soil, landscape, and erosion relationships in a northwest illinois watershed

Soil, Landscape, and Erosion Relationships in a Northwest Illinois WatershedWilliam R. Kreznor, Kenneth R. Olson,* Wayne L. Banwart, and Donald L. Johnson

ABSTRACTThe purpose of this study was to establish the relationship be-

tween soil taxonomic description, landscape form and position, anderosion in northwest Illinois. The study area consists of a first-orderdrainage basin about 10 ha in size. The hillslopes have mean gra-dients of 6 to 10% with overland flow the primary contributor toerosion. A detailed geomorphic map of the watershed was con-structed on a 1-m topographic survey base map. Map units weredelineated based on slope shape, gradient, and USDA erosion class.Erosion classes were determined for the transect pedons by com-parison with two uneroded and uncultivated pedons in area ceme-teries with similar slope gradient, slope shape, slope length, andlandscape positions. The A horizon thickness and organic C contentdecreased and clay content increased as a consequence of cultivationand erosion. Transect data for all hillslope geomorphic units showedthe erosion classes near the shoulder to be either slightly or mod-

W.R. Kreznor, K.R. Olson, and W.L. Banwart, Dep. of Agronomy,1102 S. Goodwin Ave.; and D.L. Johnson, Geography Dep., Univ.of Illinois, Urbana, IL 61801. Funded under the terms of an agree-ment between USDA and the Illinois Agric. Exp. Stn. as part ofHatch Project no. 15-359 and in cooperation with NorthcentralRegional Project no. NC-174. Received 16 Feb. 1989. "Correspond-ing author.

Published in Soil Sci. Soc. Am. J. 53:1763-1771 (1989).

erately eroded. The lower backslopes and upper footslopes, justabove the sediment basin, were either severely or very severelyeroded, suggesting slope length to be the dominant factor affectingerosion. The geomorphic units with concave across-slope shapeswere found to be less eroded than those with convex across-slopeshapes. All cultivated pedons failed to meet the criteria for Molli-sols, which was the taxonomic placement of the uncultivated pedons.Erosion phase, landscape segment, slope shape, slope gradient, andslope length affected the taxonomic placement of the cultivated hill-slope soils when compared with uncultivated sites.

CARSON AND KiRKBY (1972) have characterizedhillslope forms in humid temperate environ-

ments as transport-limited when the rate of weather-ing exceeded that of erosion. Transport-limited hill-slope form and gradient were controlled mainly by soilcreep and rainsplash, which produced convexity at theupper part of the slope. Overland flow occurred pri-marily at the lower part of the slope and producedconcave forms.

The scientific investigation and control of acceler-ated soil erosion are impeded by the variability of soil

1764 SOIL SCI. SOC. AM. J., VOL. 53, NOVEMBER-DECEMBER 1989

erosion that results from the variability in relief andsoil properties (Nowak et al., 1985). The effect of slopeshape on soil loss and runoff has been studied undercontrolled conditions of simulated rainfall and me-chanically shaped slopes (Young and Mutchler, 1968,1969). These researchers reported that the areas ofmaximum erosion occurred lower on the linear andconvex slopes than on concave slopes. Similar studiesin small drainage basins have demonstrated signifi-cant differences in runoff and erosion among hillslopesegments and shapes (Betson and Marius, 1969;Dunne, 1978; Emmett, 1978).

Soils and landforms are closely related and are, toa large extent, dependent on the same factors: climate,biota, relief, parent material, and time (Jungerius,1985). Consequently, a number of pedologists andgeomorphologists have developed hillslope modelsthat help to explain the spatial variability of and in-terrelations among erosion, deposition, and soil prop-erties. Troeh (1964) found that land-surface shape cor-related well with soil drainage class for a single soiltype on a given hillslope. Ruhe and Walker (1968)developed a model that identified hillslope compo-nents in an open drainage system and classified eachcomponent according to water movement over andthrough it. These models were modified to includeconsideration of the location and relative rates of col-luvial deposition and sediment removal (Hole andCampbell, 1985).

