Soil-Geomorphic Relations on the Blue Ridge Front: II. Soil Characteristics and PedogenesisR. C. Graham* and S. W. Buol

ABSTRACTSoils along two mountain slope transects in mica gneiss and schist

terrane were studied to better understand soil distribution and gen-esis on the Blue Ridge Front landscape in North Carolina. Soils mayform entirely in residuum or colluvium, but on most slope positionsthey have developed in both materials. The colluvial/residual natureof soils is related to geomorphic position and has a strong influenceon soil properties and the evolution of the soils. Soils on upper slopeshave a significant residual component as a result of weathering intothe parent rock. Low slope positions are sites of accumulation, re-sulting in deep soils formed in colluvium. Dystrochrepts, or veryweakly expressed Hapludults, have developed entirely in colluvium,whereas soils with at least a partial component of residuum areHapludults. Soils on landscape positions down wear ing into freshmica gneiss or schist are in the micaceous mineralogy class, whereasthose in highly weathered colluvium and in very stable residuum(i.e., nearly level summit positions) have been depleted of mica andare in the oxidic class. A conceptual model relating slope processesto pedogenesis is proposed in which colluvial transport interruptsthe orderly in situ progression of residual soil development. Thegenesis and distribution of soils are best understood when studiedin landscape contexts, rather than at the level of individual pedonsor classification units.

THE BLUE RIDGE FRONT marks the junction of thePiedmont and the Blue Ridge Mountains in the

southeastern United States. Drainages with high relief,broad ridges, and a humid temperate climate providea wide range of soil forming conditions allowing thedevelopment of both Inceptisols and Ultisols. Pre-

vious studies of soil sequences on mica gneiss andschist in the Piedmont and lower Blue Ridge Moun-tains suggest that Dystrochrepts occur on steep side-slopes and Hapludults on gently sloping ridge tops(Rebertus and Buol, 1985b; Schumacher and Perkins,1987). Soils in the higher Blue Ridge Mountains areprimarily Dystrochrepts and Haplumbrepts (Mc-Cracken et al., 1962; Daniels et al., 1987), thoughLosche et al. (1970) report an aspect dependence withDystrochrepts on north facing slopes and Hapludultson south aspects.

No soil genesis studies have been published for theBlue Ridge Front (Daniels et al., 1984). Graham et al.(1990) described geomorphic processes and resultantdistributions of regolith, including colluvium, soil re-siduum, and saprolite, in an area of the Blue RidgeFront. Our objectives in this study were to describesoil characteristics and interpret soil genesis in thisBlue Ridge Front landscape.

MATERIALS AND METHODSThe study area is located immediately south of the Blue

Ridge Parkway in the Thurmond Chatham Gameland,Wilkes County, North Carolina (Graham et al., 1990, Fig.1). Eight pedons that represent changes in geomorphologyR.C. Graham, Dep. of Soil and Environmental Sciences, Univ. ofCalifornia, Riverside, CA 92521-0424; and S.W. Buol, Dep. of SoilScience, North Carolina State Univ., Raleigh, NC 27695-7619. Pa-per no. 11873 of the Journal Series of the North Carolina Agric.Res. Serv. Received 13 Oct. 1988. 'Corresponding author.

Published in Soil Sci. Soc. Am. J. 54:1367-1377 (1990).

1368 SOIL SCI. SOC. AM. J., VOL. 54, SEPTEMBER-OCTOBER 1990

and parent materials were sampled from areas with micagneiss and schist bedrock. Soils were described and sampledusing conventional procedures (Soil Survey Staff, 1981; SoilConservation Service [SCS], 1984). Samples were air driedand passed through a 2-mm sieve. Particle-size distributionwas determined by the pipet method, with centrifugation todetermine fine clay (SCS, 1984). Bulk density was deter-mined on Saran-coated clods and organic C content wasestimated using a modified Walkley-Black procedure (SCS,1984). Soil pH was measured in a 1:1 soil/water suspension,or a saturated paste for horizons with much organic matter.Exchangeable bases were leached from samples using 1 MNH4OAc, pH 7.0. Sodium and K were measured by flamephotometry, Ca and Mg by atomic absorption spectroscopy.Exchangeable acidity was measured by leaching with pH 8.2BaCl2-triethanolamine and titrating with 0.2 M HC1 (SCS,1984). Cation exchange capacity (CEC) was calculated as thesum of bases plus exchangeable acidity. Free Fe and Mnoxides were removed with a citrate-bicarbonate-dithionite(CBD) extraction (SCS, 1984) and analyzed by atomic ab-sorption spectroscopy.

To obtain clays for mineralogical analyses, soils weretreated with pH 9.5 NaCIO (5.25% w/w) to remove organicmatter (Anderson, 1963) and dispersed with an ultrasonicprobe. Clay was separated by sedimentation and decantationas prescribed by Jackson (1979). Sodium-saturated, freeze-dried clay was used for differential thermal analysis to quan-tify gibbsite and kaolin contents. Samples were heated inflowing N2 gas from 50 to 850 °C using a DuPont Series 99thermal analyzer at a rate of 50 °C min-1. Freeze-dried clayswere also analyzed with Mossbauer spectroscopy to quantifyhematite and goethite contents as described by Graham etal. (1989a, 1989b). For x-ray diffraction analysis, Mg- andK-saturated clays were smeared on glass slides (Theisen andHarward, 1962). Thin sections were prepared from soil clodsimpregnated with plastic resin (Buol and Fadness, 1961).Terminology used to describe micromorphology is that ofBrewer (1976).

A computer program using least squares to fit a con-strained 4th degree polynomial was used to identify sufficientclay increases with depth to meet argillic horizon criteria (F.Moorman and S.W. Buol, 1985, unpublished computer pro-gram). The program assesses the clay distribution from asmoothed curve fitted between midpoints of horizons. Theresults of this method may differ from those obtained bysimple visual comparison of tabulated eluvial and illuvial

horizon clay content values. For example, Pedons CF andDE fail the argillic requirements for clay increase with depthvia the computer plotting program (i.e., clay increase is toogradual), but qualify using visual comparison of numericaldata. In the visual comparison method, all parts of a givenhorizon are assumed to have the same clay content, excepttransitional zones indicated by horizon boundaries. Thecomputer program, however, calculates a distribution of claywithin each horizon based on overall profile trends; accord-ingly, part, or parts, of a horizon may have more clay thanthe tabulated value and part(s) may have less. We used theresults of the experimental computer plotting program forpedon classification.

