soil-landscape relationships in virginia: i. soil variability and parent material uniformity

DIVISION S-5-SOIL GENESIS, MORPHOLOGY& CLASSIFICATION

Soil-Landscape Relationships in Virginia: I. Soil Variabilityand Parent Material Uniformity

M. H. Stolt,* J. C. Baker, and T. W. Simpson

ABSTRACTRelationships between soil development and parent material or

landscape position can often be hidden or confounded by soil varia-bility. Variability in particle size and elemental composition was ex-amined in soils within four representative toposequences in Virginia.The objectives were to partition total variability, describe lateral var-iability within horizons of the same pedon, and evaluate parent ma-terial uniformity. Total variability was partitioned among study sites,landscape positions, horizons, and random effects. Variability attrib-uted to landscape position was minimal (<8%), suggesting that parentmaterial differences or horizon differentiation may be more importantin explaining spatial variability in soils than landscape position. Lat-eral variability was substantially higher in C horizons, than in asso-ciated Bt and near-surface horizons. These differences are related tothe weathering and pedogenesis occurring in these horizons resultingin an increase in the degree of order of the soil constituents. Meancoefficients of variation ranged from 8 to 34% for particle size data,and 10 to 30% for elemental data, indicating considerable lateralvariability and supporting the need for multiple sampling within ho-rizons. Lithologic discontinuities were difficult to recognize withoutobvious field evidence. Differences in distributions of clay-free sandand Zr, or ratios of Ti/Zr and sand/silt for residual soils were relatedto differential weathering rates or slight variations in the parent rock.

SPATIAL VARIABILITY is universal to all soils and canhave many possible sources. Often spatial varia-

bility is divided into systematic and random compo-nents. Systematic variability is the result of recognizabledifferences in weathering rates, lithology, topogra-phy, or hydrology. Random variability is attributed tounrecognizable differences in these parameters, as well

Contribution of the Dep. of Crop and Soil Environmental Sci-ences, Virginia Polytechnic Institute and State Univ., Blacksburg,VA 24061-0404. Received 9 July 1991. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 57:414-̂ 21 (1993).

as differences due to sampling and laboratory error(Wilding and Drees, 1983).

Statistical methods employed to separate, distin-guish, and report systematic and random variabilitydepend on the type and objectives of the study. Ed-monds et al. (1985) and Thomas et al. (1989) usednested designs to examine variability within mappingunits. Sources of variability were reported using analysisof variance tables. Campbell (1978) employed semi-variance to examine relationships between variabilityand distance in two adjacent map units. Variance wasreported in semivariograms. Many studies examiningsampling designs have used CVs to describe soil var-iability (Ball and Williams, 1968; Harradine, 1949;Drees and Wilding, 1973; Mausbach et al., 1980).Coefficients of variation are a means by which vari-ability for a set of parameters can be expressed andcompared regardless of units or orders of magnitude.

Knowledge of the degree of parent material uni-formity is essential for studies in which changes dueto pedogenesis are quantitatively being evaluated. Par-ent material unconformities are attributed to stratifi-cation in parent rock resulting in variability in particlesize or mineralogy (Brewer, 1976), or to lithologicdiscontinuities resulting from colluvial, alluvial, oreolian additions. Clay-free particle-size distributionsare the most commonly used method to distinguishdiscontinuities (Rostad et al., 1976; Brewer, 1976;Barshad, 1964; Rutledge et al., 1975). Illuviation re-sults in redistribution of clay particles and systematicvariability between horizons.

In most instances, index mineral or elemental trends(usually zircon or the elemental equivalent Zr, or Zr/Abbreviations: CV, coefficient of variation; PSD, particle-sizedistribution; DCB, dithionite-citrate-bicarbonate; ICP, inductivelycoupled plasma spectrometry; XRF, x-ray fluorescence; SD, stan-dard deviation.

STOLT ET AL.: SOIL-LANDSCAPE RELATIONSHIPS IN VIRGINIA: I. 415

* STUDY SITE

Fig. 1. Location of study area and sites.