The correlation of specific soil properties with land-scape position included the properties of organic mat-ter content and A-horizon thickness (Furley, 1968;Kleiss, 1970; Malo et al., 1974; Davidson, 1977; Gre-gorich and Anderson, 1985), B horizon thickness anddegree of development (Norton and Smith, 1930;Daniels and Jordan, 1966; Vreeken, 1973), soil mot-tling (Reimer, 1957; McCaig, 1984), pH and depth tocarbonates (Daniels and Jordan, 1966; Furley, 1968;Vreeken, 1975; Gregorich and Anderson, 1985), andwater storage and movement (Hoover and Harsh,1943; England and Holton, 1969; Q'Loughlin, 19,81;Sinai et al., 1981; Hanria et al., 1982).

The recognition of close relationships between soilproperties and slope position has led a number of re-searchers to partition soil bodies according to geo-morphic factors prior to any analysis of soil variabil-ity. Bouma (1985) has stated "certain landscapefeatures are so clearly evident that (geo)statistical tech-niques are not needed to distinguish them. Hencethese features can be used to define subpopulations interms of areas which can be characterized by consid-ering point data within these areas." Briggs and Shis-hara (1985) reported that, in general, separation bygeomorphical land classes provided survey units moreuniform in terms of their soil properties than the land-scape as a whole. Some properties exhibited a morepredictable pattern than others, so in some cases geo-morphic partitioning was not any more successfulthan a random-sampling scheme. Vreeken (1973) em-phasized that geomorphic partitioning is necessary, asin the case of erosionally induced age differences, be-tween a hillslope summit and backslope.

Walker et al. (1968) used multiple regression anal-ysis to evaluate the relationship between soil proper-ties and landform parameters such as slope shape andgradient, aspect, and elevation. Correlation was im-

proved when sample sites were partitioned to convexand concave delineations. They suggested that the re-maining unaccounted-for variation may be attribut-able to the carryover of persistent features from a pre-vious land surface or drainage regime, or to significantbiopedoturbation. Jungerius and van Zon (1983) re-ported that fauna! activity such as that by worms andmoles reduced differences in soil properties affectingerodibility among landscape units in a forested basin.Johnson et al. (1987) demonstrated that biopedotur-bation can act to create or increase both differentiationand simplification in soils and their properties.

A simple two-dimensional consideration of slopelength and gradient was found inadequate for describ-ing soil-landscape relationships (King et al., 1983).These researchers found that soil variation was mosteffectively recognized and mapped when the land-scape was partitioned into units based on slope length,gradient, and shape. Partitioned sampling was consid-ered more appropriate than simple random samplingin an analysis of accelerated soil erosion by overlandflow (McCaig, 1983). Wilding (1985) recommendedrandom sampling only when differences in a soil bodyare not evident. Partitioned sampling along a transectperpendicular to drainage was considered more effi-cient and unbiased.

Nizeyimana and Olson (1988) measured the effectsof degree of erosion on the chemical, mineralogical,and physical properties of seven Illinois soils. Mod-erately and severely eroded phases of a soil series oc-curred as a consequence of slight differences in slopegradient, slope shape, or slope length within a land-scape position.

The objectives of this study were: (i) to establish therelationships between soil taxonomic description,landscape form and position, and erosion in an opencultivated watershed in northwest Illinois and (ii) todetermine the effects of landscape components on thesoil properties and taxonomic placement of these cul-tivated hillslope soils when compared with unculti-vated sites.

METHODSThe Area of Study

The study area is located in Sec. 29, T. 15N, R. 6E in thesouthwestern part of Bureau County in northwestern Illinois(Fig. 1). It consists of a cultivated, first-order drainage basinabout 10.5 ha in size. The study area is located on the un-dulating, loess-covered Illinoian till plain about 10 km be-yond the limit of the Wisconsin glaciation. Researchers(Marcus, 1980) have denned a first-order drainage basin asthe surface area that contributes water and sediment to anunbranched stream channel. It is comprised of two majorregions: the headwater and the channelway. The headwaterregion has no permanent channel and flow lines tend toconcentrate water in a semielliptical zone. The channelwayregion has a permanent channel in which flow lines termi-nate. Physiographically, the area is within the GalesburgPlain (Leighton et al., 1948). This region is characterized byan undulating plain moderately dissected by the surfacedrainage network of the Spoon River and its tributaries.Based on railroad and cemetery records, settlement and ag-ricultural development of the area began about 1854.

