Strength characteristics of spodic horizons in soils of the Atlantic Coastal Plain, U.S.A.

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Strength characteristics of spodic horizons in soils of the Atlantic Coastal Plain, U.S.A. M.C. Rabenhorst and R.L. Hill

University of Maryland, College Park, MD 20742, USA ABSTRACT Rabenhorst, M.C. and Hill, R.L.,1994. Strength characteristics of spodic horizons in soils of the Atlantic Coastal Plain, U.S.A. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology,Townsville, Aus@alia,July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 855-863.

During late Pleistocene and Holocene times, eolian sands were redistributed as dunal deposits along portions of the Atlantic Coastal Plain. Under poorly drained conditions, soils formed in these sandy deposits commonly have spodic horizons. Spodic horizons of these Haplaquods (Alaquods) exhibit a range in moist consistence with some containing strongly indurated ortstein. This investigation was undertaken to elucidate the cause of ortstein induration. Field estimates of moist consistence were compared with measurements of soil strength using modulus of rupture for air dry samples and unconfined penetrometer under saturated conditions. The strength of the ortstein samples is not due to cementation by either Si or Fe, and soil strength appears to be inversely related to total organic C. Strength appears to be most closely related to the presence of monomorphic coatings of fine materials bridging framework sand grains. A pyrophosphate extractable Al phase may be present in the bridging material, but silicate clays identified in coatings in the ortstein may be partially responsible for the greater strength. Overall, the microfabric is more informative concerning the strength of the materials, than chemical characteristics of the samples. INTRODUCTION On the lower Delmarva peninsula there are areas with sandy deposits which have water tables at or near the land surface for significant periods each year. In such areas, soils may develop spodic horizons or subsoil zones enriched with illuvial organic materials which would lead to their classification as Aquods (Soil Survey Staff, 1990). In some instances these zones (Bh or spodic horizons) become strongly indurated (presumably cemented) which has important implications for the use of these soils. These soils are of limited extent on the Delmarva Peninsula but they occur throughout the Atlantic coastal plain and, in some states, are very extensive (Daniels et al., 1975; Collins, 1992). On the Delmarva Peninsula, Aquods seem to occur primarily in sediments which were initially of fluvial origin, but were later reworked during late Pleistocene or Holocene times by eolian activity. These eolian sands are reported to have been deposited during a major period of climatic change in late Pleistocene time (10 - 30,000 years ago) (Denny and Owens, 1979).

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There is, however, some evidence that eolian activity has occurred on the Delmarva Peninsula at various points throughout the Holocene. Ortstein is a common occurrence in Aquods. Lee et al., (1988a) observed the presence of ortstein associated with spodic horizons in Aquods in the coastal plain of Florida (lo00 km south of Maryland) and sought to determine the cause of the induration. They concluded that the principal reason for the induration was cementation of the coarser sand grains by Al-rich materials, and that the Al in the cemented horizons was mainly complexed by fulvic acid (Lee et al., 1988b). McHardy and Robertson (1983) also identified Al as a major constituent of coatings in cemented spodic horizons in the U.K., but in addition found lower levels of Fe and higher levels of Ca than in more weakly cemented materials. In a recent review of microscopy in Spodosols, McSweeney and FitzPatrick (1990) cite the work of several others who have related cementation in spodic horizons (development of ortstein) to various microfabrics. In general, the presence of coatings of organic-rich material (presumably complexed with Fe or Al) are indicative of a spodic horizon, and are nearly always present if the sample is indurated. Based on the work of others as well as our own preliminary research, hypotheses on the cause of induration in these spodic horizons were developed. Possible cementing agents, include Fe or Al compounds, Si compounds or complex organic compounds. The objective of this study was to test these hypotheses regarding the major cause of cementation in indurated spodic horizons by correlating chemical extractions and micromorphological observations to strength measurements of spodic horizon materials. METHODS AND MATERIALS Samples for this study were collected from sites identified during an earlier study of Aquods occurring on dunal landscapes of the lower Delmarva Peninsula (Fig. 1) (Condron, 1990). The two pedons (termed sites A and B) from which samples were collected for this study were both classified as sandy, siliceous, mesic Typic Haplaquods (Soil Survey Staff, 1990) or as Typic Alaquods following recent proposed revisions to the classification of Spodosols (ICOMOD, 1991). The soils in this study were similar in nature to those examined by Daniels et al., (1975) in that they possessed very strongly expressed spodic horizons, some of which were >2 m in thickness (Condron, 1990). Within the upper meter of the spodic horizons, the soil materials ranged in moist consistence from friable through very firm to indurated. The indurated zones were considered to be ortstein and were generally discontinuous within the horizons. Soil zones of intermediate consistence (fm-brittle) were also identified. Clods, approximately 15 to 20 cm in diameter, were collected and grouped into one of three classes, "friable", "intermediate"or "cemented." Three to six clods from each of the two pedons, were collected from each consistence class. Using a circular saw with an abrasive blade, moist clods were cut into slabs and trimmed to a size (c. 1 x 3 x 7 cm) appropriate for strength measurements using the modulus of rupture (MOR) equipment. Three to eight slabs were prepared from each clod and air dried. Strength determinations were made on each of the slabs using the modulus of rupture method of Reeve (1965). The mean air dry moisture content of the soils during this determination was 1.6% and ranged from 0.3 to 4.9%. Once the slabs were broken during the MOR determination, half of