RESULTS AND DISCUSSIONField Morphology

The locations of the studied pedons are shown inFig. 1 and are related to regolith type in Table 1, aswell as by Graham et al. (1990). Morphologic prop-erties of the pedons are presented in Table 1. All soilshave ~5 cm-thick Oi horizons composed of freshleaves and twigs overlying 3 cm-thick Oe horizons,except on the cross-dip footslope (Pedon CF), whereleaf litter was blown off (Graham et al., 1990). Re-gardless of geomorphic position, the organic-rich min-eral surface soil is consistently divided into two ho-rizons, typically the 0- to 2-cm and 2- to 8-cm depths,designated A and E, respectively. Subsoil horizonationincludes Bt or Bw horizons and is less consistent be-tween pedons. Although both residual and colluvialcomponents are present in most pedons (Graham etal., 1990), the lithologic origins of these materials werenot considered sufficiently different to warrant desig-nation of discontinuities. No buried horizons werefound in any of the soils examined.

The top two mineral horizons of all the soils have10YR hues and dark colors because of high organic-matter contents. Maximum redness is achieved in sub-soils and ranges from 10YR to 2.5YR hues. Soil red-ness is strongly dependent on easily weathered maficminerals in the parent rock that alter to hematite. Il-luvial accumulation of hematite further enhances sub-

800

750-

JUJ 700Ul

650-

TYPIC HAPLUDULTS

RUPTIC-LITHIC-ENTIC HAPLUDULTS,

& ROCK OUTCROP,

TYPICDYSTROCHREPTS/oxjdjc

0sw

200 400DISTANCE (meters)

"555"

B

TYPIC HAPLUDULTS

TYPICDYSTROCHREPTS

oxidic

0NE NW

200 400DISTANCE (meters) SE

Fig. 1. Pedon locations and soil distribution by subgroup and mineralogy class along the transected mountain slopes, (A) cross-dip slope, (B)dip slope. Pedon abbreviations and characteristics are as in Table 1.

GRAHAM & BUOL: SOIL-GEOMORPHIC RELATIONS ON THE BLUE RIDGE FRONT: II. 1369

soil redness (Graham et al., 1989b). Most saprolite hascolors similar to those of the parent rock. Primarymineral weathering is less extensive in saprolite thanin soils, and secondary Fe oxides occur largely as pseu-domorphs of the parent minerals rather than beingdisseminated to pigment the matrix, as in the soil(Graham et al., 1989a).

Loam is the most common textural class in thesesoils. The coarsest textures are loamy sands and veryfine sandy loams in Cr horizons. All of the soils havevery friable fine or very fine granular structure in theorganic-rich A horizon. Most B horizons are friableand have weak or moderate subangular blocky struc-ture. Angular blocky structure is present only in theBt3 horizon of Pedon DU, which also has the highestclay content. The C horizons are massive or have weakstructure, and Cr horizons have rock-controlled struc-ture. The soils are very porous, with well-developedpore continuity throughout. In all cases the Oe horizonis bound to the mineral soil (A horizon) by a densenetwork of very fine roots.

Soil depths, corresponding to the depth of colluviumplus soil residuum, range from 60 cm on the ridge top

(Pedon R) to more than 265 cm on the dip-slope hol-low (Pedon DH). Deepest soils are formed where col-luvium accumulates (Pedons CF and DH). Upperslope positions experience continual losses of surficialmaterial (Graham et al., 1990), and soils there are notas deep (Table 1, Pedons CB, CS, DU, and DE). PedonR, on the ridge top, is shallowest, although it is un-derlain by relatively thick saprolite. The downwardextension of soil in this completely residual situationis the result of bioturbation of the chemically weath-ered saprolite.

MicromorphologyThe B horizons of all soils had relatively high pro-

portions of silt-size grains and silasepic fabrics. Re-bertus and Buol (1985a) found this to be the charac-teristic fabric of fine-loamy Hapludults formed frommica gneiss.

Field detection of argillans was tenuous in most pe-dons because the soils are not strongly developed.Nevertheless, argillans of some kind were found in Bhorizons of all but Pedons CB and DH. In several

Table 1. Morphologic properties of the mineral soil horizons and pedon classifications.

HorizonPedon CF,

AElE2Bwl,2Bw3,4Bw5BCCl

C2,3Crl-3

Pedon CL,AEBtlBt2Bt3Bt4Crl-8

Pedon CB,AEBtlBt2CCr

Depth, cm Color (moist) Texturef

cross-dip footslope, 40% gradient0-3 10YR 3/1 13-9 10YR 3/3 si9-42 10YR 5/4 si

42-98 10YR 4/4 si98-137 10YR 4/4 vstl

137-165 10YR 4/4 1165-183 10YR 4/4 vstl183-200 10-7.5YR 5/6 vstl

200-234 10-7.5 YR 4/4 vfsl234-270 2.5Y 6/4-5Y 5/3 si

lower cross-dip backslope, 35% gradient0-2 10YR 3/1 12-9 10YR 3/3 19-32 10YR 5/4 vstvfsl

32-65 10YR 4/4 165-85 7.5YR 5/6 vfsl85-105 7.5YR 5/8 vfsl

105-300 2.5Y 6/4-5Y 5/3 vfslcross-dip backslope, 58% gradient

0-2 10YR 3/2 si2-9 10YR 3/4 gsl9-16 10YR 4/4 gsl

16-52 10YR 5/6 gvfsl52-75 2.5Y 5/4 vfsl

Structure]:

2vf, fgr2fsbk2f, msbk2msbk2msbk2msbkIf, msbkm

2vf, fgr2fsbk2fsbk2f, msbk2f, msbk2fsbkrock struc.