Ti ratios) with depth are examined in conjunction withparticle-size distributions to assess parent materialuniformity. Zirconium and Ti should be examined infractions > 2 ^tirn because of clay illuviation (Rostadet al., 1976; Brewer, 1976; Smeck and Wilding, 1980;Milnes and Fitzpatrick, 1989). The use of Ti as anindex, however, may be restricted because Ti can oc-cur as a secondary mineral such as leucoxene-coatedsand grains (Chapman and Horn, 1968; Kaup andCarter, 1987), or in weatherable minerals such as bio-tite, hornblende, and sphene (Kaup and Carter, 1987).

According to Brewer (1976), uniform parent ma-terial is material that shows less variability than thatrelated to sampling and laboratory determination ofthe soil constituents. Evaluation of variability withinsampling units such as horizons, however, has beenlimited. Drees and Wilding (1973) examined elemen-tal variability within sampling units of soils, and Smeckand Wilding (1980) reported values for particle-sizefractions within pedons formed in glacial deposits.Similar studies regarding residual soils have not beenpublished.

The overall focus of this study was to evaluate therelationships between landscape position and soil gen-esis. In order to examine these relationships, effectsrelated to random variability had to be recognized andseparated from the systematic variability associatedwith parent material, pedogenesis, or landscape po-sition. Therefore, this study was designed with thefollowing objectives: (i) to estimate and describe lat-

30-j

CO 20-i_CD

-4—'0>

E ioH

SITE 4

Bedrock

50 100 150meters

200

Fig. 2. Cross-section of Site 4. Vertical exaggeration is fivetimes. Depth to bedrock could not be determined for theentire distance between the summit and backslope, and thedashed line indicates the general trend.

B

SAMPLING DESIGNFig. 3. Diagram of sampling scheme within individual pedons

(after Dress and Wilding, 1973).

eral variability of particle size and elemental compo-sition within horizons of the same pedon; (ii) to evaluatethe degree of parent material uniformity within sitesand individual pedons; and (iii) to partition the vari-ability of the overall study into several sources, andestimate the contribution of each source to the totalvariability.

MATERIALS AND METHODSField Methods

More than 50 soil-landscape associations were examined inthe Piedmont physiographic province and Blue Ridge High-lands region of Virginia in an attempt to find relatively undis-turbed soils in which the residual portion of the soil had formedfrom a uniform parent material. Reconnaissance sites werelocated in old churchyards, cemeteries, or homesteads on soilsformed from gneissic or schistose rocks. Morphology of soiland saprolite at summit positions was examined with a bucketauger to a depth of 3.5 m or lithic contact. Samples werecollected from summit soils showing evidence of residual na-ture, uniform parent material, and minimal cultural distur-bance. Soils at backslope and footslope positions were alsoexamined. Samples were collected from these soils if thesaprolite (residual parent material) showed a similar natureto the saprolite at the summit. In the lab, PSD and sand andsilt mineralogy of the saprolite were examined to determineif the residual parent materials of the summit, backslope,and footslope soils were similar, and the saprolite was uni-form. Soil morphology and PSD were examined to establishrepresentative soils for these positions. Clay-free PSD ofsummit soils was examined for indications of lithologic dis-continuities.

Four representative toposequences were chosen for detailedstudy. Sites 1 and 2 are located in the Piedmont, and Sites 3and 4 in the Blue Ridge Highlands (Fig. 1). At each site, soilpits were excavated at the summit, backslope, and footslopepositions. Figure 2, a cross-section of Site 4, is representativeof the landscape associations studied. Profiles were describedusing the procedures described in Soil Survey Staff (1951) andGuthrie and Witty (1982). Undisturbed clods were collectedfor bulk density measurements. Bulk samples were collectedfrom each of four faces 1 m apart and labeled A, B, C, andD (Fig. 3). Samples collected from the horizon just below theA horizon, the Bt horizon of maximum clay content, and low-est C horizon accessible within the pit were bagged separately.Samples collected from the other horizons were combined,forming a composite sample for each horizon.