STRENGTH CHARACTERISTICS OF SPODIC HORIZONS

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Fig. 1. Location of study site on the Delmarva Peninsula, Atlantic Coastal Plain. each broken slab was brought to saturation with distilled water, and a measurement of unconfined strength was made using a hand held penetrometer. Selected slabs were analyzed for bulk density using the paraffin-coated clod method. Total organic C was determined using dry combustion. Aluminum was determined on 0.2M sodium pyrophosphate (pH 9.5) extracts using atomic absorption spectroscopy. Soil thin sections were prepared from selected slabs representing the median strength determination for each clod. Air dried samples were impregnated using a 60:40 mixture of Casto1ite:rnonomeric styrene. After about 1 week, the samples were hardened by exposing them to a 5 Mrad dose of y radiation. Thin sections were analyzed by transmitted light microscopy and scanning electron microscopy using a JEOL model JXA-840A electron microscope/microprobe equipped with an energy dispersive spectrometer. The statistical design used in data analysis was completely randomized in which a group composed of a friable, an intermediate, and a cemented clod was treated as a replication. The location sites and the qualitative strength field assessments ( i e . friable, intermediate or cemented) were the main class variables. Multiple determinations from the slabs obtained from the same clods were used in the analysis. The determination error associated with the multiple determinations was partitioned from the general experimental error so as not to bias the analysis. Determination and experimental error were pooled when they were not significantly different (p > 0.25). A general linear model (GLM) procedure was used to perform analysis of variance on penetrometer, MOR, C, Al and bulk density measurements. Type I11 sums of squares were used for comparisons in the analysis of variance to account for unequal sample sizes where it was not possible to have complete groupings of five replications of friable, intermediate, and cemented clods. A minimum of three replications were used at all times.

M.C. RABENHORST AND R.L. HILL

858 Table 1 Summary of Statistical Significance.

Penetrometer Modulus of Total Pyrophosphate Strength Rupture Carbon Extractable Bulk Density Strength Aluminum

** ** ** ** * Site ** ** ** * * Consistence Group ** * NS ** Site x Group Interaction *, ** indicates statistical significance at 0.05 and 0.01 respectively; ns indicates nonsignificance at the 0.05 level.

