soil–landform relationships on a loess-mantled upland landscape in missouri

Soil-Landform Relationships on a Loess-Mantled Upland Landscape in MissouriF. J. Young and R. D. Hammer*

ABSTRACTSoil survey users are requesting statistically valid distributions of

soil attributes that are important for management and land use. Thehypothesis that many soil attributes vary predictably with landscapepositions was tested with 257 pedons from point transects in a 40-haupland Missouri setting. The effect of landscape position on the centraltendencies of selected soil properties was examined. Most soil proper-ties were similar between ridge and shoulder positions. Differenceswere minimal within the backslope. Backslopes differed from ridgesand shoulders, with more argillic horizon clay, thinner epipedons, andless organic C, lower pH and base saturation, and less silt on a clay-free basis. Color patterns suggest that backslopes are wetter thanridges and shoulders, with more redoximorphic activity and organicmatter accumulation on ped faces. Differences among the ridge-shoulder pedons and backslope pedons may be caused by differinghydrologic patterns as a result of interactions between topographyand the underlying glacial till.

A IMPORTANT CONCEPTUAL MODEL of soil variability isthat soils and geomorphic processes are synergistic

with resulting associations of soils with geomorphic fea-tures. Thus, specific soils are associated with specificlandforms, and soil patterns are repeating and predict-able (Simonson, 1959; Ruhe, 1975; Daniels and Ham-mer, 1992). Many workers have applied this model tolocal landscapes and have investigated relationshipsamong soils and geomorphic surfaces.

Early soil-geomorphic observations often were con-ducted in the context of the catena, a concept that de-scribes the areal associations of soils along hydrologicsequences or valley sides (Milne, 1936). Bushnell (1942)divided the soil catena into several components, eachcharacterized by a specific soil with features related tospecific erosional or hydrological conditions. The catenamodel was not precise—it included both uniform andmultiple parent materials—and was envisioned both as"a classification grouping" (Bushnell, 1942) and a "unitof mapping convenience" (Milne, 1936). Some catenaapplications, particularly those which failed to recognizethe dynamic nature of soil-geomorphic relationships,were inconsistent or inappropriate (Hall, 1983).

Ruhe (1956) expanded and refined the model by de-scribing relationships among geomorphic surfaces, un-derlying materials and soils. He related specific soils tospecific surfaces in multi-layered glacial drift in Iowaand clearly defined relationships among surface andmaterial ages with expression of certain soil attributes.Acton (1965), working in glacially modified terrain inCanada, related different soils to ice disintegration as

R.D. Hammer, Dep. of Soil and Atmospheric Sci., Univ. of Missouri,Columbia, MO 65211; and F.J. Young, USDA-NRCS, Lincoln Univ.,Jefferson City, MO. Contribution from the Missouri Agric. Exper.Stn. Journal Ser. no. 12 514. Received 2 Sept. 1996. "Correspondingauthor ([email protected]).

Published in Soil Sci. Soc. Am. J. 64:1443-1454 (2000).

affected by different landscape positions. Landscape po-sitions were characterized by differences in length,shape, gradient, and relative position of the individualslope segment. Dan and Yaalon (1968) related specific"pedomorphic forms" to "pedomorphic surfaces,"which locally controlled hydrology in Israel. They recog-nized that soils and relief are "genetically and evolution-arily interdependent" in this setting. Parsons (1978) de-scribed soil-geomorphic relationships in Oregon andnoted that degree of expression of important soil attri-butes was a function of age and intensity of weathering,both of which were locally correlated with specific geo-morphic surfaces. Pregitzer et al. (1983) documentedchanges in soils along a topographic gradient, and re-lated these changes to vegetation and nutrient status.These applications all recognized that "landform" in-cludes the underlying materials, and that a hillslopetransect may include materials of different ages andsources.