2vf, fgr2fgr2fsbk2f, msbkImsbk

RegolithBoundary§ typell Classification

cwgwgwddcw

cwgwgwgwdidi

awcwgwai

75+ Intact dipping bedrock slabs interrupt all horizons, except

cccccccc

cs

cccccrs

ccccr

s

Coarse-loamy,oxidic, mesicTypic Dystrochrept

(Typic Hapludult ifargillic horizon isdetermined by visualcomparison of tabulatedclay values)

Fine-loamy,oxidic, mesicTypic Hapludult

Coarse-loamy, micaceous,mesic Ruptic-Lithic-Entic Hapludult

Oxidicratio

(horizon) Mica*

0.26 22(Btl)

0.24 15(Bt2)

0.26 48(Bt2)

the A horizon, on a scale of > 1 meter.Pedon CS, cross-dip shoulder, 36% gradient

AABBtlBt2Bt3CCrR

0-2 10YR 3/2 12-12 10YR 4/4 gl

12-32 7.5YR gl32-53 7.5YR 4/6 gl53-96 7.5YR 5/6 gl96-110 2.5YR5/4 gvfsl

110-122 2.5YR5/4 vgsl122+

2vfgr2fgr, sbk2fsbk2f, msbk2msbkmrock struc.

cwcwgwgbcbcb

ccccrrs

Fine-loamy, micaceous, mesicTypic Hapludult

0.37 61(Btl)

t v = very, g = gravelly, 1 = loam, si = sandy loam, scl = sandy clay loam, Is = loamy sand, fsl = fine sandy loam, st = stony.j 1 = weak, 2 = moderate, 3 = strong; vf = very fine, f = fine, m = medium, co = coarse; gr = granular, sbk = subangular blocky, abk = angular blocky,

—» = parting to.§ a = abrupt, c = clear, g = gradual, d = diffuse, w = wavy, i = irregular, b = broken.1 c = colluvium, r = soil residuum, s = saprolite.# Fine-sand grain count frequency in the control section.

1370

Table 1. (cont.)

Horizon Depth, cm

SOIL

Color (moist)

SCI. SOC. AM. J., VOL. 54, SEPTEMBER-OCTOBER 1990

Texturef Structure^ Boundary§Regolith

typeH

Oxidicratio

Classification (horizon) Mica*

Pedon R, ridge top, 3% gradientAEBtlBt2BCCrlCr2Cr3

Pedon DU,AEBABtlBt2Bt3CCr

Pedon DE,AlA2Bwl,2BCCCr

Pedon DH,AEBwl-3

Bw4,5Bw6Bw7Bw8-10C

0-22-88-21

21-4242-6060-8484-127127+

10YR 2/110YR 4/47.5YR 4/47.5YR 4/67.5YR 4/6SYR 4/47.5YR 4/4

111glglvgslIs

2vfgr2msbk2msbk2msbk2msbkrock struc.rock struc.

awcwgbgididi

rrrrrsss

Fine-loamy, oxidic, mesic 0.38Typic Hapludult (Bt2)

36

upper dip slope, 38% gradient0-22-7

7-2020-3434-5454-9595-110110+

10YR 3/210YR 4/37.5YR 3/47.5YR 4/4SYR 4/62.5YR 4/6SYR 5/6

vglglg>glgsclgsclgsl

2vf, fgr2f, mgr2cogr, fsbk2f, msbk2msbk2f, mabkm-lvfabk

awcwcwciai

cccrrrrs

Fine-loamy, micaceous, mesic 0.38Typic Hapludult (Btl)

(possibly a Kanhapludult,but appropriate cation-exchange capacity dataare not available)

61

dip slope escarpment, 68% gradient0-22-88-70

70-9797-120120+

dip slope hollow,0-22-88-85

85-145145-185185-205205-255255-265

10YR 2/110YR 3/27.5YR 4/47.5YR 4/47.5YR 4/6

19% gradient10YR 2/210YR 4/37.5YR 4/4

7.5YR 4/67.5YR 4/610YR 5/610YR 5/4-4/410YR 5/6

vglglgl-gslvgslvgsl

11gsl

gsl-11scl1-sclgscl

2fgr2vf, fgr2msbkl-2fsbkl-2fsbk

2vfgr2fsbk2f, msbk

2-3msbk3msbk3msbk

cwdwgbaw

cwcw

dwcw

cccccs

ccc

ccc

c

Coarse-loamy, micaceous, mesic 0.37Typic Dystrochrept (Bw2)

(Typic Hapludult ifargillic horizon isdetermined by visualcomparison of tabulatedclay values)

Coarse-loamy, oxidic, mesic 0.28Typic Dystrochrept (Bw2)

63

24

t v = very, g = gravelly, 1 = loam, si = sandy loam, scl = sandy clay loam, Is = loamy sand, fsl = fine sandy loam, st = stony.11 = weak, 2 = moderate, 3 = strong; vf — very fine, f — fine, m = medium, co = coarse; gr = granular, sbk = subangular blocky, abk = angular blocky,

—» = parting to.§ a = abrupt, c = clear, g = gradual, d = diffuse, w = wavy, i = irregular, b = broken.H c = colluvium, r = soil residuum, s = saprolite.# Fine-sand grain count frequency in the control section.

pedons, argillans extended along foliation planes intothe saprolite and weathered bedrock.

Strong continuous channel ferriargillans were abun-dant in the Bt3 horizon of Pedon DU, a largely residualHapludult with the reddest Bt horizon (2. SYR 4/6)and highest clay content (31%) of any examined. Pe-don CF, formed in colluvium, just barely meets therequirements for an argillic horizon by visual com-parison of clay content values, but fails to meet thecriteria via computer plotting. We have identified thispedon as a Dystrochrept, but recognize that it may beconsidered a minimally developed Hapludult. The Bhorizons appeared weakly developed in the field, butabundant, strongly oriented channel argillans were ob-served in thin section. No other soils had channel ar-gillans, but all had free grain cutans or micaceous silt-and clay-size particles on sand-size quartz grains.

Physical PropertiesSoils that are at least partially residual (Pedons DU,

R, CS, CB, and CL) had the most distinct clay-en-

riched zones, while entirely colluvial soils (Pedons CF,DE, and DH) had more gradual clay increases (Table2). The highest clay contents in Pedon DH occurredwithin the zone of a fluctuating water table, probablya result of in situ weathering. Clay contents decreasedsharply in the Cr (saprolite) horizons. Soft saprolite(e.g., Pedon CL) contained 5 to 10% clay, while hardsaprolite had <5% clay (e.g., Pedons R and CS). Sap-rolite was considered soft if it could be crushed in onehand and hard if two hands were required to break it(Graham et al., 1990).

