matrix through chemical and/or physical processes. This stage is characterized by the formation of vertical gray seams (redox depletions) with Fe- and/or Mn- oxide accumulations within the brittle matrix subjacent to the seam/matrix interface (2b). Continuous channels may be lined with argillans and/or surrounded by redox depletions superjacent to the Fe accumulations
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Fig. 3 Schematic of Stage 1 (a) macromorphological, (b) micromorphological (Fig. 7 is the key to schematics): The fragipan perches water, and most of the fragipan and the horizons above it are seasonally saturated. Saturation and oxidation of C from roots in the pores and vertical seams accelerate the chemical reduction of Fe and Mn. The mobilized Fe and Mn diffuse into the fragipan peds, encounter trapped O2, and precipitate as Fe- and Mn-oxides (Fig. 3a). During this stage, planes and ped faces exhibit Fe and Mn accumulations (hypocoatings) and clay accumulation as argillans or coatings. Redox accumulations (quasicoatings) associated with depletions in the ped matrix are rare. These depletions are relatively fine, occur in close proximity to primary structural units (seams), and rarely coalesce; thus the fragipan prisms do not appear to be fragmented (Plate 9 and Fig. 3b)
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Fig. 4 Schematic of Stage 2 (a) macromorphological, (b) micromorphological (Fig. 7 is the key to schematics): Redox concentrations and fragipan prism fragmentation (Fig. 4a). Redox depletions coalesce causing the fragipan prisms to appear fragmented. Redox concentrations occurring at ped (fragment) surfaces are accentuated. Seams widen and often contain fragipan fragments (>10 mm) and remnant nodules (<10 mm) from ped degradation (Plates 1 and 8, and Fig. 4b)
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Fig. 5 Schematic of Stage 3 (a) macromorphological, (b) micromorphological (Fig. 7 is the key to schematics): Discrete fragipan fragments (nodules) visible in horizons above the fragipan (Bt and E'/Btx or E'c horizons) (Fig. 5a). Redox depletions (albic material) continue to expand and become the matrix in which Btx fragments are now recognizable primarily as subrounded to rounded nodules although some larger (>10 mm), angular peds are still present (Plates 6 and 7, Fig. 5b)
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Fig. 6 Schematic of Stage 4 (a) macromorphological, (b) micromorphological (Fig. 7 is the key to schematics): Complete degradation of the upper fragipan and weathering of nodules (Fig. 6a). Prism fragments in the E' are rare; only nodules remain. These remnants may be spaced over several cm in the lower Bt or Bw horizons but are more closely spaced in the E' horizons. Some of the nodules contain redox depletions suggesting that they are being altered (Plates 2–5, Fig. 6b)
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Copyright © 2000 by the Soil Science Society of America.