Some studies have investigated hillslope processes,particularly slope length, gradient, and distance fromsummit, as they affect distributions of soil organic C,clay, and nutrients. These include classic work by Aan-dahl (1948), Ruhe and Walker (1968), Walker and Ruhe(1968), Kleiss (1970), and Malo et al. (1974). All ofthese studies revealed that distributions of particularsoil attributes vary as functions of geomorphic and hy-drologic processes that differentially distribute water,sediments, and dissolved materials. Most studies, how-ever, have used a few "representative pedons" or other-wise relatively small sample sizes. These studies wereimportant in identifying cause-and-effect relationshipsin soil landscapes; however, they left unanswered thequestions related to magnitudes and patterns of vari-ances of soil attributes within specific soils and land-forms.

Our hypothesis is that many soil properties vary pre-dictably with landscape position, and that magnitudesof variation should vary among but are somewhat pre-dictable within landforms. Our objective is to use a largedata set to test the effect of landscape position on therelationships of soil properties with landforms within arelatively small, loess-mantled upland landscape.

METHODSStudy Area

The study area is a 40-ha, upland, tall fescue (Festuca arundi-nacea Schreb.) pasture in northwestern Boone County, Mis-souri. The field has been in pasture since before World WarII, and fertilization and liming have been minimal. Relief is18 m from the interfluve divide on the western border of thearea to the perennial stream thalweg on the east. The site wasclassified (stratified) into three landforms, "ridge," "shoul-

Abbreviations: CEC, cation-exchange capacity.

1443

1444 SOIL SCI. SOC. AM. J., VOL. 64, JULY-AUGUST 2000

Table 1. Descriptions of variables analyzed for differencesamong landforms.

Variable Description

Scale (m)

0 100 200 300Fig. 1. Geomorphic surfaces within the study area, and locations of

point transects used for sampling.

der," and "backslope" for sampling purposes (Fig. 1). Theselandforms were chosen because they are easily identified andare the most extensive in the watershed. Footslopes are inex-tensive and discontinuous and are included within the back-slope for sampling. Ridge slope gradients range from 1 to 3%,and shoulders range from 2 to 4%. Backslope gradients aregenerally 4 to 8%, with a maximum of =45%.

Soils formed in loess that varies from about 1-m thick onlower backslopes to more than 2 m on finger ridges. Thesubstratum is pre-Illinoian glacial till and includes discontinu-ous remnants of paleosols presumed to be Late Sangamon inage. Hillslope sediments of variable thickness and compositionare between the loess and the till. Most of the site is mappedin the cooperative, in-progress soil survey, as Arisburg (fine,montmorillonitic, mesic Aquertic Argiudolls), with inclusionsof Armstrong (fine, montmorillonitic, mesic Aquollic Hap-ludalfs).

SamplingThe sampling strategy was to obtain large numbers of sam-

ples from each landform while trying to equalize numbersof observations within landforms. Interlocking transects wereplaced to traverse landforms both parallel and normal to slopegradients (Fig. 1). Transect placement and sampling intervalsalong transects were determined subjectively to capture thefull range of soil variability within landforms (Young et al.,1992). Transects were straight lines, inflected where necessaryto conform to landforms. Sampling intervals along transectswere 15 m except on ridges, which had smaller areal extent,so sampling density was increased, and ridges were sampledat 7.5-m intervals from multiple, parallel transects 7.5-m apart.A total of 257 pedons was obtained as cores taken with aGiddings hydraulic soil probe (5-cm-diam. tube). Sampledepth (120 cm, the length of the sampling tube) contained

Pedon-specificMOTLDEPHCLAYMAX

CONTCLAY

ARGDEPHMOLLIC

DEPMAX

MINMAX

ORGC

Horizon-specificECEC

CEC7

OCPHBS7

MG

CASICLCLAY

Depth to iron depletionsMaximum clay content in the argillic

horizonClay content in the particle-size control

section (top SO cm of argillic)Depth to the argillic horizonThickness of the mollic epipedon (or colors

meeting mollic criteria)Depth to maximum clay content in the

pedonDifference in clay content between the

horizon with the smallest amount of clayand the horizon with the largest amountof clay

% organic carbon by weight in the upper100 cm of the pedon

Effective cation-exchange capacity,derived as the sum of the bases plusaluminum

Cation-exchange capacity by ammoniumacetate (at pH 7)

% organic carbon, by weightpH in 1:1 soil-water pasteBase saturation (at pH 7), derived as the

sum of the bases divided by the cation-exchange capacity

Ammonium acetate-extractablemagnesium

Ammonium acetate-extractable calcium% silt on a clay-free basis% clay

the taxonomic control sections. All samples were assumed tobe independent.