416 SOIL SCI. SOC. AM. J., VOL. 57, MARCH-APRIL 1993

Table 1. Description of detailed study sites.Position

SummitBackslopeFootslope




Parent materials

saprolitesaprolite

local alluvium over rock

saprolitecolluvium over saprolite

colluvium over rockSite 3

saprolitecolluvium over saprolitecolluvium over saprolite

Site 4saprolitesaprolite

colluvium over saprolite

SlopeSite 1 (Piedmont, mica gneiss)

<2% (planar)18% (planar)6% (concave)

Site 2 (Piedmont, mica gneiss)<2% (planar)7% (planar)

5% (concave)JBjue Ridge Highlands, augen gneiss)

<2% (planar)18% (planar)6% (concave)

(Blue Ridge Highlands, gneissic schist)<2% (planar)14% (planar)6% (concave)

Soil classification

clayey, oxidic, mesic Typic Hapludultclayey, oxidic, mesic Typic Hapludult

fine-loamy, mixed, mesic Typic Hapludult

clayey, oxidic, mesic Typic Hapludultclayey, oxidic, mesic Typic Hapludult

fine-loamy, mixed, mesic Typic Hapludult

fine-loamy, mixed, mesic Typic Hapludultfine-loamy, mixed, mesic Typic Hapludultfine-loamy, mixed, mesic Typic Hapludult

fine-loamy, mixed, mesic Typic Hapludultfine-loamy, mixed, mesic Typic Hapludultfine-loamy, mixed, mesic Typic Hapludult

Laboratory MethodsBulk samples were air dried, ground, and passed through a

2-mm sieve. Particle-size distribution was determined by pi-pette (Gee and Bauder, 1986), with fine clay determined aftercentrifugation. Bulk density was determined by the clod method(Brasher et al., 1966). Dithionite-citrate-bicarbonate-extract-able Fe and Al were removed following the procedures ofHolmgren (1967) and analyzed by ICP. The ICP was calibratedwith standards diluted in the same matrix as the soil extracts.Elemental Zr, Ti, Fe, Ca, and K were determined from pelletsusing XRF techniques (Jones, 1982). Pellets were preparedfrom ground silt or sand mixed in a ratio of 1:1 with boricacid. Silt and clay fractions were separated by gravimetric andcentrifugation methods and the mineralogy determined follow-ing the procedures of Jackson (1956). Kaolinite and gibbsitepercentages were determined by differential scanning calori-metry using poorly crystalline Georgia kaolinite and a Rey-nold's synthetic for kaolinite and gibbsite standards, respectively(Tan and Hajek, 1977). Percentages of the remaining mineralswere estimated using relative peak areas.

Table 2. Total soil variability explained by site, position, horizon,and error.

Variable Site Position Horizon Error

Very coarse sandCoarse sandMedium sandFine sandVery fine sandCoarse siltFine siltCoarse clayFine claySandSiltClayDCBt FeDCBt AlSand ZrSilt ZrSandTiSilt TiSand FeSilt FeSandKSilt K

4349464944474744

1756

425

5826661380

415

0000000000000020230070

———————48444646534246939278349391882644197873922869

97853

117345

10377

1329146

198

2526

Statistical MethodsThe value for an individual variable (Yljtl) can be explained

by the ideal statistical model:

= JU + St,+ PtJ + Hfj + Eijkl

t DCB = dithionite-citrate-bicarbonate extractable.

where /x represents the population mean; S, the effects due tosite; Pi} the effects due to landscape position; Hijk the effectsdue to horizon; and Eijkl is that which is due to random error.The percentage of the variability explained by each effect wasestimated using a nested design (Webster, 1977). The per-centage of the total variance attributed to each component wasestimated by dividing the component variance by the total var-iance (SAS Institute, 1985).

Analysis of variance was used to test if sample means of Chorizons were significantly different at the 0.05 level betweenlandscape positions. Significantly different means were indi-cated by least significant difference. Confidence intervals werecalculated for selected parameters at the 95% confidence level.

