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Soil Science Society of America Journal 64:1443-1454 (2000)
© 2000 Soil Science Society of America

DIVISION S-5-PEDOLOGY

Soil–Landform Relationships on a Loess-Mantled Upland Landscape in Missouri

F.J. Youngb and R.D. Hammera

a Dep. of Soil and Atmospheric Sci., Univ. of Missouri, Columbia, MO 65211 USA
b USDA-NRCS, Lincoln Univ., Jefferson City, MO USA

hammerr{at}missouri.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Methods
 Results
 Discussion
 REFERENCES
 
Soil survey users are requesting statistically valid distributions of soil attributes that are important for management and land use. The hypothesis that many soil attributes vary predictably with landscape positions was tested with 257 pedons from point transects in a 40-ha upland Missouri setting. The effect of landscape position on the central tendencies of selected soil properties was examined. Most soil properties were similar between ridge and shoulder positions. Differences were minimal within the backslope. Backslopes differed from ridges and shoulders, with more argillic horizon clay, thinner epipedons, and less organic C, lower pH and base saturation, and less silt on a clay-free basis. Color patterns suggest that backslopes are wetter than ridges and shoulders, with more redoximorphic activity and organic matter accumulation on ped faces. Differences among the ridge–shoulder pedons and backslope pedons may be caused by differing hydrologic patterns as a result of interactions between topography and the underlying glacial till.

Abbreviations: CEC, cation-exchange capacity


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Methods
 Results
 Discussion
 REFERENCES
 
AN IMPORTANT CONCEPTUAL MODEL of soil variability is that soils and geomorphic processes are synergistic with resulting associations of soils with geomorphic features. Thus, specific soils are associated with specific landforms, and soil patterns are repeating and predictable (Simonson, 1959; Ruhe, 1975; Daniels and Hammer, 1992). Many workers have applied this model to local landscapes and have investigated relationships among soils and geomorphic surfaces.

Early soil–geomorphic observations often were conducted in the context of the catena, a concept that describes the areal associations of soils along hydrologic sequences or valley sides (Milne, 1936). Bushnell (1942) divided the soil catena into several components, each characterized by a specific soil with features related to specific erosional or hydrological conditions. The catena model was not precise—it included both uniform and multiple parent materials—and was envisioned both as "a classification grouping" (Bushnell, 1942) and a "unit of mapping convenience" (Milne, 1936). Some catena applications, particularly those which failed to recognize the dynamic nature of soil–geomorphic relationships, were inconsistent or inappropriate (Hall, 1983).

Ruhe (1956) expanded and refined the model by describing relationships among geomorphic surfaces, underlying materials and soils. He related specific soils to specific surfaces in multi-layered glacial drift in Iowa and clearly defined relationships among surface and material ages with expression of certain soil attributes. Acton (1965), working in glacially modified terrain in Canada, related different soils to ice disintegration as affected by different landscape positions. Landscape positions were characterized by differences in length, shape, gradient, and relative position of the individual slope segment. Dan and Yaalon (1968) related specific "pedomorphic forms" to "pedomorphic surfaces," which locally controlled hydrology in Israel. They recognized that soils and relief are "genetically and evolutionarily interdependent" in this setting. Parsons (1978) described soil–geomorphic relationships in Oregon and noted that degree of expression of important soil attributes was a function of age and intensity of weathering, both of which were locally correlated with specific geomorphic surfaces. Pregitzer et al. (1983) documented changes in soils along a topographic gradient, and related these changes to vegetation and nutrient status. These applications all recognized that "landform" includes the underlying materials, and that a hillslope transect may include materials of different ages and sources.

Some studies have investigated hillslope processes, particularly slope length, gradient, and distance from summit, as they affect distributions of soil organic C, clay, and nutrients. These include classic work by Aandahl (1948), Ruhe and Walker (1968), Walker and Ruhe (1968), Kleiss (1970), and Malo et al. (1974). All of these studies revealed that distributions of particular soil attributes vary as functions of geomorphic and hydrologic processes that differentially distribute water, sediments, and dissolved materials. Most studies, however, have used a few "representative pedons" or otherwise relatively small sample sizes. These studies were important in identifying cause-and-effect relationships in soil landscapes; however, they left unanswered the questions related to magnitudes and patterns of variances of soil attributes within specific soils and landforms.