Cores were subdivided for description and laboratory analy-sis as follows:HorizonA-lA-2B-lB-2B-3B-4

Variable; =20 cm max.Variable; rest of A horizonUpper 15 cm of argillic horizonNext 15-cm depth incrementNext 20-cm depth incrementNext 20-cm depth increment

Sampling by genetic horizons is preferable under most con-ditions, but sampling by depth increments below the A horizonensured data base uniformity. We wanted to avoid an unbal-anced design, which would have resulted from sampling hori-zons by their thicknesses. The incremental depth samplingapproach did not mix unlike materials because changes withinthe argillic horizons were gradual. The A-l horizon is roughlyequivalent to the Ap horizon. The A-2 horizon was not in allpedons, so its sample size is smaller. Clay films distinguishedlower A from upper B horizons. Color and structure werehelpful but were not diagnostic. Most B horizons are Bt'sor Btg's, although many of the dark B-l horizons might beconsidered B/A horizons.

Analyses and VariablesAll pedons were classified in the field as being within a

ridge, shoulder, or backslope. Surface shapes (convex, plane,

1 Mention of a specific trade or product name does not necessarilymean the endorsement by the University of Missouri or the exclusionof other products.

40-

Ridge Shoulder Backslope

Slope Position

v>0)ffl

o•o0)a

oS Ridge Shoulder Backslope

Slope Position

80-i

co1Q.fl>Q ̂o Eu. u

Q.0)Q

20-

co(8 «—^

o°§O '-

.S o

0> 3

no>Ridge Shoulder Backslope

0.75-

0.5-

0.25-

Slope Position


Slope Position

50-

2-* 40-+*m.E 30->.oE3E'5m

20-

10-


co^u0)

U)

oo

oo

o

40-

30-


Slope Position s,ope positionFig. 2. Median values among landforms: (a) depth to argillic horizon, (b) mollic epipedon thickness, (c) depth to Fe depletions, (d) organic C

in the upper 100 cm of the pedon, (e) maximum clay content in the argillic horizon, (f) clay content of the particle-size control section (upper50 cm of the argillic horizon).


Table 2. Summary of nonparametric tests for significant differ-ences among landforms for medians of variables representingpedon properties.

Clay Cont Min Dep Arg MotlLand max clay max max deph Mollic Orgc deph

over1 vs. 2Ivs. 32 vs. 3

1

11

1

11

1

1

I

SI

1

11

1

11

1 = significant at 1% level; 5 = significant at 5% level; "Over" = overalltest among ridge, shoulder and backslope; "Ivs2" = ridge-shouldercomparison; "Ivs3" = ridge-backslope comparison; "2vs3" = shoul-der-backslope comparison.

or concave both in plan and profile) and slope positions (up-per, mid, and lower) were noted for all backslope pedons.

Morphological observations used standard methods (SoilSurvey Division Staff, 1993), except for "Fe depletion" and"Fe-manganese stains and concretions" codes. A "none" cate-gory was added to each, and a "gray matrix" category wasadded to the "Fe depletion" coding.

After morphological observations, all samples were air-dried and ground to pass a 2-mm sieve for the followinganalyses:

• (i) Particle size: clay, two silt fractions (coarse and fine),two sand categories (very fine sand, and fine sands andcoarser)

• (ii) Organic C• (iii) NH4OAc-extractable bases (Ca, Mg, K, Na)• (iv) CEC by NH4OAc• (v) pH (water)

These are the standard characterization analyses for a pedonsampled within the Missouri National Cooperative Soil Survey(NCSS). All analyses were by the University of Missouri SoilCharacterization Laboratory except particle-size determina-tion, which was by the modified pipette method (Indoranteet al., 1990). Soil pH was in a 1:1 soil solution suspension usingan Orion digital ionalyzer/501 pH meter1 (Orion Research,

Beverly, MA) with a combination electrode. Total soil C wasdetermined with a Leco CR 12 carbon analyzer1 (Leco Corp.,St. Joseph, MI).