Comparison of fine-clay to total-clay ratios (FC/TC)within a profile may help identify eluvial and illuvialhorizons (Buol et al., 1980). Ratios of FC/TC de-creased from the thin A horizon to the E horizon andthen increased to a maximum in the B horizons (Table2), even in soils with only minor B horizon clay con-tent increases (e.g., Pedon DE). Illuviation of clay is,or has been, an active process in all of the soils.

The bulk density of unweathered mica gneiss froma cross-dip slope rock outcrop was 2.64 Mg m-3. The

GRAHAM & BUOL: SOIL-GEOMORPHIC RELATIONS ON THE BLUE RIDGE FRONT: II. 1371

Table 2. Physical and chemical properties of the soils.

CoarseDepth fragments

Horizon (cm) (>2 mm)

Pedon CFA# 0-3El# 3-9E2# 9-42Bwl# 42-67Bw2# 67-98Bw3# 98-115Bw4# 115-137Bw5# 137-165BC# 165-183Cl# 183-200

Pedon CLA# 0-2E# 2-9Btl# 9-32Bt2# 32-65Bt3# 65-85Bt4 85-105Crl 105-165Cr2 165-200

Pedon CBA# 0-2E# 2-9Btlf* 9-16Bt2# 16-52C 52-75Cr 75-80

Pedon CSA# 0-2AB# 2-12Btl# 12-32Bt2# 32-53Bt3 53-96C 96-110Cr 110-122

% (v/v)

47323

35ft35tt4

35Tt35ft

76

45ft1368

NDND

141815191044

11232229222736

Particle-sizedistributionfFinesand

Totalsand Silt Clay FC/TCf

% (w/w)

22.424.526.424.723.120.821.321.425.428.6

20.822.623.322.122.621.825.228.4

26.226.126.624.920.834.2

15.915.915.814.514.319.827.1

50.854.258.354.552.449.348.348.757.463.0

48.649.753.851.855.853.265.269.3

58.857.659.358.756.674.9

44.645.046.246.946.657.972.2

36.438.330.830.432.532.232.529.827.223.7

37.740.229.231.225.027.927.525.1

33.436.530.927.132.317.3

42.441.333.029.529.632.724.4

12.87.5

10.915.115.118.519.221.515.413.3

13.710.117.120.119.218.97.35.6

7.85.99.8

14.211.17.8

13.013.720.823.623.89.42.9

0.480.21ND0.61NDNDND0.47NDND

0.420.29ND0.73NDNDNDND

0.580.29ND0.860.97ND

0.42NDND0.65NDNDND

Bulk Organicdensity C

Mgnr3

ND1.021.301.451.461.521.611.571.481.44

ND.07.32.43.48.50.31.38

NDND1.131.251.45ND

ND1.08.34.32.45.40.75

gkg-

803354321111

7838531111

902812331

8529104221

pH

4.24.75.35.55.65.35.35.35.35.4

4.14.65.05.55.35.55.45.4

4.14.64.75.14.95.1

4.44.44.44.55.34.75.0

ExchangeableBases

1.50.40.81.41.51.31.61.91.40.4

0.90.50.41.31.10.50.10.2

1.00.40.40.80.60.2

1.30.60.91.21.30.50.2

Acidity

cmolc kg

31.117.32.84.52.01.92.83.33.31.6

26.913.04.42.84.02.400

17.810.14.43.62.41.2

24.114.26.04.84.41.66.0

CEC§(pH 8.2)

Basesatura-

tion

CBD-extractableUFe Mn

i ———— % —— g kg-i ——

32.617.73.65.93.53.24.45.24.72.0

27.813.54.84.15.12.90.10.2

18.810.54.84.43.01.4

25.414.86.96.05.72.16.2

52

2224434136373020

348

322117

100100

548

181914

54

13202324

2

8.48.57.4

11.09.6

11.712.412.310.410.4

9.510.19.5

12.313.814.43.33.0

6.69.28.58.07.21.7

16.018.820.025.026.310.74.7

0.01tr00tr

0.010.010.020.010.01

00trtrtrtr0tr

trtrtrtrtrtr

trtrtr

0.010.010.010.01

t Fine sand = 0.25-0.1 mm, total sand = 2-0.05 mm, silt = 0.05-0.002 mm, clay <0.002 mm.t Ratio of fine to total clay.§ CEC = cation exchange capacity.H CBD = citrate-bicarbonate-dithionite; for Mn, tr= detected, but <0.005.# Colluvial material.tt Includes 30% flat stones.ND = not determined.

bulk density of hard saprolite from a Cr horizon was1.75 Mg nr3 and for soft saprolite, 1.3 to 1.4 Mg m~3

(Table 2). Bulk density decreases during the transfor-mation of rock to saprolite, because chemical weath-ering releases elemental components while the fabricremains physically undisturbed. In some B horizons,bulk density was as great as 1.62 Mg nr3, particularlyin colluvial material. This may have been caused bya closer packing of grains in the quartz-rich colluvialmaterial than in the saprolite or residual soils thatcontain much sand-size, low-density, weathered bio-tite (Graham et al., 1989a). Bulk densities were verylow in A and E horizons, ~ 1.0 Mg m~3.

Chemical PropertiesChemical properties of the soils are summarized in

Table 2. Organic C content of the 2 cm-thick, dark Ahorizons was 80 to 170 g kg"1, decreasing to 30 to 50

g kg-1 in the second horizon and to <5 g kg-1 in thelower horizons. The pH of these soils was 3.7 to 4.9in the surface, 4.5 to 5.8 in the subsoil, and 5.0 to 5.7in the saprolite. The dominant exchangeable baseswere K and Mg. Low levels of Ca were present, butessentially no Na was detected. Exchangeable baseswere greatest in the surface horizon, as a result of nu-trient cycling, and in B horizons, because of the in-creased CEC associated with the higher clay contentthere. Exchangeable acidity was relatively high (10-47cmolc kg-1) in the A and E horizons, the result of H+

displaced when the acid organic matter is buffered atpH 8.2. Exchangeable acidity is much lower in the Bhorizons (2-6 cmolc kg-1), where clay minerals providethe exchange sites.