Our hypothesis is that many soil properties vary predictably with landscape position, and that magnitudes of variation should vary among but are somewhat predictable within landforms. Our objective is to use a large data set to test the effect of landscape position on the relationships of soil properties with landforms within a relatively small, loess-mantled upland landscape.


   Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
Results
 Discussion
 REFERENCES
 
Study Area
The study area is a 40-ha, upland, tall fescue (Festuca arundinacea Schreb.) pasture in northwestern Boone County, Missouri. The field has been in pasture since before World War II, and fertilization and liming have been minimal. Relief is 18 m from the interfluve divide on the western border of the area to the perennial stream thalweg on the east. The site was classified (stratified) into three landforms, "ridge," "shoulder," and "backslope" for sampling purposes (Fig. 1) . These landforms were chosen because they are easily identified and are the most extensive in the watershed. Footslopes are inextensive and discontinuous and are included within the backslope for sampling. Ridge slope gradients range from 1 to 3%, and shoulders range from 2 to 4%. Backslope gradients are generally 4 to 8%, with a maximum of {approx}15%.



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  		Fig. 1 Geomorphic surfaces within the study area, and locations of point transects used for sampling

 
Soils formed in loess that varies from about 1-m thick on lower backslopes to more than 2 m on finger ridges. The substratum is pre-Illinoian glacial till and includes discontinuous remnants of paleosols presumed to be Late Sangamon in age. Hillslope sediments of variable thickness and composition are between the loess and the till. Most of the site is mapped in the cooperative, in-progress soil survey, as Arisburg (fine, montmorillonitic, mesic Aquertic Argiudolls), with inclusions of Armstrong (fine, montmorillonitic, mesic Aquollic Hapludalfs).

Sampling
The sampling strategy was to obtain large numbers of samples from each landform while trying to equalize numbers of observations within landforms. Interlocking transects were placed to traverse landforms both parallel and normal to slope gradients (Fig. 1). Transect placement and sampling intervals along transects were determined subjectively to capture the full range of soil variability within landforms (Young et al., 1992). Transects were straight lines, inflected where necessary to conform to landforms. Sampling intervals along transects were 15 m except on ridges, which had smaller areal extent, so sampling density was increased, and ridges were sampled at 7.5-m intervals from multiple, parallel transects 7.5-m apart. A total of 257 pedons was obtained as cores taken with a Giddings hydraulic soil probe (5-cm-diam. tube). Sample depth (120 cm, the length of the sampling tube) contained the taxonomic control sections. All samples were assumed to be independent.

Cores were subdivided for description and laboratory analysis as follows:

Sampling by genetic horizons is preferable under most conditions, but sampling by depth increments below the A horizon ensured data base uniformity. We wanted to avoid an unbalanced design, which would have resulted from sampling horizons by their thicknesses. The incremental depth sampling approach did not mix unlike materials because changes within the argillic horizons were gradual. The A-1 horizon is roughly equivalent to the Ap horizon. The A-2 horizon was not in all pedons, so its sample size is smaller. Clay films distinguished lower A from upper B horizons. Color and structure were helpful but were not diagnostic. Most B horizons are Bt's or Btg's, although many of the dark B-1 horizons might be considered B/A horizons.

Analyses and Variables
All pedons were classified in the field as being within a ridge, shoulder, or backslope. Surface shapes (convex, plane, or concave both in plan and profile) and slope positions (upper, mid, and lower) were noted for all backslope pedons.

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

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

These are the standard characterization analyses for a pedon sampled within the Missouri National Cooperative Soil Survey (NCSS). All analyses were by the University of Missouri Soil Characterization Laboratory except particle-size determination, which was by the modified pipette method (Indorante et al., 1990). Soil pH was in a 1:1 soil solution suspension using an Orion digital ionalyzer/501 pH meter1 (Orion Research, Beverly, MA) with a combination electrode. Total soil C was determined 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), whereas others are horizon-specific (Table 1) .


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  		Table 1 Descriptions of variables analyzed for differences among landforms

 
Statistical Methods
Soil property differences among landscape classes were examined for (i) landform (ridge, shoulder, and backslope); (ii) plan and profile curvature (convex, plane and concave, for pedons sampled on the backslope only); and (iii) position along the slope gradient (upper, mid, lower, and footslope, for pedons sampled on the backslope only).