The climate of the area is temperate and humid. The av-erage annual air temperature is 284 K with an average dailymaximum temperature of 290 K and an average daily min-imum temperature of 278 K. Average annual precipitation

KREZNOR ET AL.: SOIL, LANDSCAPE, AND EROSION RELATIONSHIPS 1765

is 880 mm, with nearly two-thirds of this total occurringbetween the months of April and September. Average an-nual snowfall is about 710 mm. The average frost-free periodis about 180 d (Wernstedt, 1972).

The geologic history of the study area and the nature ofthe surficial materials were reported by Willman and Frye(1970), Wascher et al., (1971), Willman et al. (1975), Piskinand Bergstrom (1975), and Fehrenbacher et al. (1986): Theloess is primarily Peoria loess deposited 12 500 to 22 000 yrbefore present (BP) and the underlying Roxana silt inter-preted as a mixed zone of gritty material dated elsewhere asolder than 22 000 yr BP (Willman and Frye, 1970). The claymineral suite of both these silty units is dominated by smec-tite. Total loess thickness ranges from 0 to greater than 400cm. Beneath the Peoria loess and the Roxana mixed zoneis the Radnor Till Member of the Glasford Formation. Thisis a gray, pebbly, silty clay loam till of Illinoian age with ahigh illite content. The thickness of the till is variable, rang-ing from 12 to over 27 m. Interbedded limestone, sandstone,and shale bedrock of the Pennsylvania-age Modesto For-mation underlies the glacial drift. The glacial materials ofthe study area contain several readily identifiable buriedsoils. The upper one is the Farmdale soil formed in Roxanasilt (mixed zone) with a radiocarbon date from 22 000 to28 000 yr BP (Willman and Frye, 1970). It is a weakly de-veloped soil, but is easily recognized by a dark-colored Abhorizon and a cambic Bb horizon with weak structure. Be-neath the Farmdale soil is the Sangamon soil formed in theIllinoian till (Follmer, 1982). It is a strongly developed pa-leosol containing a dark-colored Ab horizon and a firm,clayey Btb horizon characterized by prominent clay coatingsand well-developed soil structure. Both of these paleosolstypically are leached of primary carbonates, although theymay be enriched in places with secondary carbonates. Oc-casionally, both of these soils may be truncated in varyingdegrees by erosion subsequent to their formation.

Most of the modern soils in the area formed mainly inthick loess under tall-grass prairie vegetation (Fig. 2). Thesoils in the sediment source area include Tama, Elkhart, andAssumption soils, which are members of the fine-silty,mixed, mesic Typic Argiudolls family. Soils of the Tamaseries occur on upland interfluves, summits, and backslopes,

have slopes of 0 to 10%, and make up about 75% of thebasin. Tama soils are leached of carbonates to a depth of

/ LAKEI MICHIGAN

%:T^T Study Area

Bureau County

Fig. 1. Location of the study area in Bureau County, Illinois.

^^ /Fig. 2. Relationships among soils, landscape position, and parent materials in the study area.


>100 cm. Of minor occurrence on severely eroded uplandbackslopes are members of the Elkhart and Assumption soilseries. These soils have slopes of 10 to 15%. Elkhart soilscontain free carbonates at a depth of 50 to 100 cm. As-sumption soils formed in 50 to 100 cm of loess and theunderlying Illinoian glacial till. The nearly level colluvial-alluvial areas along drainageways consist of Radford soils(fine-silty, mixed, mesic Fluvaquentic Hapludolls). Thesesoils formed in dark-colored silty alluvium and slopewashunderlain by a dark-colored buried soil.