Eight of the variables considered in this study are pedon-specific (i.e., are applicable to the pedon as a whole), whereasothers are horizon-specific (Table 1).

Statistical MethodsSoil property differences among landscape classes were ex-

amined for (i) landform (ridge, shoulder, and backslope); (ii)plan and profile curvature (convex, plane and concave, forpedons sampled on the backslope only); and (iii) positionalong the slope gradient (upper, mid, lower, and footslope,for pedons sampled on the backslope only).

Most properties were not normally distributed (Young etal., 1999), so the nonparametric Kruskal-Wallis method (Dan-iel, 1990) was used to detect statistically significant differencesamong landscape classes. Trial-and-error transformationswere not attempted because transformations do not alwaysnormalize data effectively (Young et al., 1992). All tests wereconducted with SYSTAT (Wilkinson, 1992) software.

RESULTSRelationships Among Landforms

Depth to the argillic horizon, mollic epipedon thick-ness, depth to Fe depletions, and organic C content wereall less for backslope pedons than for ridge and shoulderpedons (Fig. 2a-d). The maximum argillic horizon claycontent and the particle-size control section clay con-tents were greater on backslope soils than ridges andshoulder soils (Fig. 2e and f). Pedons on shoulders andridges generally were similar for these properties. Dif-ferences among landforms were highly significant forsix of the eight pedon-specific variables (Table 2). Ofthe significantly different variables, all differed between

Table 3. Summary of nonparametric tests for significant differences among landforms for medians of variables representing horizon prop-erties.

Hor

A-l

A-2

B-l

B-2

B-3

B-4

Land

overIvs. 2Ivs. 32 vs. 3over

Ivs. 2Ivs. 32 vs. 3over

Ivs. 21 vs. 32 vs. 3over



Ivs. 2Ivs. 32 vs. 3

Clay

5

1

1

111

111

151

11

Sicl1

111

111

111

111

111

11

Ca

1

155

5

5

55

111

Mg ECEC15111 1S1 1

S

151

11111

1

CEC7

1

1

1

1S

55

S

BS-71

1

1

111511151S

1

11

oc5

55

1

111

111

11

PH1

111

111

111

11

1

1S

1 = significant at 1% level; 5 = significant at 5% level; "Over" = overall test among ridge, shoulder, and backslope; "1 vs. 2" = ridge-shoulder comparison;"1 vs. 3" = ridge-backslope comparison; "2 vs. 3" = shonlder-backslope comparison.

25-

C.4-ta

.01

75-

100

o-

A1 *

A2

Sample sizes:Ridge: 99 pedonsShoulder: 32 pedonsBackslope: 106 pedons

~*— Ridge Mg

o Shoulder Mg

•-o—- Backslope Mg

-x? B 1 *

o B2*

B3*

20 25 30 35

Clay Content

40

25"

CL0) -̂Q £_ o'oU)

50-

75-

100

P A1

Sample sizes:Ridge 99 pedonsShoulder 32 pedonsBackslope: 106 pedons

B1

VP B 2 '

18 20 22 24 26 28

CEC by NfyOAc (cmol c kff 1 )

25-

aa) -—.0 E 50

u'oCO

75'

100

A 1 * Q

A 2

B1

B2* c

B3

B 4 * .•,.-•''

,-j.-''

.0 off

f' \

Sample sizes: iRidge: 99 pedons \Shoulder: 32 pedons \Backslope: 106 pedons ̂

o Eo

91 92 93 94 95 96

75


100O t>

5.5 6 6.5

O)pH (1:1 H2

Fig. 3. Continued.


25-

O.Q> -̂N

O £O 501

'5V)

75~

100

25'

Sample sizes:Ridge: 99 pedonsShoulder: 32 pedonsBackslope: 106 pedons /

B1 *

B2*

Q.o>

o(0

50

75B3

B4

15 16

25"

Q.o -^o E

u 50"oen

751

NH4OAc - extractable Ca

100

A2 .*?