Base saturation was low in the acid surface horizons(Table 2). Subsoil base saturation was related to land-scape position and drainage. Pedon R, a residual soilon the ridge top, had <10% base saturation through-

1372

Table 2. (cont.)

SOIL SCI. SOC. AM. J.

\_\JtUO'K

Depth fragmentsHorizon (cm) (>2 mm)

Pedon RA 0-2E 2-8Btl 8-21Bt2 21-42BC 42-60Crl 60-84Cr2 84-105

Pedon DUA# 0-2E# 2-7BA# 7-20Btl 20-34Bt2 34-54Bt3 54-95C 95-110

Pedon DEAl# 0-2A2# 2-8Bwl# 8-35Bw2# 35-70BC# 70-97C# 97-120Cr 120+

Pedon DHA# 0-2E# 2-8Bwl# 8-35Bw2# 35-60Bw3# 60-85Bw4# 85-113Bw5# 113-145Bw6# 145-185Bw7# 185-205Bw8# 205-222Bw9# 222-242BwlO# 242-255C# 255-265

898

26224442

36301827302518

38212528414550

8131811181511129554

21

VOL. 54, SEPTEMBER-OCTOBER 1990

Particle-sizedistributionf

Finesand

11.915.716.215.216.024.728.6

12.613.914.814.718.118.525.2

13.714.816.915.315.818.322.2

12.716.919.818.419.420.518.817.820.012.616.816.219.7

Totalsand

32.839.441.842.047.868.778.9

43.844.649.349.152.752.560.2

45.549.652.055.162.163.781.3

37.146.558.954.455.756.151.948.155.350.147.346.862.1

Silt

47.642.036.533.832.523.518.7

40.041.135.130.522.316.122.0

43.240.234.630.923.121.311.1

43.740.227.232.129.929.831.431.824.030.728.126.421.1

Clay

19.618.621.724.219.77.82.4

16.214.315.620.425.031.417.8

11.310.213.414.014.815.07.6

19.213.313.913.514.414.116.720.120.719.224.626.816.8

FC/TCt

0.450.21ND0.580.37NDND

0.390.18NDND0.660.490.47

ND0.090.25ND0.42NDND

0.500.15ND0.21ND0.28ND0.31NDND0.46NDND

Bulkdensity

ND1.09.17.25.46.39.37

ND.21.32.42.35.36.39

ND0.981.151.381.441.40ND

NDND.36.26.33.50.62.62.59

NDNDNDND

OrganicC

16142216310

10151157433

11853103432

171357644321

NDNDNDND

txcnangeaoie

PH

3.74.34.44.65.15.15.0

4.44.34.74.95.25.65.4

4.84.75.05.55.65.65.7

4.24.95.25.65.45.45.25.15.35.65.65.65.8

Bases

1.80.60.30.50.60.30.1

1.60.80.30.51.21.30.5

6.91.30.61.21.61.91.1

5.10.90.61.00.50.50.50.82.02.02.62.82.1

Acidity

46.524.012.16.97.75.20.8

30.517.56.03.63.74.03.9

30.421.08.16.14.94.44.4

38.213.44.43.53.62.02.05.34.54.73.93.91.3

fHfR*-£•«-§(pH 8.2)

48.324.612.47.48.35.50.9

32.118.36.34.14.95.34.4

37.322.38.77.36.56.35.5

43.314.35.04.54.12.52.56.16.56.76.56.73.4

Basesatura*1

tion

422775

11

545

12242512

1967

16253020

126

1122122019133029404261

CBD-extractablell

Fe

18.629.129.435.433.824.118.8

22.126.727.530.637.957.646.3

14.318.220.119.720.722.220.1

14.920.020.318.818.818.218.119.126.940.635.035.346.9

Mn

0.010.020.010.010.050.080.10

0.040.060.030.020.020.020.01

0.160.080.040.040.040.030.05

0.020.050.040.070.040.040.050.090.100.370.180.300.86

t Fine sand = 0.25-0.1 mm, total sand = 2-0.05 mm, silt = 0.05-0.002 mm, clay <0.002 mm.i Ratio of fine to total clay.§ CEC — cation exchange capacity.11 CBD = citrate-bicarbonate-dithionite; for Mn, tr= detected, but <0.005.# Colluvial material.tt Includes 30% flat stones.ND = not determined.

put, whereas in other pedons base saturation increasedin B horizons. The increases in B horizon base satu-ration were particularly pronounced on lower slopepositions (Pedons CL, CF, and DH), where cationscarried downslope by throughflow drainage are ex-pected to accumulate (Hall, 1983).

The soils derived from gneiss (Pedons CB, CL, andCF) had less CBD-extractable Fe (free Fe oxides) thanthose derived from schist (Table 2; Pedons CS, R, DU,DE and DH). The schist has a higher proportion ofFe-bearing minerals, including readily weathered al-mandine (Graham et al., 1989a). The greatest increasein subsoil Fe oxides occurred in the partially to whollyresidual Hapludults on the upper slopes (Pedons DU,R, and CS). Soils lower on the slopes, largely devel-oped in colluvium, had a more uniform distributionof free Fe oxides, resulting from the colluvial hompg-enization of preweathered soil material. Iron oxidecontent increased below 2 m in Pedon DH, where Fe2*

transported in the groundwater oxidizes and precipi-tates in the zone of the fluctuating water table.

The CBD-extractable Mn content was also lower inthe gneiss-derived soils than in those derived fromschist (Table 2). Manganese oxide content increasedwith depth into the saprolite in Pedon R, perhaps asa result of the predominantly vertical translocationsexpected on the broad, nearly level ridge top. The high-est Mn oxide content was associated with the watertable zone below 2 m in Pedon DH. This zone ofconcentrated Mn oxides had black mottles, particu-larly around rock fragments, that effervesced with 30%H202.

MineralogyThe gneiss and schist parent rocks both contain

muscovite, biptite, chlorite, quartz, and untwinnedplagioclase. Micas and chlorite are more abundant in

GRAHAM & BUOL: SOIL-GEOMORPHIC RELATIONS ON THE BLUE RIDGE FRONT: II. 1373

Table 3. Components of the clay (<2 ion) and fine sand (0.1-0.25 mm) fractions of selected soil horizons.Clay fraction

Horizon Depth X-ray diffraction! Kaolin* Gibbsitei

2:1 Dithionite-phyllosilicates extractableand chlorite§ Goethiteti Hematitefl Fe

Fine-sand fraction

Quartz Mica.. . (U. - . ... (V...