Most properties were not normally distributed (Young et al., 1999), so the nonparametric Kruskal-Wallis method (Daniel, 1990) was used to detect statistically significant differences among landscape classes. Trial-and-error transformations were not attempted because transformations do not always normalize data effectively (Young et al., 1992). All tests were conducted with SYSTAT (Wilkinson, 1992) software.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results
Discussion
 REFERENCES
 
Relationships Among Landforms
Depth to the argillic horizon, mollic epipedon thickness, depth to Fe depletions, and organic C content were all less for backslope pedons than for ridge and shoulder pedons (Fig. 2a–d) . The maximum argillic horizon clay content and the particle-size control section clay contents were greater on backslope soils than ridges and shoulder soils (Fig. 2e and f). Pedons on shoulders and ridges generally were similar for these properties. Differences among landforms were highly significant for six of the eight pedon-specific variables (Table 2) . Of the significantly different variables, all differed between ridge and backslope, but none differed between ridge and shoulder. Shoulder and backslope differed on five of the eight.








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  		Fig. 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 (upper 50 cm of the argillic horizon)

 

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  		Table 2 Summary of nonparametric tests for significant differences among landforms for medians of variables representing pedon properties

 
Three quarters of the nine horizon-specific properties tested in six horizons were significantly different among landforms (Table 3) . Most pairwise differences were between pedons on ridges and pedons on backslopes, and between pedons on shoulders and pedons on backslopes. As with the pedon-specific properties, most horizon properties did not differ between ridge and shoulder positions.


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  		Table 3 Summary of nonparametric tests for significant differences among landforms for medians of variables representing horizon properties

 
Medians of soil properties are plotted as depth distribution curves, with separate curves for each landform (Fig. 3) . Backslope pedons are higher in clay relative to ridge and shoulder pedons, which are similar (Fig. 3a). These differences are not consistently reflected in the cation-exchange capacity (CEC) (Fig. 3b). Backslope pedons have less silt (on a clay-free basis) than ridge and shoulder pedons (Fig. 3c). Soil pH differences are more complex, with lower pH in the epipedons and upper argillic horizons of backslope pedons, and higher pH with depth (Fig. 3d). Calcium generally follows the pH trend (Fig. 3e), but Mg does not (Fig. 3f). Organic C differences are small but significant, with larger concentrations in backslope A-1 horizons, but lesser concentrations with depth (Fig. 3g).









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  		Fig. 3 Depth distributions, using median values from each horizon, with ridge, shoulder, and backslope pedon medians plotted separately: (a) clay, (b) cation-exchange capacity (CEC), (c) silt on a clay-free basis, (d) pH, (e) Ca, (f) Mg, (g) organic C. * Significant differences at the 5% level. The backslope differed significantly from the ridge and shoulder for all variables at all depths except extractable Ca in the A1 horizons (e); clay, CEC and organic C in the A2 horizons (a,b, and g); pH and extractable Ca in the B3 horizon (e). Shoulder CEC was significantly higher in value in the B4 horizons than the ridge and backslope (b), and ridge Ca was significantly less than Ca in shoulder and backslope B4 horizons (e)

 
Most argillic horizon color values differ among landforms (Fig. 4) . Ped surfaces (Fig. 4a) and ped interiors (Fig. 4b) are lighter in most backslope B horizons compared to ridges and shoulders. Furthermore, the difference between interior and surface color values is greater for most of the backslope B horizons. Very little variation exists in the colors of the A-1 and A-2 horizons, which are uniformly dark on this landscape.





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  		Fig. 4 Munsell color value means by B-horizons among landforms: (a) ped surfaces, (b) ped interiors, (c) difference between interior and surface. * Significant differences at the 5% level. Backslope ped surface color values were higher in the B2 and B3 horizons (a). Ped interior color values were significantly lower in the B1 horizon shoulder and higher in the B2 horizon backslope (b). Color values for ped interiors were statistically different among all landforms in the B3 horizons, and backslope ped interior values statistically exceeded ridges and shoulders in the B4 horizons (b). The difference between mean ped interior and mean ped exterior color values was significantly higher for the backslope landform in B1 and B2 horizons. All mean color value differences differed statistically from one another in the B3 and B4 horizons (c). = ridge; = shoulder; = backslope