Study Site DescriptionsStudy sites are located in the Piedmont (Sites 1 and 2) and

Blue Ridge Highlands (Sites 3 and 4). Soils have formed incolluvium, local alluvium, saprolite, or rock (Table 1). Sap-rolite of the Piedmont sites is derived from a mica gneiss (Lov-ingston Formation, Bloomer and Werner, 1955). Saprolite,derived from an augen gneiss (Pilot Gneiss, Lewis, 1975) ora gneissic schist component of the Blue Ridge Complex Die-trich, 1954), is the parent material for residual soils at Sites 3and 4, respectively. The genesis and morphology of saprolitederived from the Lovingston Formation and Pilot Gneiss arediscussed in Stolt et al. (1992). Study sites are situated inmature woodlands. Vegetation and soil morphology suggestthat selective cutting was the only cultural disturbance affect-ing summit and backslope soils. Summit soils have slopes <2%.Slopes of backslopes range from 7 to 18%. Footslope soilsoccur on landscapes with 5 and 6% slopes. Stone lines com-posed of angular quartz gravels occur in each footslope soil.These stone lines could be traced upslope to the backslopepedons of Sites 2 and 3. The likely sources of the quartz grav-els are quartz veins that commonly occur within the parentrock, saprolite, and soil material.

RESULTS AND DISCUSSIONTotal variability is attributed to differences among study

sites, landscape positions within sites, horizons within


Table 3. Sample means of elemental content for the sand (S) and silt (Si) fractions of the residual parent material (C horizons)at each landscape position.

Position SZr SiZr STi SiTi SFe SiFe S K SiK





0.03atO.lOb0.19c

O.lSa0.14bO.lSab

O.OSa0.06a0.06a

0.04ab0.03aO.OSb

0.16aO.lOa0.12a

O.Ola0.02a0.42b

0.02a0.02a0.02a

0.02aO.Ola0.02a

0.30a0.66b0.66b

1.03a1.04a0.69a

0.95a1.03a1.17a

0.96al.OSab1.23b

0.40a0.70bl.OOc

0.60a0.69a0.67a

0.47a0.33aO.SOa

0.48a0.87b0.75c

Sitel

Site 2

Site 3

Site 4

1.88a4.74b2.62a

S.lla4.48a4.36a

0.91a1.57b2.27c

1.29a2.89b2.85b

2.55a4.49b3.54b

3.15a3.3 lab3.86b

5.32ab4.2Sa6.06b

1.72a5.33b4.27c

4.54a4.15a4.19a

3.74a3.87aS.OSa

S.lSaS.OSa4.90a

3.59a4.51b3.13a

3.03a3.21a2.85a

3.97a4.12a2.58b

2.72a2.75a2.55a

1.92a2.83b2.31c

t Sample means with different letters are significantly different at the 0.05 level.

positions, and random error (variability within horizons,and sampling and lab error). The percentage of the totalvariability contributed by landscape position is minimalto negligible (Table 2). The data suggest that, for theseupland soils in which study sites were spread amongdifferent physiographic regions and parent materials, parentmaterial and regional climate were more important thanlandscape position in explaining soil development for theparameters tested.

Of the variables examined, only sand Zr and Ti, andtotal silt have >50% of their total variability explainedby site. Sand and silt fractions have approximately equalamounts of variability explained by site and horizon (Ta-ble 2). Therefore, even though the soils occur in twodifferent regions and parent materials vary from gneissesto schists, soils and parent materials are similar. Varia-

bility in total, fine, and coarse clay; sand; DCB-extract-able Fe and Al; silt Zr, Ti, Fe, and K; and sand Fe isexplained primarily by horizon (generally >50% of totalvariability). These results are expected for Hapludults inwhich considerable weathering, leaching, and clay illu-viation have occurred.

Variability of Residual Parent Materialsamong Landscape Positions

Residual parent materials (C horizons) within sites haveformed from the same parent rock and are very similarin respect to morphology. In order to describe variationsin these horizons among landscape positions, differencesin clay-free particle size and elemental composition wereexamined for C horizons within sites (Tables 3 and 4).

Table 4. Sample means of clay-free (cf) particle-size fractionst of the residual parent material (C horizons) at each landscapeposition.

Position


cfS

83ai76a77a

CfVCS

7a3blla

cfCS

17a8b15a

cfMS

13alOalla

cfFS—— % ————

Sitel28a32a25b

cfVFS

18a23bISa

cfSi

17a24a23a

cfCSi

19a14b19a

cfFSi

7alOb14b


66a62a71a

2ala4a

5a4a8b

7a6a

Site 230a28a28a

Site 3

22a22a22a

34a38a29a

14a14aISa

20a23a14a



62a69b74c

56a58aSOa

ISa19a26b

19a17a14a

18a18a21b

13a13alla

9a9a9a

Sa6aSa

BabISb12a

Site 4lOallalla

7a7a5b

8aliblOb

38a31b26c

4a4aSa

8a7a5b

18a22a18a

30a24b21b

26a20b31c

t Abbreviations for the clay-free (cf) fractions include: S = sand, VCS = very coarse sand, CS = coarse sand, MS = medium sand, FS = finesand, VFS = very fine sand, Si = silt, CSi = coarse silt, FSi = fine silt.

t Sample means with different letters are significantly different at the O.OS level.