Pedon CFEl# 3-9Bwl# 42-67Cl# 183-200

Pedon CLE# 2-9Bt2# 32-65Bt4 85-105Crl 105-165

Pedon CBE# 2-9Bt2# 16-52C 52-75

Pedon CSftAB# 2-12Bt2 32-53C 96-110

Pedon RftE 2-8Bt2 21-42Crl 60-84

Pedon DUftE# 2-7Bt3 54-95C 95-110

Pedon DEftA2# 2-8Bw2# 35-70C# 97-120

Pedon DHftE# 2-8Bw2# 35-60Bw7# 185-205

HIV,K,G,M,C,B/VHIV,K,G,M,C,B/VK,HIVw,G,M,B/V,C

HIV,K,G,M,B/V,CHIV,K,G,M,B/V,C

—G,K,B/V,M,HIVw,C

HIV,G,K,M,B/V,CHIVw,G,B/V,K,M,CG,B/V,HIV,K,M,C

HIV,G,M,K,C,B/VG,HIVw,M,K,B/V,CG,M,K,HIVw,B/V,C

K,HIV,G,M,C,B/VK,HIVw,G,M,B/V,CK,G,V,M

HIV,K,G,M,C,B/VG,K,HIVw,MG,K,B/V,HIVw,M

HIV,K,B/V,C,G,MHIV,G,K,M,B/V,CHIVw,G,K,M,B/V,C

HIV,K,M,G,C,B/VHrV,K,M,G,C,B/VHIV,K,M,G,B/V,C

89

17

211149

61617

988

222131

121616

51014

106

15

101619

7152361

101827

152237

51721

42425

61713

386

_

61-

_6147

=£30

_

52-

_49-

_38-

623229

_52-

_67-

^14-

_1314-

—14-

_19-

_21-

172022

_

17-

_15-

_

0-

_

02-

_0-

_2-

_3-

588

_4-

_4-

_8.9-

_8.4

10.0-

_9.0-

_13.2

—

_15.2-

13.618.019.2

_

13.6-

_12.2-

817657

87817262

704830

483917

776255

762122

553534

727556

172342

12162737

264766

506183

223845

247776

446464

373443

t Minerals are listed in order of decreasing peak intensity. B/V = interstratified biotite/vermiculite, C = chlorite, G = gibbsite, HIVvermiculite, HIVw = weakly hydroxy interlayered vermiculite, K = kaolin, M = mica, V = vermiculite.

t Determined by differential thermal analysis.§ Percentage shown = 100% - (kaolin + gibbsite + goethite + hematite) %.H Determined by Mossbauer spectroscopy (Graham et al., 1988b).# Colluvial material.ft Weathered almandine present in 1-5 mm fraction.

hydroxy interlayered

the schist, which also contains almandine and tracesof magnetite. The gneiss contains more magnetite andno almandine (Graham et al., 1989a).

All of the primary minerals in the parent rocks occurin the sand fraction of the soils (Graham et al., 1989a).Quartz and micaceous minerals (mainly biotite andits pseudomorphic weathering products, but also in-cluding muscovite and chlorite) made up 95 to 100%of the fine sand (Table 3).

In the saproite, biotite is pseudqmorphically alteredto interstratified biotite/vermiculite, vermiculite, ka-olinite, and a minor amount of gibbsite (Graham etal., 1989a). In grain counts, both unaltered and alteredbiotite grains were counted as mica. Fine-sand quartz-to-mica (Q/Mi) ratios (Fig. 2) were consistently muchlower, commonly <1, in the saprolite (Cr horizons)than in the overlying soil horizons. Both residual andcolluvial portions of soils have experienced biotur-bation, and colluvium has been disrupted further by

mass movement (Graham et al., 1990). Such physicaldisruption comminutes the biotite weathering prod-ucts and thus depletes them relative to the much morestable quartz in the fine sand fractions. In all but PedonDH, A horizons had the highest Q/Mi ratios, as highas 8 for Pedon CL (Fig. 2). This results from both moreintense chemical weathering in the acid, organic mat-ter-rich surface horizons, and greater physical weath-ering compared to subsoils. The slight decrease in Q/Mi ratios in the upper 35 cm of Pedon DH may resultfrom an overwash of mica-enriched material from thenearby spring-fed creek that drains the hollow.

The comminution of biotite weathering productsstrongly influences the clay fraction composition (Gra-ham et al., 1989b). Hydroxy interlayered vermiculite(HIV) or kaolinite produced the predominant x-raydiffraction peaks in all colluvial soil horizons (Table3). Gibbsite becomes a larger clay fraction componentin residual portions of soils. This was particularly true

1374 SOIL SCI. SOC. AM. J., VOL. 54, SEPTEMBER-OCTOBER 1990

QUARTZ/MICA(fine sand)

0 2 4 6 0 2 4 6 8 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4

1

2-

CrCr Cr

2 4 6 0 2 4 6 8 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4CF CL CB CS DU DE DH

Fig. 2. Depth distributions of fine-sand (0.1-0.25 mm) quartz-to-mica ratios for the sampled pedons. Pedon abbreviations and characteristicsare as in Table 1.

in Cr horizons, where gibbsite comprised up to 61%of the clay fraction (Table 3, Pedon CL). Much of thegibbsite probably originated from the weathering ofplagioclase; as suggested by Calvert et al. (1980), al-though traces of gibbsite were identified in weatheredbiotite and almandine grains from saprolite (Grahamet al, 1989a). Concurrently, Al solubilized and leachedfrom the very acid surface horizons may precipitateas gibbsite in the higher-pH environments of the lessintensively weathered residual horizons and saprolite.It is probable that Al solubility relationships, the di-lution effect of comminuted phyllosilicates and illu-viation all influence the distribution of gibbsite withinthe profiles.