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





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  		Fig. 5 Munsell color chroma means by B-horizons among landforms: (a) ped surfaces, (b) ped interiors, (c) difference between interior and surface. * Significant differences at the 5% level. The mean ped surface chroma in the ridge soil profiles was significantly more than the shoulder and backslope for all horizons, but all horizons differed significantly from one another (a). The mean ped interior chromas were statistically higher in all ridge horizons except the B2. In the B4 horizon, all ped interior mean chroma colors were statistically different (b). The difference between mean ped surface and mean ped interior chromas was statistically different among all landforms in all horizons (c). = ridge; = shoulder; = backslope

 
Iron–manganese stains and concretions are more abundant in the upper B horizons of backslope pedons relative to ridge and shoulder pedons and follow a similar (but nonsignificant) trend with depth (Fig. 6a) . Iron depletions are more abundant in each backslope B horizon than in corresponding ridge pedon B horizons (Fig. 6b), with shoulder pedons intermediate.




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  		Fig. 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 stains and concretions were in the backslope B1 and B2 horizons (a). Iron depletions were significantly more abundant in all the backslope soils throughout all horizons. In the B2 and B3 horizons, the shoulder had signifcantly more Fe depletions than the ridge (b). = ridge; = shoulder; = backslope

 
Relationships Within the Backslope
Relatively few differences exist within the backslope among pedons grouped by curvature class, either plan or profile. Median plan curvature values for calcium and base saturation increase in the sequence convex–plane–concave in deeper B horizons (Table 4) . Differences among profile curvature classes are related to particle size. Silt is more abundant in some B horizons on plane surfaces relative to concave surfaces.


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  		Table 4 Medians of variables that are significantly different (P < 0.05) among plan and profile curvature classes within the backslope. Pairwise significance is shown, where 1 = convex, 2 = plane, and 3 = concave

 
Grouping backslope pedons by position along the slope (upper, mid, lower, and footslope) resulted in only a few significant differences among the measured properties. Footslope pedons have more clay and less silt in B horizons than backslope pedons (Fig. 7a and b) . Soil pH, base saturation, and calcium all have larger values in lower B horizons on footslopes relative to upslope pedons (Fig. 7c; only pH is shown).





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  		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 landscape position than the footslope within the B1 and B4 horizons (a). Silt on a clay-free basis was signifantly less abundant in all footslope horizons except the A2 (b). The pH in water was significantly higher in the backslope B1 and B2 horizons, and in the footslope B3 and B4 horizons

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results
 Discussion
REFERENCES
 
Pedogenesis has been similar on ridge and shoulder landforms in the study area. The ridges are narrow, broadly convex, and have hydrological and pedogenic characteristics similar to the shoulders.

Pedogenesis has differed between ridge–shoulder pedons and backslope pedons. Differences in lessivage are reflected in more clay in the argillic horizon in backslope pedons, in contrast to observations by Walker and Ruhe (1968). Leaching and stratigraphic differences are reflected in pH, base saturation and calcium, which indicates that argillic horizons in backslope pedons are more leached in the upper part, but, with increasing depth, are more strongly influenced by calcareous glacial till. Parent materials have also influenced the (clay-free) silt content, which is less in the hillslope sediments of the backslopes than in the loess on the ridges. Melanization and erosional differences between landforms are indicated by higher concentrations of organic C in ridge pedons, which also have thicker epipedons and darker colors deeper into the argillic horizons. Patterns of melanization differ between landforms as well, with increased concentrations of soil profile organic matter in the A-1 horizon of backslope pedons, less with depth, and more accumulation on ped faces relative to pedons on ridges. Gleization is more obvious within backslope pedons, whereas ferrugination is more pronounced within ridge–shoulder pedons. Wider color differences between ped interiors and surfaces within backslope pedons reflects greater redox activity on ped surfaces within backslope pedons. Chemical analyses of Fe and Mn compounds could substantiate these observations.

The CEC7 only partially follows the trend established by more direct measures of clay. Organic C and mineralogy affect CEC. Lesser organic C concentrations in backslope argillic horizons relative to ridge pedons may offset the higher clay content within backslopes.