B1

B2

B3*:Sample sizes:Ridge: 99 pedonsShoulder: 32 pedonsBackslope : 106 pedons

0.5 1.5 2.5

Organic C

100

Fig. 3. Depth distributions, using median values from each horizon,with ridge, shoulder, and backslope pedon medians plotted sepa-rately: (a) clay, (b) cation-exchange capacity (CEC), (c) silt on aclay-free basis, (d) pH, (e) Ca, (f ) Mg, (g) organic C. * Significantdifferences at the 5% level. The backslope differed significantlyfrom the ridge and shoulder for all variables at all depths exceptextractable Ca in the Al horizons (e); clay, CEC and organic Cin the A2 horizons (a,b, and g); pH and extractable Ca in the B3horizon (e). Shoulder CEC was significantly higher in value in theB4 horizons than the ridge and backslope (b), and ridge Ca wassignificantly less than Ca in shoulder and backslope B4 horizons(e).


1 2 3 4 5 6 7

NH4OAc - extractable Mg (cmo{.kg-

YOUNG & HAMMER: SOIL-LANDFORM RELATIONSHIPS ON AN UPLAND LANDSCAPE 1449

4-1

3-

(0Eo>_£O

B1 B2 B3 B4

2.5-1

Q>OCOI_

0>

1.5-

1 -

0.5-

coEo

HorizonFig. 4. Munsell color value means by B-horizons among landforms:

(a) ped surfaces, (b) ped interiors, (c) difference between interiorand surface. * Significant differences at the 5% level. Backslopeped surface color values were higher in the B2 and B3 horizons(a). Ped interior color values were significantly lower in the Blhorizon shoulder and higher in the B2 horizon backslope (b). Colorvalues for ped interiors were statistically different among all land-forms in the B3 horizons, and backslope ped interior values statisti-cally exceeded ridges and shoulders in the B4 horizons (b). Thedifference between mean ped interior and mean ped exterior colorvalues was significantly higher for the backslope landform in Bland B2 horizons. All mean color value differences differed statisti-cally from one another in the B3 and B4 horizons (c). D = ridge;• = shoulder; • = backslope.

ridge and backslope, but none differed between ridgeand shoulder. Shoulder and backslope differed on fiveof the eight.

Three quarters of the nine horizon-specific propertiestested in six horizons were significantly different amonglandforms (Table 3). Most pairwise differences werebetween pedons on ridges and pedons on backslopes,and between pedons on shoulders and pedons on back-slopes. As with the pedon-specific properties, most hori-zon properties did not differ between ridge and shoul-der positions.

Medians of soil properties are plotted as depth distri-bution curves, with separate curves for each landform(Fig. 3). Backslope pedons are higher in clay relativeto ridge and shoulder pedons, which are similar (Fig.3a). These differences are not consistently reflected in

the cation-exchange capacity (CEC) (Fig. 3b). Back-slope pedons have less silt (on a clay-free basis) thanridge and shoulder pedons (Fig. 3c). Soil pH differencesare more complex, with lower pH in the epipedons andupper argillic horizons of backslope pedons, and higherpH with depth (Fig. 3d). Calcium generally follows thepH trend (Fig. 3e), but Mg does not (Fig. 3f). OrganicC differences are small but significant, with larger con-centrations in backslope A-l horizons, but lesser con-centrations with depth (Fig. 3g).

Most argillic horizon color values differ among land-forms (Fig. 4). Ped surfaces (Fig. 4a) and ped interiors(Fig. 4b) are lighter in most backslope B horizons com-pared to ridges and shoulders. Furthermore, the differ-ence between interior and surface color values is greaterfor most of the backslope B horizons. Very little varia-


0)3re

2.5-1

0)uc0)I_0)

1.5-

0.5-

B3

Horizon

B4

Fig. 5. JMunsell color chroma means by B-horizons among landforms:(a) ped surfaces, (b) ped interiors, (c) difference between interiorand surface. * Significant differences at the 5% level. The meanped surface chroma in the ridge soil profiles was significantly morethan the shoulder and backslope for all horizons, but all horizonsdiffered significantly from one another (a). The mean ped interiorchromas were statistically higher in all ridge horizons except theB2. In the B4 horizon, all ped interior mean chroma colors werestatistically different (b). The difference between mean ped surfaceand mean ped interior chromas was statistically different amongall landforms in all horizons (c). D = ridge; • = shoulder; • =backslope.

tion exists in the colors of the A-l and A-2 horizons,which are uniformly* dark on this landscape.