Weathering of Fe-bearing primary minerals in thesoils produces goethite and hematite (Graham et al.,1989a). Goethite was prevalent (13-22%) in all ana-lyzed clay fractions of these soils (Table 3). Most goe-thite originates from biotite weathering, but it was alsodetected in weathered almandine grains by Mossbauerspectroscopy (Graham et al., 1989a).

Finely divided hematite has a strong red-pigmentingeffect in soils (Schwertmann and Taylor, 1977). Red-ness of soils analyzed in this study is highly correlatedwith hematite content (Graham et al., 1989b). He-matite was consistently present and most abundant insoils derived from almandine-bearing schist (Table 3),whereas soils developed from gneiss, which lacked al-mandine, contained little or no detectable hematite.Apparently, almandine weathers relatively rapidly, re-leasing Fe in concentrations greater than the solubilityproduct of ferrihydrite, which, according to Schwert-mann (1985), is the necessary precursor of hematite.

Mineralogy ClassAll of the pedons examined fell into one of two min-

eralogy classes: oxidic or micaceous (Table 1). Theoxidic class requires <90% quartz, <40% any othermineral used to define another mineralogy class, andan oxidic ratio >0.2. The oxidic ratio is the sum ofextractable Fe2O3 and gibbsite divided by clay (all ofthese factors being percent of the <2 mm fraction).The micaceous class requires >40% mica by weightin the 0.02- to 20-mm fraction, but, in practice, per-centage by grain count frequency is used (Soil SurveyStaff, 1975; Harris et al., 1984; Rebertus and Buol,1989).

Control-section oxidic ratios and mica contents of

the pedons analyzed in this study are presented inTable 1. Every pedon had an oxidic ratio >0.2 and,if it were not for the high mica content of some of thepedons, all of the soils would be in the oxidic min-eralogy class.

The mineralogy classes of the soils depend on theweathering history of the regolith in which the soilsformed. Their distribution by geomorphic position isshown in Fig. 1. Residual soils on the broad, nearly-level ridge top have undergone in situ weathering tosuch an extent that mica has been depleted from thesand fractions. Because the control-section mica con-tent was <40%, the soils are in the oxidic class (PedonR, Table 1). Colluyium low on the slopes has also beendepleted of sand-size mica, largely by weathering be-fore deposition (Graham et al., 1990). Thus, soils thathave control sections developed in these deposits arealso in the oxidic mineralogy class (Pedons DH, CL,and CF). Soils in the micaceous mineralogy class oc-curred on convex or linear upper slope positions over-lying fresh micaceous parent material (Pedons CB, CS,DU, and DE).

PedogenesisSoil properties used to distinguish between argillic

and cambic horizons result from pedogeriic pathwaysstrongly influenced by the geomorphic history of theregolith. Rebertus and Buol (1985a) proposed a con-ceptual model of soil genesis in residuum for the up-land mica gneiss and schist terrane of western NorthCarolina. A more comprehensive explanation of soilgenesis must take into consideration slope instabilityand the prevalence of colluvium in this area. In Fig.3, we attempt such a comprehensive explanation bymodifying the model of Rebertus and Buol (1985a) toaccount for mass movement and pedogenesis in col-luvium.Pedogenesis in Residuum

Soils in stable residuum, where there are essentiallyno losses or gains of material by mass movement, fol-low the developmental path described by Rebertus andBuol (1985a). Their study suggested that in fresh re-siduum an initial flush of clay occurs when readilyweathered minerals, such as Ca plagioclase, alter toclay minerals (Fig. 3, R-l). This clay is translocatedand illuviation cutans form, but the diagnostic sub-surface horizon is a cambic, because the clay content

GRAHAM & BUOL: SOIL-GEOMORPHIC RELATIONS ON THE BLUE RIDGE FRONT: II. 1375

PEDOGENEStS IN COLLUVIUM

C-1 INITIAL DEPOSIT Mass movement

^Stabilization

- soil material homogenized- physical disruption of weathered minerals

produces clay-size material- illuviation cutans possible- CAMBIC

Mass movement _C-2 SUBSEQUENT DEPOSITS

- soil material homogenized- weatherable minerals

depleted by prior weathering- clay depleted from surface

horizons by prior eluviation- with each colluvial cycle

(movement-deposit) illuviationbecomes less significant

- CAMBIC

PEDOGENESIS IN RESIDUUM

R-1 INITIAL FLUSH OF CLAY- easily weathered minerals (e.g., plagioclase)

produce clay- illuviation cutans form- CAMBIC

R-2 FIRST PERIOD OF MINIMAL ILLUVIATION- easily weathered minerals depleted- biotite alters pseudomorphically- pedoturbation disrupts illuviation cutans- ARGILLIC

R-3 ILLUVIATION RESUMES- biotite extensively altered (kaolinized)- comminution triggered by pedoturbation

produces abundant illuviation cutans- ARGILLIC

C-34 SECOND PERIOD OF MINIMAL ILLUVIATION

- easily comminuted, altered biotite depleted- pedoturbation destroys illuviation cutans

faster than they form- ARGILLIC

MINIMAL ILLUVIATION '- easily weathered & comminuted minerals (

(e.g., plagioclase and kaolinized biotite) idepleted

- rate of clay illuviation depends on in situweathering

- pedoturbation disrupts illuviation cutans- ARGILLIC

Fig. 3. Pathways of soil genesis related to diagnostic subsurface horizon formation in mica gneiss and schist terrane of western North Carolina.The R-1 to R-4 (Pedogenesis in Residuum) follow Rebertus and Buol (1985a).

increase is insufficient to produce an argillic horizon.By the time the easily weathered minerals are depleted,an argillic horizon has formed (Fig. 3, R-2). The re-sidual soils then enter a period during which illuvia-tion is minimal. Biotite alters pseudomorphically tovermiculite and kaolinite, but the grains remainlargely intact because they retain sufficient elasticityto resist extensive breakage; thus, production of clay-size material is minimal. At this stage of development,illuviation cutans are often sparse or absent, becausethey have been disrupted by pedoturbation. A secondperiod of illuviation begins when the biotite becomesextensively kaolinized (Fig. 3, R-3). The altered grainshave lost their elasticity and are readily broken downinto the clay size fraction by pedoturbation. Illuviationof this clay results in abundant illuviation cutans andthe further development of the argillic horizon. Whenthe easily comminuted, altered biotite is depleted, il-luviation again becomes minimal (Fig. 3, R-4). Theargillic horizon persists, but pedoturbation destroysilluviation cutans faster than they form. Another pe-riod of illuviation follows the kaolinization of mus-covite (Rebertus and Buol, 1985a). The illuviation oc-curs in intermittent periods because the susceptibilityof the primary minerals to weathering is not a contin-uum; thus, production of clay-size material does notoccur at a constant rate.Pedogenesis in Colluvium