Field MethodsStandard surveying techniques and equipment (Evett,

1979) were used to construct a topographic map of the drain-age basin (Fig. 3). The map was constructed at a scale of1:1800 with a contour interval of 1 m. It was used to identifyand delineate geomorphic mapping units based on land-scape position, slope, shape and gradient, and to preciselylocate the watershed boundary. The map was useful formore-detailed partitioning of three main landscape units: (i)hillslope summits, (ii) erpsional backslopes and upper foot-slopes, and (iii) depositional footslopes and channel intopresumably more-homogeneous subunits. The erosionalhillslopes were separated into slightly eroded summit andshoulder positions where overland flow was considered aminor contributor to erosion, and the more-eroded back-slopes and upper footslopes where overland flow was con-sidered the primary contributor to erosion.

Erosional backslopes and upper footslopes were parti-tioned into subunits (Fig. 4) based on classes of slope shape(both parallel and perpendicular to the slope gradient) ac-cording to the hillslope configuration classification modeldescribed by Ruhe (1975). Morphometric variables of the

basin are summarized in Table 1. Within each unit, transectswere laid out parallel to the hillslope gradient. Observationinterval varied among the transects from 5 m to slightly lessthan 12m depending on the transect length, and consistedof 6 to 10 observations per transect. Measurements and ob-servations including slope, surface soil color, thickness of Aand Bt horizons (defined as that part of the B horizon con-taining illuvial clay as detected with a 10X hand lens),solum, and loess, and depths to the upper and lower bound-aries of Bt horizons, carbonates, and chroma of 2 or less (asmatrix or mottles) were recorded. Notes were also maderelative to soil and rock stratigraphic units observed or in-ferred. This included the depths to and thicknesses of pa-leosols and surficial geologic materials such as Peoria loess,Roxana mixed zone (Fehrenbacher et al., 1986), and Illi-noian glacial drift (Willman and Frye, 1970). A single pedonrepresenting average conditions for the subunit containingit was selected for each of the different subunits present inthe watershed. These pedons were described in detail andselected properties characterized in the laboratory.

Two pedons of relatively uneroded Tama soils were lo-cated as close to the study area as possible while satisfyingthe following requirements: they had been relatively undis-turbed since settlement of the area, they were on backslopepositions having gradients approximating the soils in thestudy area, and they formed in thick Ipessial material over-lying Illinoian glacial till. Pedons meeting these criteria wereavailable in two area cemeteries, one pedon was locatedabout 5 km northwest of the study area, and the other wasabout 28 km southwest. The cemetery pedons were de-scribed in detail and samples were analyzed for selectedproperties in the laboratory. These pedons provide an in-dication of how thick the original A horizons flight havebeen prior to cultural erosion of the hillslope.

Watershedboundary

Intermittentdrainage

.Perennialdrainage

Fig. 3. Topographic map of the study-area watershed.

NSCALE 1:1800

50 0 100 METERS

Fig. 4. Geomorphic map of the source area and location of transectsand sample sites. (L = linear, V = convex, and C = concave).


Laboratory ProceduresThe particle size distribution of the <2-mm fraction was

determined by the pipette method and sand sieving (SoilSurvey Staff, 1984). The organic C was determined using amodification of the Walkley-Black wet oxidation procedure(Nelson and Sommers, 1982).Statistical Methods

An analysis of variance was performed in order to com-pare the means of selected soil properties among the geo-

Table 1. Morphometric variables of the study area-watershed.

morphic units. Significant differences between means werecomputed by Fisher's least significant difference test. Dif-ferences between means were reported as significant if thedifference was significant at the 0.10 level.