Table 2 Comparisons of means of various parameters arranged by site and field moist consistence grouping. Values within the same row which are followed by the same letter are not significantly different at the 0.05 level. Site B

Parameters

Site A

Friable Intermediate Cemented Friable Intermediate Cemented Penetrometer Strength (kg/cm2) Modulus of Rupture Total Carbon Aluminum (pyrophosphate) Bulk Density

0. 126a

0.217a

0.533b

0. 184a

0.582b

1.377c

0.415ab

0.232a

0.7OOc

O.65Oc

0.464h

0.843d

1.9lbC 0.39a

1.20a 0.35a

1.40ab 0.40a

3.40d 0.76b

3.57d 0.96c

2.04c 0.94c

1.54h

1.57h

1.7Y

1.28a

1.38ab

1.58h

Means were compared following a significant F-statistic using student's t-tests. A correlation analysis was performed to determine if consistent patterns were occurring among the dependent variable measurements. All statistical procedures were performed using the SAS system for analysis (SAS, 1985). RESULTS AND DISCUSSION Strength Measurements

When penetrometer strength measurements (saturated soils) were compared for the various field (moist) consistence groups from two sites, sigdicant differences (a= 0.05) were found between the groups (Table 2). At a given site, the strength of groups were ordered, as expected, as cemented > intermediate > friable. There were also significant differences between the sampling sites (Table 1). As previously suggested, we postulate that the increased strength in the indurated samples is due to some sort of cementation.

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STRENGTH CHARACTERISTICS OF SPODIC HORIZONS

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Fig. 2. Pyrophosphate extractable aluminum vs. strength (kg/cm2) of saturated soil as determined by penetrometer. The correlation coefficient was only 0.54, but still signifcant at the 0.01 level.

Significant differences between groups were also observed in strength determinations using Modulus of Rupture (Table 2). At a given site, the "cemented" samples always had significantly greater strength than the other two groups. The friable group, however, had greater strength (though not signficantly so) than the intermediate group. This contrast in strength measurements between the penetrometer and the modulus of rupture is likely related to differences in the moisture content at the time of measurement. The penetrometer determinations were done at saturation while the MOR determinations were done at air dryness. It is postulated that greater quantities of organic matter (Table 2) or clay in the friable (when moist) samples may yield additional strength under air dry conditions. Chemical Extractions Silica Some workers have suggested that some forms of Si, such as allophane, may be constituents of grain coatings in spodosols. Sihca was therefore measured in selected samples which were extracted using the dithionite-citrate-buffer (DCB) procedure (Kittrick and Hope, 1963). The quantities of Si determined were generally below the detection limit. Therefore, the hypothesis that Si might be an important cementing agent was discarded and Si determinations were not made for the remainder of the samples.

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M.C. RABENHORST AND R.L. HILL

Fig. 3. Intergrain microaggregate microstructure and an enaulic related distribution pattern of friable sample from site A; plane polarized light (PPL); frame width (Fw)1.2 mm.

Iron Because Fe is a common constituent in spodic horizons of better drained spodosols, some have postulated that secondary Fe may contribute to the cementation and strength characteristics of these soils. The poorly drained (reducing) and highly permeable (sandy) nature of the Aquods on the Delmarva Peninsula have resulted in extractable Fe levels (DCB, pyrophosphate and acid ammonium oxalate) which commonly approach the detection h i t of the analysis (4 l g. kg-l). Levels in the horizons sampled in this study ranged from 0.0 to 0.2 g kg-'. We concluded, therefore, that Fe was not responsible for the induration observed in these materials. Total Carbon As commonly occurs in spodic horizons, the C content in these materials was high (Table 2) (mean %C = 2.2 and ranging from 0.6 to 5.7%). Within a given site there appears to be an inverse relationship between total C and moist (penetrometer) strength. Furthermore, the strongest significant differences in organic C levels were found between sites (Table 1). Therefore, while total organic C is an important component of spodic horizons, this parameter cannot explain the induration of ortstein in these soils.