Table 5. Range and average coefficients of variation for particle-size fractions! by horizon across all four study sites.Horizon Sand VCS CS MS FS VFS Silt CSi FSi Clay CC FC

EBtC

1-84-181-17

6-275-55

15-87

2-125-647-47

2-592-267-64

1-112-223-65

1-114-141-19

1-152-29

Average

6-324-438-70

0-102-384-35

3-361-459-79

3-631-66

10-78

5-633-75

18-128

EBtC

3910

162843

61821

1511219

4917

61010

41115

151822

51218

111038

151136

241560

t Abbreviations include: VCS = very coarse sand, CS = coarse sand, MS = medium sand, FS = fine sand, VFS = very fine sand, CSi = coarsesilt, FSi = fine silt, CC = coarse clay, and FC = fine clay.

For the C horizons of Site 1, sand Zr and silt Ti are theonly elemental fractions significantly different at all threelandscape positions (Table 3). The other elemental frac-tions, and all particle-size fractions (Table 4), are eitherstatistically equivalent across the landscape, or one po-sition shows some variation. The C horizons in Site 2are the most uniform across the landscape, with no meanssignificantly different between all three landscape posi-tions, and few differing only between two landscape po-sitions (Tables 3 and 4).

The C horizons at Site 3 are saprolite formed from anaugen gneiss. Sand Fe is the only elemental componentthat differs significantly between all three landscape po-sitions (Table 3). Six of the other elemental fractionsshowed no difference between landscape positions. Clay-free sand and silt are significantly different between allthree positions (Table 4). Clay-free coarse and fine siltand the various sand fractions show less variability. Thesedata suggest that, although some variation in the particlesize occurs between landscape positions, elemental com-position and therefore sand and silt mineralogy are sim-ilar.

Residual parent material for the soils at Site 4 is sap-rolite formed from a gneissic schist. Particle size data ismore uniform across the landscape than elemental, withseven out of nine particle size variables showing no sig-nificant difference at the 0.05 level across the entirelandscape. Silt Ti, Fe, and K, as well as fine silt differsignificantly between the three landscape positions (Ta-ble 3), suggesting that the residual parent material variesslightly depending on landscape position.

Variability within HorizonsVariability within horizons of the same pedon (lateral

variability) was examined for three horizons within eachpedon. The C horizon has the, highest mean CV for everyparticle-size fraction analyzed (Table 5). In seven out ofthe 10 DCB and elemental parameters, C horizons alsohave the highest CV (Table 6). These data indicate thatC horizons are more variable than E or Bt horizons.Similar findings were reported by Mausbach et al. (1980).These trends are expected for Hapludults. Surface, near-surface, and Bt horizons have undergone substantialchange and ordering due to pedogenesis and weathering.Thus, these horizons show less variability than C hori-zons.

Several of the CVs for elemental Zr and fine clay inC horizons, and elemental Fe in near-surface horizons,are very high. Coefficients of variation are applicable tomost data except when a direct relationship occurs be-tween the magnitude of the parameter measured and theassociated standard deviation (Wilding and Drees, 1983).This problem arises when the values determined are atthe detection limits of the instrument or individual pro-cedure. Zirconium was determined by XRF, which hasa reported detection limit of 0.01%. Several of the Btand C horizons have Zr levels at or close to the detectionlimit. This may also explain the high CVs for fine clayin C horizons. All fine clay CVs above 75% occur inhorizons with <3% fine clay With carefully controlledconditions, total clay can be measured within 1% (Geeand Bauder, 1986). This does not, however, account for

Table 6. Range and average coefficients of variation by horizon for elemental sand (S) and silt (Si) and dithionite-citrate-bicarbonate (DCB) extractable variables across all four study sites.