RESULTS AND DISCUSSIONTransect data on a geomorphic-unit basis (Kreznor,

1987) show a trend toward decreasing thickness of thesolum, the mollic epipedon, and the Bt horizon withincreasing distance along the hillslope from the sum-mit to the footslope. In addition, there is a decreasein depth to the lower boundary of the Bt horizon, tofree calcium carbonate, to soil mottle color chroma of2 or less, and to matrix color value of more than 4.Along some transects identified in Fig. 4 as LC-1, LL-1, and VV-1, this trend reverses itself at the lowermostpedon: thicknesses and depths were greater than oneor more of the upslope pedons (Kreznor, 1987). Thiswas mainly due to a noticeable decrease in slope gra-dient at the lowermost pedon as the erosional upperfootslope and the deppsitional toeslope merged. Pre-sumably, runoff velocity would decrease and infiltra-tion increase in this part of the hillslope. The resultwould be a decrease in erosion and enhancement ofthe soil-development processes there.

Partitioning of the basin into geomorphic unitsbased on slope shape and gradient has little effect onreducing the variability in the soil properties withinthese units. There is no significant difference at the10% level among the geomorphic units (Table 2) forthe properties of thickness of either the solum or theBt horizon, and depth to the lower boundary of eitherthe Bt horizon or to carbonates. The mean thicknessof the mollic epipedon for the convex-concave (VC)

Table 2. Summary of transect data for selected soil properties within each of the geomorphic unit classes represented in the study-areawatershed. __ ___ __ _ _____

Geomorphic unit classf

Geomorphic unit

Depositional area(lower footslope andtoeslope)

Erosional backslope andupper footslopeLC-1LC-2LC-3LC-4LL-1LV-1LV-2LV-3VC-1VC-2VL-1VL-2VV-1VV-2VV-3VV-4

SubtotalSummit and shoulderTotal watershed area

Area

m2

24900

840023002200250049004000500030001 8002400260035002800270032003700

5500025500

105 400

Mean slopegradient

%

1

669899

10567

1088798

2

Slopelength

m

—

811316698949984

12311699748199

10771

137

32

Property

Slope%

MeanSD

n

Linear-concave

(LC)7.12.225

Linear-linear(LL)

9.01.87

Linear-convex(LV)

8.02.520

Convex-concave

(VC)6.71.8

13

Convex-linear(VL)

8.93.212

Convex-convex(VV)

7.73.221

Thickness of:Solum!cm

Bt horizonjcm

Mollic epipedon!cm

Bt lowerboundary:):cmCarbonate!cm

Chroma!< 2cmValue!>4cm

MeanSD

nMean

SDn

MeanSD

nMean

SDn

MeanSD

nMean

SDn

MeanSD

n

146.4a27.3

1780.3a30.821

12.6b12.025

100.3a32.421

119.3a48.4

1856.8ab28.12141_

1

121.6a56.8

756.8a40.6

712.6b8.87

85.7a34.6

689. la58.2

640.7b14.1

774_1

140.5a36.8

1466.3a34.416

12.9b9.920

96.8a27.7

14112.7a57.416

89.4a53.31662—

1

147.9a40.6

1373.6a34.113

21.9a6.313

99.7a38.1

13127.5a65.4

1353.0b15.41388_1

151.6a38.212

79.8a39.6

1211. 7b10.812

108.9a34.1

11116.7a56.2

1249.4b24.8

1266_1

143.8a36.421

85.5a37.121

10.9b9.521

105.8a35.621

128.6a56.121

43.2b10.52188—

1t Downslope shape listed first and followed by across-slope shape.! Values for any given property followed by a common letter are not significantly different at the 0.10 level by Fisher's least significant difference test.


units is greater than that of all the other units. Themean depth to chroma of 2 or less for the linear-con-vex (LV) units is greater than the convex-concave(VC), convex-linear (VL), convex-convex (VV), andlinear-linear (LL) units. There is no significant differ-ence between the LV and LC units, or among the LC,LL, VC, VL, and VV units for this property. The var-iation among the units having convex shapes in thedirection parallel to the slope gradient is less than thatamong the units having linear-shaped slopes (Table2). The coefficient of variation (CV) ranged from alow of 19% in solum thickness for the LC unit to ahigh of 95% in the mollic epipedon thickness for theLC unit (data not given). Over two-thirds of the CVvalues were between 20% and 50%. The thickness ofthe mollic epipedon is the most-variable property; themean CV for all units is 75%. The thickness of thesolum is the least-variable property, the mean CV forall units being 28%.