Chromas are consistently higher (brighter) in all hori-zons for ridge pedons than for backslope pedons, bothfor ped surfaces (Fig. 5a) and for ped interiors (Fig.5b). However, the differences between ped surface andinterior chromas are greater for backslope pedons (Fig.5c). Shoulder pedons do not follow a consistent trend.

Iron-manganese stains and concretions are moreabundant in the upper B horizons of backslope pedonsrelative to ridge and shoulder pedons and follow a simi-lar (but nonsignificant) trend with depth (Fig. 6a). Irondepletions are more abundant in each backslope B hori-zon than in corresponding ridge pedon B horizons (Fig.6b), with shoulder pedons intermediate.

Relationships Within the BackslopeRelatively few differences exist within the backslope

among pedons grouped by curvature class, either planor profile. Median plan curvature values for calciumand base saturation increase in the sequence convex-plane-concave in deeper B horizons (Table 4). Differ-ences among profile curvature classes are related toparticle size. Silt is more abundant in some B horizonson plane surfaces relative to concave surfaces.

Grouping backslope pedons by position along theslope (upper, mid, lower, and footslope) resulted in onlya few significant differences among the measured prop-erties. Footslope pedons have more clay and less silt inB horizons than backslope pedons (Fig. la and b). SoilpH, base saturation, and calcium all have larger values


ucnT3c3-Q

.2+*o

5-1

4-

3-

2-

1 -

0)OCn•DC3

ji>**+*o

B1 B2 B3

Horizon

B4B1 B2 B3

HorizonFig. 6. Means by horizon among landforms of the abundance of: (a) Fe-Mn stains and concretions, and (b) Fe depletions. Ordinal values are

scaled from 1 (none) to many (4), and for Fe depletions, 5 (gray matrix). * Significant differences at the 5% level. Significantly more stainsand concretions were in the backslope Bl and B2 horizons (a). Iron depletions were significantly more abundant in all the backslope soilsthroughout all horizons. In the B2 and B3 horizons, the shoulder had signifcantly more Fe depletions than the ridge (b). D = ridge; H =shoulder; • = backslope.

in lower B horizons on footslopes relative to upslopepedons (Fig. 7c; only pH is shown).

DISCUSSIONPedogenesis has been similar on ridge and shoulder

landforms in the study area. The ridges are narrow,broadly convex, and have hydrological and pedogeniccharacteristics similar to the shoulders.

Pedogenesis has differed between ridge-shoulderpedons and backslope pedons. Differences in lessivageare reflected in more clay in the argillic horizon in back-slope pedons, in contrast to observations by Walker andRuhe (1968). Leaching and stratigraphic differences arereflected in pH, base saturation and calcium, which indi-cates that argillic horizons in backslope pedons are moreleached in the upper part, but, with increasing depth,are more strongly influenced by calcareous glacial till.Parent materials have also influenced the (clay-free) siltcontent, which is less in the hillslope sediments of thebackslopes than in the loess on the ridges. Melanizationand erosional differences between landforms are indi-cated by higher concentrations of organic C in ridgepedons, which also have thicker epipedons and darkercolors deeper into the argillic horizons. Patterns of mela-nization differ between landforms as well, with in-creased concentrations of soil profile organic matter inthe A-l horizon of backslope pedons, less with depth,and more accumulation on ped faces relative to pedonson ridges. Gleization is more obvious within backslopepedons, whereas ferrugination is more pronouncedwithin ridge-shoulder pedons. Wider color differencesbetween ped interiors and surfaces within backslopepedons reflects greater redox activity on ped surfaceswithin backslope pedons. Chemical analyses of Fe andMn compounds could substantiate these observations.

The CEC7 only partially follows the trend establishedby more direct measures of clay. Organic C and mineral-ogy affect CEC. Lesser organic C concentrations inbackslope argillic horizons relative to ridge pedons mayoffset the higher clay content within backslopes.