The orderly progression of residual soil develop-ment, proposed by Rebertus and Buol (1985a) andoutlined on the right side of Fig. 3, can be interruptedat any point by mass movement. This is particularlyimportant on ihe Blue Ridge Front, where mass move-ment affects almost all of the landscape, and most ofthe regolith mantle is at least partly colluvium (Gra-

ham et al., 1990). Physical disruption during colluvialmovement homogenizes the transported material andproduces clay-size particles by breaking down chem-ically weathered mineral grains. Illuviation begins af-ter deposition and, if the colluvium is sufficiently sta-ble, illuviation cutans form (Fig. 3, C-1). Pedogenicalterations are sufficient only for the development ofa cambic horizon. However, depending on residencetime, degree of preweathering, and initial clay content,an argillic horizon can form (Fig. 3, C-3). In such cases,illuviation cutans may be absent or sparse. Easilyweathered and comminuted minerals that serve assources of clay are depleted, and illuviation cutans aredisrupted (though they may be preserved below thezone of pedoturbation).

As colluvium moves downslope, physical and chem-ical processes deplete it of the readily weathered min-erals that serve as sources of clay production. Surfacehorizons are depleted of clay-size particles by the con-tinuing processes of eluviation and, possibly, winnow-ing by raindrop impact. Illuviation becomes less sig-nificant in colluvium that has been depleted of clayand easily weathered minerals (Fig. 3, C-2). If suchhighly weathered colluvium is finally stabilized (Fig.3, C-3), argillic horizon development may be a rela-tively slow process. Argillic horizons seem to formmore quickly in the mineralogically fresh material up-slope.Illustrations of the Residual and ColluvialPedogenic Pathways

The pedons of this study can be interpreted by themodel proposed in Fig. 3. Illuviation cutans referredto in this discussion are channel argillans or ferriar-gillans detected in thin section. Pedon R is an entirelyresidual soil, with an argillic horizon but no illuviation

1376 SOIL SCI. SOC. AM. J., VOL. 54, SEPTEMBER-OCTOBER 1990

cutans. It apparently is approaching the second periodof minimum illuviation (Fig. 3, R-4). Pedon DU hasa mantle of creep colluvium, but is mostly residualand has an argillic horizon with abundant channel fer-riargillans. It appears to be in the second period ofilluviation (R-3). Pedon CS has an argillic horizonwith abundant mica and no illuviation cutans. It cor-responds most closely to the first period of minimalilluviation (R-2), even though the upper part of thepedon is colluvium (suggesting C-3). Pedon DE is insteep, unstable colluvium and has low Q/Mi ratiosindicating that the soil is not highly weathered. It hasa cambic or very weakly developed argillic horizonwithout illuviation cutans (C-l, trending toward C-3).Pedon CF is in a relatively recent, but stable colluvialdeposit. It has abundant illuviation cutans in its cam-bic or weakly developed argillic horizon (C-l, trendingtoward C-3). Pedons CB and CL have developedlargely in colluvium that has stabilized sufficiently forargillic horizon formation. These soils lack illuviationcutans and are apparently in a period of minimal il-luviation (C-3). Finally, Pedon DH has formed in sta-bilized, but highly weathered, colluviurn. It has neitherilluviation cutans nor an argillic horizon and illus-trates the condition in highly weathered colluvium,where illuviation has become relatively insignificant(C-2).

Although time-interval information is missing, thediscussion above demonstrates how the proposed ge-netic pathways may help interpret soil morphologyand genesis. Additional refinement and evidence ob-viously is needed.

SUMMARY AND CONCLUSIONSSoils along two mountain-slope transects were stud-

ied to interpret soil genesis on the Blue Ridge Frontlandscape. Parent material composition was impor-tant in determining soil color. Soils developed on al-mandine-bearing schist were much redder than thoseon gneiss, which lacked almandine. Hematite, a highlyeffective red pigmenting agent, is largely a product ofalmandine weathering in this soilscape.

Soils may form entirely in residuum or colluvium,but on most slope positions they develop in both ma-terials. In this study of two slope transects, soils de-veloped entirely in colluvium were Dystrochrepts, orvery weakly developed Hapludults, while those de-veloped at least partly in residuum were Hapludults.The colluvial/residual nature of the soils was relatedto geomorphic position and had a strong influence onsoil properties and the evolution of the soils. Soils onupper slopes had significant residual components as aresult of weathering into the parent rock. In contrast,low slope positions were sites of accumulation, re-sulting in deep soils formed in colluvium. Colluvialtransport interrupts the orderly in situ progression ofresidual soil development. Colluvial movement pro-motes the physical breakdown of primary phyllosili-cates and weathering products. This can provide aflush of illuviated clay in relatively recent colluvialdeposits; but, if the colluvium has been subject to pre-vious transport-deposition cycles, it may be so de-pleted of easily weathered minerals that clay produc-

tion and illuviation are minimal. Argillic horizonsseem to form more quickly in mineralogically freshregolith upslope than in highly weathered colluviumon low slope positions.

All soils had oxidic ratios >0.2 and all would be inthe oxidic mineralogy class if not for the high micacontent of some soils. Soils on landscape positionsoverlying fresh mica gneiss or schist were in the mi-caceous mineralogy class, whereas those in highlyweathered colluvium and in very stable residuum (i.e.,the nearly level ridge top) had been depleted of micaand were in the oxidic class.

Soils in this steeply sloping terrain often have col-luvial and residual components that are not lithol-pgically distinct. The history of these soil materials,including their weathering and redistribution by slopeprocesses, plays an important role in determining soilproperties and routes of soil development. Soils arebest understood when studied in landscape contexts,not simply as individual pedons or classification units.

CHOPART & VAUCLIN: TESTING A MODEL FOR WATER BALANCE ESTIMATION 1377