Wilding (1985) reported that a CV of 35% or moreis not uncommon for properties such as solum thick-ness, depth of carbonate leaching, and depth to mot-tling. Within the study area basin, there are three mainconsiderations relative to the observed variability inthose properties measured. The first relates to thesampling method and the topography of the deeperhorizon boundaries. The observations along each ofthe transects were made on soil cores 10 cm in di-ameter. It was not possible to determine the locationof each observation relative to the waviness of thehorizon boundaries. Hence, this source of variation isincluded in the measurements.

The second source of variability relates to through-flow. Hillslope seeps were observed periodically on thelower parts of some of the sideslopes. These may haveformed as a result of water moving initially downwardthrough the soil and encountering a layer having lowerpermeability as a result of a pore-size discontinuity.Typically, this discontinuity consists of paleosols

formed in the Roxana mixed zone or Illinoian till.Such a zone could slow the downward movement ofwater, and lateral flow along the surface of the dis-continuity would be enhanced. The flow lines couldeventually intersect the land surface where the dis-continuity outcrops along the eroded sideslope. Thismechanism for laterally translocating materials in sus-pension and solution and its contribution to soil de-velopment was described by Huggett (1976) andMcCaig (1984). Such a mechanism might increase thedepth of carbonate leaching, thicken the Bt horizon,and decrease depth to mottling in the soils on theslopes just above the outcrop compared with soils onhillslopes where throughflow does not occur.

Lastly, the contribution of the paleosols themselvesto soil variability cannot be overlooked. On the lowerparts of some severely eroded backslopes, the presenceof paleosols may have complicated some of the ob-servations. Several observations were made where thelower solum of the Tama soil was pedogenically ov-erprinted upon the leached solum of the Farmdale soilor, to a lesser extent, upon the deeper lying Sangamonsoil. This made it appear that the Tama soil had de-veloped more deeply. It is possible that in some casesthis situation was not detected in the field inasmuchas degree of overprinting is sometimes difficult to as-certain, for example when the palesol lies just belowthe lowest depth of observation or where the paleosolis weakly developed, truncated, or enriched with sec-ondary carbonates.

Erosion classes, as defined by USDA (Soil SurveyStaff, 1981), relates to the percentage loss of originalA horizon of a pedon. While uncultivated and uner-oded reference pedon could be found in the study-areawatershed. Two area cemeteries, which included slop-ing land, were located with similar slope gradients,slope shapes, slope lengths, and landscape positions(Table 3). These reference pedons were described inthe field and analyzed in the laboratory (Table 4). The

Table 3. Landscape characteristics at the uneroded and uncultivated reference sites and hillslope geomorphic units at the cultivated study site.

Soil Erosionseries phase Slope gradient

Tama uneroded

Tama uneroded

Tama moderate

Tama severe

Tama severe

Tama moderate

Tama severe

Tama moderate

%

8

9

6

10

10

7

11

12

Slope shape

Downf Acrossf

Reference pedon BU11convex convexReference pedon ST5convex linear

LC map unitlinear concave

LL map unitlinear concave

LV map unitlinear concave

VC map unitconvex concave

VL map unitconvex linear

VV map unitconvex convex

Slope length

m

60

50

120

60

60

100

90

40

Landscape position Aspect (facing)

lower backslope

lower backslope

lower backslope

middle backslope

middle backslope

lower backslope

lower backslope

middle backslope

NW

SE

sw

w

w

sw

E

E

t Down represents the shape of the slope length. Across represents the shape of the slope width.


mean thickness of the two reference pedon A horizons(Table 4) was inferred as representing the original A-horizon thickness when determining the soil loss viaerosion and the erosion classes of the cultivated pe-dons at our study site. The reference pedons havewell-developed mollic epipedons (10YR 3/1 and10YR 3/2 colors) whereas the cultivated pedonslacked them; both sets of pedons have argillic hori-zons.

