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DIVISION S-5-PEDOLOGY

Fragipan Degradation and Nodule Formation in Glossic Fragiudalfs of the Lower Mississippi River Valley

^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

david_lindbo{at}ncsu.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
Glossic Fragiudalfs comprise several million hectares of agronomically important soils within the silty uplands of the lower Mississippi River Valley (Major Land Resource Area [MLRA] 134). These soils are typified by a fragipan within 100 cm of the surface overlain by a 5-cm-thick, or more, glossic horizon containing bleached coatings (albic material) along primary ped faces, indicating fragipan degradation. Concentrations of Fe–Mn nodules also occur in horizons above the fragipan. There is limited documentation regarding the in situ morphology of the nodules or their relationship to the underlying fragipan. The objectives of this study were to document (i) the profile distribution of nodules, (ii) variations in nodule morphology with depth, and (iii) the role of fragipans in nodule formation. Nodules were most common and largest directly above the fragipan horizons. Field and micromorphological observations suggested the nodules formed in remnants of the brittle fragipan matrix as a result of the conversion of Btx material to E' material (albic material) that isolated and surrounded brittle fragipan peds. The brittle peds that are the precursors to nodules appeared to decrease in size and became more highly impregnated with Fe and Mn oxides with distance above the fragipan. Spatial relationships of Fe concentrations and argillans within the nodules illustrated that the nodules were being formed in place and were not transported despite having distinct borders. With time the glossic horizon expands at the expense of the fragipan, resulting in a horizon consisting of Fe–Mn nodules and a few isolated brittle peds in a matrix of albic material.

Abbreviations: MLRA • Major Land Resource Area

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
GLOSSIC FRAGIUDALFS comprise several million hectares of agronomically important land in the silty uplands of the lower Mississippi River Valley (MLRA 134, USDA-SCS, 1981). These soils have been severely eroded in the past 150 yr due to a combination of climatic, geomorphic, and cultural factors. Erosion has been so severe in most areas that the majority of the original A, E, and much of the Bt or Bw horizons have been removed. In these eroded soils, the current Ap horizons are composed mostly of B horizon materials and often contain abundant Fe–Mn nodules (Rhoton and Tyler, 1990). In fact, erosion classes have been defined based on depth to the fragipan and concentration of Fe–Mn nodules in the surface horizons (Rhoton and Tyler, 1990; Rhoton et al., 1991).

Grenada soils (fine-silty, mixed, thermic Glossic Fragiudalfs) are common in the region and, when uneroded, are typified by an eluvial (E') horizon containing abundant Fe–Mn nodules directly above the fragipan. Previous studies suggested that fragipans are degraded by encroachment of overlying E horizons (Grossman et al., 1959; Ray, 1967; Miller et al., 1971a, 1971b; Bartelli, 1973; Buntley et al., 1977; Bullock et al., 1974). As the E horizon expands into the underlying Btx horizon, Btx horizon peds become isolated in a matrix of albic material (uncoated silt and sand grains) containing Fe–Mn nodules. The formation of albic material and fragipan degradation has commonly been attributed to redox fluctuations associated with seasonal saturation above the fragipan (Rhoton et al., 1993). Nodules formed by concentration of Fe–Mn oxides have also been considered products of redox processes; however, no direct linkage to preexisting fragipan features has been established.

Descriptions of nodules and concretions in fragipan and nonfragipan soils from the Inner Blue Grass region of Kentucky suggested two types of nodule fabric (Phillippe et al., 1972). The nodules and concretions observed in fragipan soils had undifferentiated fabric with distinct to diffuse external boundaries, whereas concretions from the nonfragipan, well-drained soils were differentiated, concentric, and spherical with sharp boundaries. These results suggest different modes of development for the nodular materials associated with fragipans.