Aluminum Previous work had shown that sodium pyrophosphate and acid ammonium oxalate both extract similar amounts of Al from these spodosols (Condron, 1990) Pyrophosphate extractable Al was significantly correlated with penetrometer strength although there were clear effects caused by differences between sites (Fig. 2 and Tables 1 and 2). While this relationship was significant at the 0.01 level, the correlation coefficient was only 0.54 indicating that there is a great deal of variability in strength which is not related to the Al content of the samples. As indicated in Table 2, the mean values for extractable Al were not significantly different between consistence classes at site B although there were significant differences between the cemented and friable samples in site A. Aluminum levels in samples analyzed from sites B and A in this study ranged from 0.39 to 0.76 and from 0.40 to 1.38, respectively. These values were generally higher and more variable than those observed by Lee et al., (1988a) for samples in Florida. They observed a significant difference in Dithionite-Citrate-Bicarbonate (DCB) extractable Al between indurated and friable materials with the ortstein ranging from 0.25 to

STRENGTH CHARACTERISTICS OF SPODIC HORIZONS

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Fig. 4. Pellicular or bridged grain microstructure and a chitonic-gefuric related distribution pattern of ortstein sample from site A; a) PPL; b) cross polarized light; F W 1.2 mm.

Fig. 5. Cracked organic-rich fine material coating sand grains in ortstein sample from site B, constituting a chitonic-gefuric related distribution pattern; PPL; FW 1.2 mm.

0.65% extractable Al and the friable materials ranging from 0.04 to 0.15% Al. Thus, many friable samples had higher levels of extractable Al than the ortstein samples in Florida. Micromorphology Microscopic observations were very enlightening and provided insight into fundamental differences between the cemented and friable samples. The friable samples were characterized by an intergrain microaggregate microstructure and an enaulic related distribution pattern (Fig. 3). The framework sand grains were largely uncoated. The fine material consisted of polymorphic organic material (or black or dark brown monomorphic fine material) with some small amounts of silt and clay. The fine material was loosely aggregated into silt and fine sand sized, irregularly shaped aggregates, which in some cases formed bridges between the framework grains. In distinct contrast, the ortstein samples had a pellicular or bridged grain microstructure and a chitonic or gefuric related distribution pattern (Fig. 4a). In plane light, the fine material appeared as brown or reddish-brown monomorphic organic material which coated the framework grains. Some of the coatings showed cracking patterns (Fig. 5 ) commonly reported by others (McSweeney and FitzPatrick, 1990). More commonly, the coatings were continuous

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M.C. RABENHORST AND R.L. HILL

and fairly thin (10 - 50 pm). In cross polarized light, the bulk of the coatings were isotropic, but within some, a thin coating that appears to be weakly oriented silicate clay was visible (Fig. 4b). This birefringent material was generally adjacent to the sand grains and interior to the amorphous organic-rich coatings. Scanning Electron Microscopy

Some workers studying ortstein have suggested that the material coating and bridging framework grains is enriched in A1 and have concluded that the cementing material is an Al rich amorphous compound (Lee et al., 1988a; McHardy and Robertson, 1983). In an attempt to test this hypotheses, thin sections were selected which had been prepared from contrasting samples collected from site A. One was from a strongly indurated ortstein (1.38 kg/cm2) and the other from a friable sample (0.14 kg/cm2). The fine materials coating (ortstein) or surrounding (friable) the framework sand grains were compared using SEM and EDX microchemical analyses. Using an analytical computer program coupled with the EDX system, Al and Si were quantified and Al:Si ratios were calculated at 8 to 12 points on each slide. The mean Al:Si ratios for the ortstein and friable samples were 0.47 (range 0.36 to 0.56) and 0.36 (range 0.27 to 0.43), respectively. Although there is considerable overlap in the range in Al:Si ratio, a means comparison test indicated that the Al:Si ratios of these samples were sigdicantly different at the 0.01 level. This observation, in conjunction with data on extractable Al, might lead one to conclude that an Al cementing agent might be involved in the greater strength of the ortstein. In examining the fluvial clay from the lamellae in a Bt horizon of a somewhat excessively drained soil formed in similar eolian sands approximately 1 km away, the Al:Si ratio of was determined to be 0.594, considerably higher than the fine materials in either of the two spodic horizons examined. Assuming that this clay was similar in mineralogy to the weakly oriented clay in the grain coatings of the ortstein sample (Fig. 4b), the higher Al:Si ratio in the ortstein coatings might be attributed, in part, to the presence of silicate clays. CONCLUSIONS The field classification of moist consistence of spodic horizon materials corresponds well with the unconfined saturated strength determinations by penetrometer. The indurated nature of the ortstein samples is not a result of cementation by either Si or Fe, and strength of the soil materials appears to be inversely related to total organic C content. Strength of the indurated materials is most closely related to the presence of monomorphic coatings of fine materials bridging framework sand grains. These coatings are expressed micromorphologically as chitonic-gefuric related distribution patterns. A pyrophosphate extractable Al phase may be present in the bridging material, but the levels of total Al (and Al:Si ratios) are not dramatically different from the intergranular fine material in the friable materials. Silicate clays identified in coatings in the ortstein may be partially responsible for the greater strength of these materials. Ignorance of the presence of silicate clays within the coatings may lead to the erroneous interpretation of EDX data, resulting in overestimations of the importance of amorphous Al compounds in the formation of ortstein. Overall, the related distribution of the coarse and fine materials (microfabric) is much more informative concerning the strength of the materials, than chemical characteristics of the samples.