The A-horizon thicknesses for all geomorphic unitsat the cultivated site (Table 4) were reduced as a con-sequence of 140 yr of cultivation and thinning viaerosion. The average A-horizon clay content of theuncultivated pedons is 27% (Table 4) while the Aphorizons of the six cultivated hillslope geomorphicunits average 30%. Organic C values (Table 4) for theAp horizon of the cultivated site were approximately50% lower than for the reference uncultivated pedonson a gram per kilogram basis, and approximately 75%lower on a total organic-C basis, that takes into con-sideration the horizon thickness differences.

The erosion classes of the pedons observed at eachtransect stop are presented in Table 5 by geomorphicmap unit (Fig. 4). Only two downhill slope shapes,convex (V) and linear (L), were delineated at the sitein combination with all three (concave [C], L, and V)across-slope shapes. The least soil loss (A-horizon ma-terials) occurred in the concave (across-slope) units(LC and VC). The uppermost pedon on each transect(immediately below the shoulder position) was eitherClass 1 (slightly eroded) or Class 2 (moderatelyeroded). The other transect points are located down-slope. The lowermost pedon was on the upper foot-slope and at the upper margin of the sediment basinand either severely or very severely eroded. Slopelength appears to be the dominant factor affecting ero-sion class.

The taxonomic placements (Soil Survey Staff, 1975)of all representative uneroded and eroded pedons, rep-resenting each geomorphic unit are provided in Table6. The uneroded reference pedons are members of thefine-silty, mixed, mesic Typic Argiudolls family towhich Tama soils are assigned. Consistent with thefindings of Lewis and Witte (1980) in Nebraska, noneof the six cultivated pedons placed in the assigned soilorder (Mollisols) as a consequence of erosion alteringthe diagnostic-epipedon properties (color and thick-ness). The diagnostic subsurface horizon (argillic) wasstill present but altered (primarily thinned by tillageequipment mixing the upper Bt into the Ap horizonand by erosion). All cultivated pedons belong to thegreat group of Hapludalfs (Table 6). The representa-tive pedons of two geomorphic units with concaveacross-slope shape (LC and VC) qualified as mollicintergrades. The LV and VL geomorphic units wereaquic and the VV and LL were typic at the subgrouplevel.

SUMMARY AND CONCLUSIONSThe relationships between erosion, landscape form

and position, and soil taxonomic description were es-tablished for a cultivated watershed in northwest Il-linois. A detailed geomorphic map with units basedon slope shape, gradient, and USDA erosion class wasprepared. Erosion classes were determined for transect

Table 4. Soil characterization data for uneroded pedons at the un-cultivated reference sites and hillslope geomorphic units at thecultivated site.

Horizon Depth

cm

OrganicC

gkg-'