An in-depth study of Grenada soils provided an opportunity to examine processes fostering Fe–Mn nodule formation in fragipan soils. The objectives of this study were (i) to determine the profile distribution of nodules, (ii) to examine variations in morphological characteristics of nodules with depth, and (iii) to examine the relationship between nodule formation and fragipan degradation.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
Site Characteristics
Six Grenada pedons exhibiting minimal erosion and displaying a range of morphological characteristics were selected within MLRA 134 (Fig. 1) . All samples and descriptions were obtained from a soil pit face following standard procedures (Soil Survey Staff, 1984). The upper sola of all pedons were derived from Peoria loess, which ranged in thickness from 130 to >400 cm (Table 1) . At both the Fayette and West Feliciana sites, the solum extended through the pre-Peoria loess into Coastal Plain sediments (Lindbo et al., 1997). All pedons occurred in pastures except the Crockett site, which was located in a virgin forest (Table 1). The pedons were located in backslope positions with slopes of 6% or less in gently sloping landscapes (Table 1).

View larger version (35K): [in this window] [in a new window]   Fig. 1 Site locations

Micromorphology
Samples were taken by inserting a 5 by 5 by 2.5 cm sheet metal frame into the pit face and extracting the undisturbed soil mass within the frame. Clod specimens were also collected. Orientation of both frames and clods was noted in the field. In the laboratory, samples were slowly air-dried to prevent possible formation of artifacts, impregnated under vacuum with Spurr low-viscosity resin (Polysciences Inc., Warrington, PA), and cured overnight in an oven at 70°C. Thin sections were cut and polished to a thickness of 30 µm. The terminology of Brewer (1976) was used to describe textural features, and that of Bullock et al. (1985) was used to describe fabric and amorphous features observed in petrographic examination of the sections. Additionally, angularity of nodules and peds was assessed by comparison to standard references (Bullock et al., 1985).

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
Soil Profile Descriptions and Nodule Concentration
All six pedons have fragipans with an overlying glossic horizon; however, the combined thickness of the horizons containing an E' component (albic material) above the fragipan ranges widely from 14 to 63 cm. Fine seams (tongues of albic material) that branch from large well-defined vertical seams and extend short distances (<5 cm) into the fragipan matrix generally occur with greatest frequency in the upper fragipan horizon (Plate 1) . The vertical seams become finer and occur at wider spacing with depth, sometimes extending below the fragipan.

View larger version (120K): [in this window] [in a new window]   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

View larger version (127K): [in this window] [in a new window]   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

Micromorphological Descriptions
Micromorphological observations support the findings regarding content and distribution of nodules as previously discussed. In A horizons, nodules are round to subround, moderately impregnated, and contain Fe and organic matter accumulations (Table 4) . Such nodules are most common in the Crockett pedon because the native conditions at that site have resulted in their preservation. Nodule characteristics change rapidly from the A to the Bt or Bw horizons. Although some Fe–organic nodules do persist with depth, most appear to be Fe-rich as indicated by their color (Plate 3) . Furthermore, the nodules within the Bt or Bw horizons are subround to subangular. Often, clay coatings around internal voids are evident within nodules in the Bt, Bw, and eluvial horizons. Argillans in these nodules formed around a void in the soil matrix prior to impregnation of this portion of the matrix by Fe oxides. This observation is consistent with reports of embedded argillans in peds from degraded horizons in New York Udalfs (Bullock et al., 1974). Redox depletions and weathering rinds (lighter-colored exterior areas) that occur on some nodules in the Bt or Bw suggest some alteration of these nodules (Plate 4) . Nodules in these horizons appeared randomly distributed and are minimally associated with voids.

View larger version (135K): [in this window] [in a new window]   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

View larger version (118K): [in this window] [in a new window]   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

View larger version (128K): [in this window] [in a new window]   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

View larger version (126K): [in this window] [in a new window]   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

View larger version (134K): [in this window] [in a new window]   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

View larger version (122K): [in this window] [in a new window]   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

View larger version (130K): [in this window] [in a new window]   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