Magnesium is poorly correlated with other variables.Differences in CA/MG ratios within soil profiles mayresult from the influence of Ca-rich glacial till with depthin some pedons.

The landscape variables explained relatively little ofthe substantial chemical and morphological variabilitywithin the backslopes. Surface shape appears not tohave greatly affected pedogenesis on the backslope.Workers on other landscapes have observed that con-cave areas are wetter (Anderson and Burt, 1978; Sinaiet al., 1981; Boyer et al., 1990), with concomitant differ-ences in drainage class (Troeh, 1964), Fe/Mn ratios (Mc-

Table4. Medians of variables that are significantly different(P < 0.05) among plan and profile curvature classes within thebackslope. Pairwise significance is shown, where 1 = convex,2 = plane, and 3 = concave.

MediansVariable Horizon Signif. Convex Plane ConcavePlan curvatureCa

OCBS7BS7

MOTLDEPH

B-4

B-2B-3B-4

_

Ivs. 32 vs. 3Ivs. 3Ivs. 3Ivs. 32 vs. 3Ivs. 2Ivs. 3

12.7

0.67477

46

13

80

65

14.7

0.58386.5

50

Profile curvatureSICLSICLCLAYCLAY

B-lB-3A-2B-4

2 vs. 32 vs. 32 vs. 31 vs. 3 32.9

94.395.428.5

939325.333.5


£o

10-

28"

44"

60-

78"

98"

A1Midslope

•*••— Footslope

Sample sizes:Midslope: 61 pedonsFootslope: 6 pedons

10

28-

^ 44-

O

§" 60O

78-

98"

A2

Sample sizes:Upper backslope: 10 pedonsMiddle backslope: 61 pedonsLower backslope: 28 pedonsFootslope: 6 pedons

B4'

20 25 30 35 40 45

Clay (%)

Eo

10"

28'

44"

60-

78-

98

«\

Sample sizes: \Midslope: 61 pedons \

v fi nerinnR \Footslope: 6 pedons

A1

-i

6 A2

B1

B2'

P B3'

5.5 6 6.5 7 7.5

O)pH (1:1 H2

Fig. 7. Depth distributions, using median values from each horizon,with different landscape positions plotted separately: (a) clay, (b)silt on a clay-free basis, (c) pH. * Significant differences at the 5%level. Clay content was significantly less in the midslope landscapeposition than the footslope within the Bl and 64 horizons (a). Silton a clay-free basis was signifantly less abundant in all footslopehorizons except the A2 (b). The pH in water was significantlyhigher in the backslope Bl and B2 horizons, and in the footslopeB3 and B4 horizons.

70 75 80 85 90 95

Silt (%)

Daniel et al., 1992), and depth to calcium carbonate(Pennock and de Jong, 1990). Concave areas usuallyare less eroded or have received hillslope sediments

(Kreznor et al., 1989), resulting in thicker A horizons(Pennock et al., 1987; Pennock and de Jong, 1990). Sucheffects were not observed in this study. Relative youth


of the landscape and low relief may be important. Per-haps the subtle convexities and concavities have devel-oped under post-settlement management during the19th and early 20th centuries.

The few differences detected along the hillslope gradi-ent are related to parent materials and reflect the influ-ence of calcareous glacial till in the lowest slope posi-tions. Other studies have demonstrated systematicchanges along slope gradients, including sedimentationprocesses and hydrologic gradients resulting in increas-ing C content downslope (Walker and Ruhe, 1968;Kleiss, 1970; Malo et al., 1974; Schimel et al., 1985;Honeycutt et al., 1990; Pennock and de Jong, 1990; Pier-son and Mulla, 1990), and particle-size differences(Ruhe and Walker, 1968; Walker and Ruhe, 1968;Walker et al., 1968; Kleiss, 1970; Malo et al., 1974).However, many of these studies were conducted inclosed systems. On hillslopes of this study area, pedonson footslope positions are not greatly influenced bydeposition, despite the decrease in slope gradient andconcave slope profile. Apparently, erosional productsare moving out of this open system.