HorizonTotal

DCBFeTotal

DCBA1 SZr SiZr STi SiTi SFe SiFe SK SiK

EBtC

EBtC

7-372-398-43

131123

5-233-»34-39

131022

5-249-1116-38

122720

3-18£425-128

112352

0-412-452-82

Average101621

3-74-183-41

58

16

4-1169-464-59

432425

1-243-192-26

11119

2-141-263-44

71014

4-182-130-14

86

10


Table 7. Confidence intervals at the 95% confidence level for means of sand/silt (S/Si), coarse sand/coarse silt (CoS/CoSi), silt Ti/Zr (Si Ti/Zr), sand Ti/Zr (S Ti/Zr), and weighted Ti/Zr (wt. Ti/Zr) ratios.

Horizon depth S/Si CoS/CoSi Si Ti/Zr S Ti/Zr wt. Ti/Zr

EBt3Cl

EBt3Cl

AB2Bt2Crt

EBt2C2

A2Bt2C3

BABt22C1

7-1648-63

245-420

7-2870-98

216-275

10-2467-100

100-135

7-1854-72

176-245

9-1627-48

170-241

18-3247-73

192-285

1.8-2.11.8-2.51.4-9.5

1.7-1.90.9-1.92.5-4.1

1.7-1.91.9-2.52.0-4.9

0.4-0.50.4-0.70.7-1.9

0.3-0.40.3-0.71.0-1.8

0.3-0.60.4-0.60.5-1.6

Site 1 Summit1.2-1.60.9-2.1O.l^t.2

Site 1 Backslope0.9-1.10.3-1.60.2-1.1

Site 1 Footslope0.8-1.71.1-1.61.3-2.2Site 4 Summit0.3-0.40.3-0.60.5-0.9

Site 4 Backslope0.2-0.40.2-0.40.4-0.8

Site 4 Footslope0.4-0.60.4-0.70.2-1.0

2.8-5.23.5-5.12.0-A2

3.9-7.74.1-7.94.9-15.3

4.7-5.55.0-9.02.9-17.6

8.4-15.412.6-22.817.4-40.6

21.4-37.823.2-65.229.1-61.1

11.4-27.417.0-48.628.4-77.2

1.7-6.85.2-9.07.3-12.5

1.2-2.22.3-5.93.8-9.1

0.3-3.51.3-3.10.1-8.1

8.3-11.34.4-19.7

16.3-26.6

15.8-18.812.0-18.621.5-41.5

17.0-25.011.5-37.918.3-28.9

3.1-5.04.1-6.35.1-6.9

2.5-4.33.5-6.05.0-7.8

2.6-4.22.4-4.91.0-9.8

7.6-14.65.9-22.1

15.0-26.8

21.2-28.223.0-27.226.9-43.9

16.3-24.216.4-40.723.0-36.2

the additional error introduced during the centrifugingprocess to determine fine clay.

Management of variability is an important aspect ofany soils study. One method to manage variability withina study is to increase the sample size. This can be seenin these two equations (Zar, 1984, p. 29-32):

CV = SD/x

SD = - x)2/N1'2

where X is the individual observation, x is the meanvalue, and N is the number of observations. Coefficients

of variation are dependent on the sample size becausethe CV is a function of the SD. Therefore, by subsam-pling within horizons the average variability about themean, described by the CV, is reduced. These resultsclearly demonstrate the need for multiple sampling ofhorizons in soils research. Multiple sampling, however,increases the amount of laboratory analysis. Compositesof each horizon can be formed by mixing equivalentamounts of the multiple samples. Laboratory measure-ments of composite samples of each horizon should closelyapproximate means calculated from multiple samplingand analysis (Wilding and Drees, 1983).

so

C1

backslope«Zr

1 0^1 0.2 I

50SBcfs

footslope

0.0 OJ 0.2 0.3

summit

Bt3

C1—— cfs- - - -SZr«—•— SiZr

50!SCfs

AB

2Bt

2Crt

Fig. 4. Sand Zr (SZr), silt Zr (SiZr), and clay-free sand (cfs)with depth for the summit, backslope, and footslope soils atSite 1.