Particle size distribution

Silt

Sand fine totalnt/O

Clayfine total

Reference pedon BU11AlA2BtlBt2Bt3Bt4BCCCg

0-1919-3535-4444-7777-9292-131

131-157157-219219-249

291712532111

2 342 341 332 322 332 311 302 321 35

716967677074818588

1820212115191387

272932312824181311

Reference pedon ST5AlA2BtlBt2Bt3Bt4BCC2Bb3Btb

0-1818-3737-4747-7474-110

110-130130-145145-183183-206206-251

3619137422222

3836363537373944

6 3816 32

75716968737678807358

15182118201515121315

24283031262321192126

Linear-concave (LC)ApBtlBt2Bt3BCCCg

0-2020-4141-8181-110

110-153153-193193-229

16642212

2 332 301 332 292 301 292 34

68717685878885

2018146557

30272313111113

Linear-linear (LL)ApBtlBt2Bt3BCCCg

0-1414-2828-7474-9898-133

133-184184-231

11431111

2 302 251 302 292 321 361 39

70737983868990

2016149654

2825201512109

Linear-convex (LV)ApBtlBt2Bt3BCC2Ab

0-1414-3434-6262-107

107-128128-223223-235

14621112

2 302 262 301 341 331 381 35

70737880888985

17161010658

28252019111014

Convex-concave (VC)ApBtlBt2Bt3BCC

0-2020-4646-8888-107

107-172172-287

1873221

2 342 331 311 362 341 33

676876828388

201914986

313023171511

Convex-linear (VL)ApBtlBt2Bt3BCCg

0-1414-3131-6666-112

112-132132-228

1864211

1 351 332 351 312 321 32

686771788687

201917968

313227211212

Convex-convex (VV)ApBtlBt2Bt3Bt4BCC

0-1313-3838-5959-8888-123

123-149149-235

16321121

1 351 351 342 281 282 341 34

70717479838489

161918141464

29282519161410


Table 5. Erosion class of transect pedons by geomorphic map unit.Erosion class}

Transectstop numberf

123456789

10Median class

Geomorphic map units§LC-1

2222

2

LC-211

1/221D

1

LC-32

3/43/442D

3

LC-4

23/43/4443

2/3D

3

LL-1233442

3

LV-1

12224242

2

LV-2

244444

4

LV-3

1/222222

2

VC-1

12

1/21/222

2

VC-20/1

12222D

2

VL-1

224444

4

VL-2

0/11234D

2

VV-1

123442

3

VV-2

11124442

2

VV-3

11234

2

VV-4

^i444444444

t Transect is downslope and starts below shoulder and ends above sediment basin (lower footslope and toeslope).\ 0 = uneroded, 1 = slightly eroded, 2 = moderately eroded, 3 = severely eroded, 4 = very severely eroded, and D = deposition.§ L = linear, C = concave, and V = convex. The first letter represents the downslope (length) shape and the second letter the across-slope (width) shape.

Table 6. Taxonomic placement of the uneroded pedons and erodedpedons of reference and study sites.Soil

series

Tama

Tama

Erosion Geomorphicphase unit

Uneroded referenceuneroded VV(BUll)

uneroded VL(ST5)

ClassificationpedonsFine-silty, mixed, mesic Typic

ArgiudollsFine-silty, mixed, mesic Typic

ArgiudollsCultivated and eroded pedons

Tama

Tama

Tama

Tama

Tama

Tama

moderate LC

severe LL

severe LV

moderate VC

severe VL

moderate VV

Fine-silty, mixed, mesicAquollic Hapludalfs

Fine-silty, mixed, mesic TypicHapludalfs

Fine-silty, mixed, mesic AquicHapludalfs

Fine-silty, mixed, mesicMollic Hapludalfs

Fine-silty, mixed, mesic AquicHapludalfs

Fine-silty, mixed, mesic TypicHapludalfs

pedons by comparing with two uneroded and uncul-tivated pedons in area cemeteries with similar slopegradients, slope shapes, slope lengths, and landscapepositions. Transect data for all hillslope geomorphicunits at the cultivated site showed the shoulders to beeither slightly or moderately eroded. The lower back-slopes and upper footslopes, just above the sedimentbasin, were either severely or very severely eroded,suggesting slope length is the dominant factor affectingerosion. The geomorphic units with concave across-slope shapes were found to be less eroded than thosewith convex across-slope shapes. As expected, A-ho-rizon thickness decreased, organic-C content de-creased, and clay content increased as a consequenceof cultivation and erosion. All cultivated pedons failedto meet the criteria for Mollisols, which was the tax-onomic placement of the uncultivated pedons on sim-ilar geomorphic units. Though originally Mollisols be-fore cultivation, the eroded hillslope soils qualified arenow Alfisols. In conclusion, erosion phase, landscapesegment, slope shape, slope gradient, and slope lengthaffected the taxonomic placement of the cultivatedhillslope soils when compared with uncultivated sites.

NORFLEET & SMITH: WEATHERING AND MINERALOGICAL CLASSIFICATION IN THE BLUE RIDGE MOUNTAINS 1771

soil, landscape, and erosion relationships in a northwest illinois watershed

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