Two hypotheses can be presented to explain pedo-genic differences between ridge-shoulder pedons andbackslope pedons. One is change in vegetation history.Ridg;s may have been predominantly prairie, whereasbackslopes may have had longer developmental periodsunder forest. Ridges are drier, more windswept, andmore susceptible to fire. Soil differences that supportthis hypothesis are (i) more abundant silt coats in theA-2 horizons of backslope pedons; (ii) higher organicC concentrations on ridges; (ii) more pronounced claymaxima in the argillic horizons of backslope pedons;and (iv) lower base saturation and lower pH in theupper argillic horizons of backslope pedons.

The vegetation history of this area is thought to havebeen dynamic (William Schroeder, personal communi-cation, 1993). Presettlement prairie maps show this areaas forest at the time of survey (Schroeder, 1981), eventhough it is referred to as the "Woodlandville Prairie,"and the soils are primarily Mollisols. It is possible thatthe forest encroached between the time that indigenouspeople were forced to leave and Euroamerican settle-ment began (Schroeder, personal communication,1992). Many soils in northcentral Missouri have mor-phological attributes imparted by both forest and prairievegetation (Hammer et al., 1994). Climate change dur-ing the Pleistocene is highly probable, and vegetationwould have changed in response (Ruhe, 1970,1984). Astable, sustained similar vegetation pattern on prairieridges and forested backslopes through the Pleistoceneseems highly unlikely.

Another hypothesis relates to the interactions amonghydrology, landforms, and parent materials. Slope andstratigraphic conditions can create soil water differences(e.g., Afyuni et al., 1993; Boyer et al., 1990; Hanna etal., 1982; McDaniel et al., 1992), and soil variabilityoften results (e.g., Bunting, 1961; Daniels and Gamble,1967; Alexander, 1986; Rosek and Richardson, 1989;Richardson et al., 1992).

Ridge surfaces in this study site are nearly level, and

are mantled with uniform thickness of permeable loess.Water movement is primarily as throughflow. Surfacerunoff may occur during prolonged or high-intensityspring storms. Ridge pedons presumably desiccate priorto backslopes in the summer, and have more uniformpatterns of water or solute movement, and of wettingand drying. Homogeneous slope gradients, surfaceshapes, and parent materials result in relatively uniformsoil properties.

Glacial till is closer to the surface on backslopes, andgeomorphic processes have produced hillslope sedi-ments with variable source textures and distributed invariable thicknesses and permeabilities. The underlying,slowly permeable till and the argillic horizon restrictvertical water movement. The combination of slope,differential permeabilities, and runon and seepage fromupslope produces variable lateral flow. More complexand variable weathering and erosion processes result.Variability in wetting and drying creates spatially andtemporally heterogeneous soil water conditions downthe backslopes. Consequently, backslope pedons aremore variable than ridge pedons. Backslope argillic ho-rizons have more clay, appear more intensely leached,and have more pronounced and more abundant redoxi-morphic features. Structural units appear more denseand less permeable, and water-driven pedogenic pro-cesses (organic deposition, redox reactions) appearmore pronounced on ped surfaces. In summary, back-slope pedons have morphological attributes, which indi-cate preferential water movement through interstitialvoids along ped faces. Soils on ridges have more homog-enous morphological attributes, suggesting that seasonaldistributions of water are more homogeneous tempo-rally and spatially.

The progressive soil survey of Boone County, Mis-souri, recognizes locally distinct ridge and backslopemap units of the Arisburg soil. The units are distin-guished by slope and erosion classes. This study indi-cates that other differences exist, and these differencescan be documented in map unit descriptions and inter-pretive tables. Many Midwestern soil series formed in1 to 2 m of loess are mapped across different geomorphicsurfaces on a wide range of slope gradients. As MajorLand Resource Area (MLRA) soil survey updates areundertaken, differences within series among geomor-phic surfaces can be sampled for and documented. Theimportance of hydrology and stratigraphy to soil vari-ability within this study area indicates the need for deepsampling and analyses of Fe and Mn variables in soilsurvey updates for northern Missouri.


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soil–landform relationships on a loess-mantled upland landscape in missouri

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