Eu,100 -

Q-150H-D

E

Bt2

backslope footslope

o.o 0.1 0.2 o.o 0.1 0.2

:A2

!Bt2

C2

0 50 1005Kcfs

C3

6 50 100sscfs

BA

Bt2

2C1

Fig. 5. Sand Zr (SZr), silt Zr (SiZr), and clay-free sand (cfs)with depth for the summit, backslope, and footslope soils atSite 4.


Table 8. Silt (2-50 jim) mineralogyt of selected horizons forthe summit and backslope soils at Site 1.

Horizon

ABt3Cl

KLN

172618

QTZ

402116

MICA

Summit5

2133

GIBB

1

FLD

271821

fflV

111312

ABt3Cl

73019

472116

51333

301920

101612

t KLN = kaolinite, QTZ = quartz, GIBB = gibbsite, FLD = feldspar,HIV = hydroxy-mterlayered-vermiculite, and t = trace.

Evaluation of Parent Material UniformityParent material unconformities were recognized in the

field at the footslope position of each study site. Materialat the surface was identified as colluvium or local allu-vium that had moved downslope. Similar materials anddiscontinuities occur at the backslopes of Sites 2 and 3.No indication of lithologic discontinuities could be foundin the field for the other six pedons. To determine ifcolluvial or alluvial additions have significantly affectedthese soils, parent material uniformity was examined.

The most common methods for distinguishing lithol-ogic discontinuities are examination of clay-free PSDs,sand/silt ratios, Ti/Zr ratios, and index mineral (or theirelemental equivalent) trends with depth. Confidence in-tervals were calculated (at 95% confidence) for ratios ofsand/silt and Ti/Zr for horizons in which lateral varia-bility was examined (Table 7). Sand and silt Zr and clay-free sand distributions were also plotted with depth (Fig.4 and 5). Site 1 from the Piedmont and Site 4 from theBlue Ridge Highlands will be discussed in detail in re-gard to parent material uniformity.

Clay-free sand content and sand and silt Zr distribu-tions of the summit and backslope soils show similartrends with depth. Clay-free sand decreases from thesurface horizon to the argillic horizon, and then increasesapproaching the C material (Fig. 4 and 5). Sand and siltZr, although somewhat variable, decreases from the sur-face horizons into the argillic horizon. Below the argillichorizon minimal change in Zr occurs with depth (Fig. 4and 5). Several explanations can account for the differ-ences in rates and direction of change with depth forclay-free sand and Zr. These explanations are the pres-ence of a lithologic discontinuity, considerable variabil-ity in parent material composition, or differentialweathering within the profile. Confidence intervals ofsand/silt, coarse sand/coarse silt, silt Ti/Zr, and sand Ti/Zr ratios of the near-surface (E or A2) and Bt horizonswithin each pedon, with the exception of the sand Ti/Zrratio in the Site 1 backslope, overlap, indicating thatsample means are not significantly different (Table 7).The primary assumption in examining clay-free PSD isthat sand and silt fractions are immobile and weather atsimilar rates. Movement of sand or silt in these soils isunlikely. Thin sections of the parent materials show thatquartz, mica, and feldspar are the most common silt- andsand-size minerals. Silt mineralogy of the summit andbackslope soils of Site 1 show a dramatic decrease inquartz from the A horizons to the Bt3 horizons (Table

8). In acid environments mica and feldspar weather muchfaster than quartz, so that in this soil environment quartzwould be expected to be concentrated in the more stronglyweathered surface horizons. Clay-free PSD data suggestthat the larger quartz grains weather at a slower rate thansilt fractions. Weathering of quartz is primarily a desil-ication process. Sand has a smaller surface area than silt,so that weathering rates and desilication are greater forsilt than sand-size quartz (Drees et al., 1989). These datasuggest that, for soils composed of a diversity of min-erals, clay-free PSD may not be a reliable index of parentmaterial uniformity.