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REFERENCES Collins, M.E., 1990. Aquods. In: J.M. Kimble and R.D. Yeck (Editors), Proceedings from the Fifth International Soil Correlation Meeting (ISCOM V) Characterization, Classification, and Utilization of Spodosols. USDA, SCS, Lincoln, Nebraska, pp. 105-122. Condron, M.A., 1990. Soils with Spodic Characteristics on the Eastern Shore of Maryland. M.S. Thesis. University of Maryland at College Park. 109 pp. Daniels, R.B., Gamble, E.E. and C.S. Holzhey., 1975. Thick Bh horizons in the N. Caroha coastal plains: Morphology and relations to texture and soil ground water. Soil Sci. SOC. Am. Proc., 39: 1177-1181. Denny, C.S. and Owens, J.P., 1979. Sand dunes on the central Delmarva Peninsula, Maryland and Delaware. U.S. Geol. Survey Prof. Paper 1067-A. ICOMOD, 1991. Circular letter #10 of the International Committee on Spodosols, with proposed modlfications to Keys to Soil Taxonomy. USDA-SCS, National Technical Center, Lincoln, Nebraska. Kittrick, J.A. and Hope, E.W., 1963. A procedure for the particle size separation of soils for XRD analysis. Soil Sci., 96: 319-323. Lee, F.Y., Yuan, T.L. and Carlisle, V.W., 1988a. Nature of cementing materials in ortstein horizons of selected Florida Spodosols: I. Constituents of cementing materials. Soil Sci. SOC.Am. J., 52: 1411-1418. Lee, F.Y., Yuan, T.L. and Carlisle, V.W., 1988b. Nature of cementing materials in ortstein horizons of selected Florida Spodosols: 11. Soil properties and chemical form(s) of aluminum. Soil Sci. SOC.Am. J., 52: 1796-1801. McHardy, W.J. and Robertson, L., 1983. An optical scanning microscopic and microanalytical study of cementation in some podzols. Geoderma, 30: 160-17 1. McSweeney, K. and FitzPatrick, E.A., 1990. Microscopic characterization of the spodic horizon. In: J.M. Kimble and R.D. Yeck (Editors), Proceedings from the Fifth International Soil Correlation Meeting (ISCOM V) Characterization, Classification, and Utilization of Spodosols. USDA, SCS, Lincoln, Nebraska, pp. 21 1-220. Reeve, R.C. 1965. Modulus of rupture. In: C.A. Black (Editor), Methods of Soil Analysis, Part 1. Agron. Monogr. 9. Am. SOC.Agron., Madison, Wisconsin, pp. 466-471. SAS, 1985. SAS User's Guide: Statistics. Version 5 Edition, Statistical Analysis Inst., Cary, North Carolina. Soil Survey Staff, 1990. Keys to Soil Taxonomy. SMSS Technical Monograph No.6, 4th editon. Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 422 pp,