Interpretations
The morphology of the nodules suggests an origin driven by redox reactions. After initial loess deposition and fragipan formation (Fig. 2) , the nodule formation begins with the reduction of Fe and Mn oxides on ped faces and along surfaces near a C source such as decaying roots (Fig. 3) . Reduction along such surfaces is evident in the fragipan. The reduced Fe and Mn then diffuse into the surrounding matrix and are oxidized. This initial stage of nodule formation is similar to the process by which Fe hypocoatings form subjacent to seams in fragipans (Gile, 1958; Lindbo and Veneman, 1993). In the present case, Fe and Mn accumulated subjacent to void surfaces in the fragipan. As the degree of redox concentration increases to the point where the entire ped face is impregnated a nodule is formed (Fig. 4 and 5) . At this stage the nodules are large, angular, and weakly impregnated, as they essentially represent the existing secondary structure of the fragipan. With time, redox depletions begin to consume the outer reaches of the nodule and the Fe–Mn oxides migrate into the interior of the ped fragment or nodule (Fig. 6) . The result is small, highly impregnated nodules surrounded by albic material. Clay coatings in some seams often roughly demarcate a subangular blocky microstructure now occupied by nodules surrounded by albic material (Plates 7 and 8) illustrate the relationship between the initial ped faces and resulting nodules.

View larger version (89K): [in this window] [in a new window]   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

View larger version (30K): [in this window] [in a new window]   Fig. 7 Key to Fig. 2–6

View larger version (86K): [in this window] [in a new window]   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)

View larger version (83K): [in this window] [in a new window]   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)

View larger version (86K): [in this window] [in a new window]   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)

View larger version (69K): [in this window] [in a new window]   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)

If we assume that the nodules represent the remnants of the fragipan horizon, then it is possible to locate approximately the original fragipan surface prior to degradation. In general, it appears that the original top of the fragipan would correspond with the horizon with the maximum nodule content. Although subjective, this allows for the approximation of erosion rates that may be useful for estimating soil loss tolerances.

	Summary

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
The distribution pattern of nodules in the E' horizon and seams within the fragipan suggest that the nodules originate as fragments of the fragipan horizon in which Fe and Mn have been concentrated. The isolated peds have fabrics and pedologic features similar to the fragipan matrix, but they are surrounded by depletion zones or siltans. Redox processes appear to control pedogenesis in the upper fragipan and superjacent horizons. Nodules are most common in the eluvial horizons. Concentrations of nodules decrease rapidly below the Btx1, although nodules are common in seams within the Btx horizons. Nodules found above the fragipan are smaller and more rounded than those at the fragipan surface and within the fragipan. These observations illustrate that the upper fragipan is degrading to brittle peds, Fe- and Mn-enriched peds, Fe–Mn nodules, and finally to an E' horizon depleted of Fe and Mn.De Coninck Favrot Tavernier Jamange 1976

	ACKNOWLEDGMENTS

 
The authors thank Mr. W.T. Brown, Mr. V.B. Campbell, Mr. F.S. Jones, and Mr. D.S. McChesney for their assistance in the field and lab. The authors also thank Ms. D.A. Kozlowski, Dr. S.W. Buol, and Dr. M.J. Vepraskas for their reviews and constructive criticism of the manuscript.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
Contribution from the USDA-ARS National Sedimentation Lab. All programs and services of the USDA are offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, marital status, or handicap.

Received for publication October 1, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 

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• Brewer R. Fabric and mineral analysis of soils. Melborne, FL: Krieger Publ. Co, 1976.
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• De Coninck F., Favrot J.-C., Tavernier R., Jamange M. Degradation dans les sols lessives hydromorphes sur materiaux argilo-sableux. Exemple des sols de la nappe detritique Bourbonnais (France). Pedologie 1976;26:105-151.
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• Lindbo D.L., Veneman P.L.M. Micromorphology of selected Massachusetts fragipan soils. Soil Sci. Soc. Am. J. 1993;57:437-442.[Abstract/Free Full Text]
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• Rhoton F.E., Bigham J.M., Schulze D.G. Properties of iron–manganese nodules from a sequence of eroded fragipan soils. Soil Sci. Soc. Am. J. 1993;57:1386-1392.[Abstract/Free Full Text]
• Rhoton F.E., Meyer L.D., McChesney D.S. Depth-of-erosion assessment using Fe–Mn nodule concentrations in surface horizons. Soil Sci. 1991;152:389-394.
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• Soil Survey Staff. Procedures for collecting soil samples and methods of analysis for soil survey. Office, Washington, DC: USDA-SCS Soil Survey Investigations Rep. 1. U.S. Gov. Print, 1984.
• USDA-SCS. Land resource regions and major land resource areas of the United States. Washington, DC: U.S. Gov. Print. Office, 1981 Agric. Handb. 296..

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