Changes in the distribution of Zr with depth do notalways indicate a change in parent material, but mayindicate a less weathered environment. Although thereis some variability, concentration of Zr can be observedin the upper 50 to 100 cm of the summit and backslopeprofiles (Fig. 4 and 5). In the soil Zr is found almostexclusively in the form of the mineral zircon, which isresistant to weathering and stable in the soil environment(Brewer, 9176; Milnes and Fitzpatrick, 1989). As othermineral forms weather from coarser fractions, zircon isconcentrated. The Zr trends suggest that the most activezone of weathering is within the upper 50 to 100 cm ofthe soils. This corresponds to the zone where clay-freesand decreases from the A to Bt horizons, and supportsthe explanation that the differences in rates or directionof change with depth of clay-free sand are primarily dueto weathering.

Confidence intervals for total sand/silt ratios for theBt3 and Cl horizons of the backslope of Site 1 do notoverlap, indicating significant differences in the meansbetween these two horizons. Distributions of clay-freesand content and sand and silt Zr show minimal changeswith depth between the Bt3 and Cl horizons. The Bt3horizon shows considerably more weathering of the siltfraction, as evidenced by the higher concentration ofmica and lower concentration of quartz and kaolinite inthe Cl horizon (Table 8). These data, as well as thedistributions of clay-free sand and Zr with depth, suggestthat differences between the Bt3 and Cl horizon are min-imal with respect to parent material uniformity.

Confidence intervals for total sand/silt ratios and sandTi/Zr ratios for the Bt2 and C3 horizons of the backslopesoil at Site 4 do not overlap, suggesting significant dif-ferences in the means. The greatest difference in ratesof change with depth for clay-free sand occurs at 72 cm(Fig. 5), which is within the BCt horizon (Stolt et al.,1993, Table 4). Evidence of relict rock structure can beobserved within this horizon, indicating the residual na-ture of this horizon and those below. Saprolite from thesesoils has formed from a gneissic schist. Thin zones (10-40 cm thick) of a more schistose, almost phylite-likematerial, are common in these soils. These zones lieparallel to the rock structure, and show less weatheringthan the primary parent materials. These thin strata mayexplain the differences in total sand/silt ratios and sandTi/Zr ratios between the Bt2 and C3 horizons.

Discontinuities were recognized in the field at the topof the 2Bt horizon in the Site 1 footslope (Fig. 4) andthe base of the Bt2 horizon of the Site 4 footslope (Fig.5). Distributions of Zr and clay-free sand, and confi-dence intervals of sand/silt or Ti/Zr ratios were not ableto establish the presence of these discontinuities. In most


cases local alluvium or colluvium is initially derived fromthe same parent rock as the residual portion of the soil,and therefore has a very similar composition. These dataand observations clearly demonstrate the difficulty in re-cognizing a discontinuity in colluvium or local alluviumwithout field evidence.

SUMMARY AND CONCLUSIONSOverall variability was distributed among study sites,

landscape positions, horizons, and random effects. Var-iability attributed to landscape position was minimal in-dicating that, for Hapludults formed in gneissic or schistosematerials, differences in parent materials or horizon dif-ferentiation may be more important in explaining spatialvariability than landscape position. Even though the soilsexamined occurred in two different regions, and parentmaterials ranged from gneiss to schist, analysis of thetotal variance indicated that the soils were similar. Theseresults are expected for Hapludults in which weatheringand pedogenesis has removed considerable evidence ofparent material differences.

Lateral variability within horizons was greatest in Chorizons. Near-surface and Bt horizons have undergonesubstantial pedogenesis, resulting in an ordering of thesoil constituents into the various horizons. Therefore,genetic horizons in developed soils have less inherentvariability than the associated parent material. Mean CVsfor several variables were >30%, indicating considera-ble lateral variability within horizons. These data suggestthat multiple sampling of horizons within a pedon is crit-ical to accurately estimate the population mean.

Examination of clay-free sand and Zr distributions withdepth, or confidence intervals for ratios of sand/silt andTi/Zr were not able to establish discontinuities in parentmaterials. Summit and backslope soils without field evi-dence of discontinuities were assumed to have formedfrom a uniform parent material. Differences in rates ordirection of change with depth for clay-free sand or Zrfor these soils were the result of differential weatheringrates, or slight variations in the parent rock. The inabilityto establish discontinuities in footslope soils with fieldevidence of multiple parent materials demonstrates thedifficulty in determining discontinuities with laboratoryindices in soils formed in colluvium or local alluvium.

soil-landscape relationships in virginia: i. soil variability and parent material uniformity

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