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

The landscape variables explained relatively little of the substantial chemical and morphological variability within the backslopes. Surface shape appears not to have greatly affected pedogenesis on the backslope. Workers on other landscapes have observed that concave areas are wetter (Anderson and Burt, 1978; Sinai et al., 1981; Boyer et al., 1990), with concomitant differences in drainage class (Troeh, 1964), Fe/Mn ratios (McDaniel et al., 1992), and depth to calcium carbonate (Pennock and de Jong, 1990). Concave areas usually are 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). Such effects were not observed in this study. Relative youth of the landscape and low relief may be important. Perhaps the subtle convexities and concavities have developed under post-settlement management during the 19th and early 20th centuries.

The few differences detected along the hillslope gradient are related to parent materials and reflect the influence of calcareous glacial till in the lowest slope positions. Other studies have demonstrated systematic changes along slope gradients, including sedimentation processes and hydrologic gradients resulting in increasing 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; Pierson 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 in closed systems. On hillslopes of this study area, pedons on footslope positions are not greatly influenced by deposition, despite the decrease in slope gradient and concave slope profile. Apparently, erosional products are moving out of this open system.

Two hypotheses can be presented to explain pedogenic differences between ridge–shoulder pedons and backslope pedons. One is change in vegetation history. Ridges may have been predominantly prairie, whereas backslopes may have had longer developmental periods under forest. Ridges are drier, more windswept, and more susceptible to fire. Soil differences that support this hypothesis are (i) more abundant silt coats in the A-2 horizons of backslope pedons; (ii) higher organic C concentrations on ridges; (ii) more pronounced clay maxima in the argillic horizons of backslope pedons; and (iv) lower base saturation and lower pH in the upper argillic horizons of backslope pedons.

The vegetation history of this area is thought to have been dynamic (William Schroeder, personal communication, 1993). Presettlement prairie maps show this area as forest at the time of survey (Schroeder, 1981), even though it is referred to as the "Woodlandville Prairie," and the soils are primarily Mollisols. It is possible that the forest encroached between the time that indigenous people were forced to leave and Euroamerican settlement began (Schroeder, personal communication, 1992). Many soils in northcentral Missouri have morphological attributes imparted by both forest and prairie vegetation (Hammer et al., 1994). Climate change during the Pleistocene is highly probable, and vegetation would have changed in response (Ruhe, 1970, 1984). A stable, sustained similar vegetation pattern on prairie ridges and forested backslopes through the Pleistocene seems highly unlikely.

Another hypothesis relates to the interactions among hydrology, landforms, and parent materials. Slope and stratigraphic conditions can create soil water differences (e.g., Afyuni et al., 1993; Boyer et al., 1990; Hanna et al., 1982; McDaniel et al., 1992), and soil variability often 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. Surface runoff may occur during prolonged or high-intensity spring storms. Ridge pedons presumably desiccate prior to backslopes in the summer, and have more uniform patterns of water or solute movement, and of wetting and drying. Homogeneous slope gradients, surface shapes, and parent materials result in relatively uniform soil properties.

Glacial till is closer to the surface on backslopes, and geomorphic processes have produced hillslope sediments with variable source textures and distributed in variable thicknesses and permeabilities. The underlying, slowly permeable till and the argillic horizon restrict vertical water movement. The combination of slope, differential permeabilities, and runon and seepage from upslope produces variable lateral flow. More complex and variable weathering and erosion processes result. Variability in wetting and drying creates spatially and temporally heterogeneous soil water conditions down the backslopes. Consequently, backslope pedons are more variable than ridge pedons. Backslope argillic horizons have more clay, appear more intensely leached, and have more pronounced and more abundant redoximorphic features. Structural units appear more dense and less permeable, and water-driven pedogenic processes (organic deposition, redox reactions) appear more pronounced on ped surfaces. In summary, backslope pedons have morphological attributes, which indicate preferential water movement through interstitial voids along ped faces. Soils on ridges have more homogenous morphological attributes, suggesting that seasonal distributions of water are more homogeneous temporally and spatially.

The progressive soil survey of Boone County, Missouri, recognizes locally distinct ridge and backslope map units of the Arisburg soil. The units are distinguished by slope and erosion classes. This study indicates that other differences exist, and these differences can be documented in map unit descriptions and interpretive tables. Many Midwestern soil series formed in 1 to 2 m of loess are mapped across different geomorphic surfaces on a wide range of slope gradients. As Major Land Resource Area (MLRA) soil survey updates are undertaken, differences within series among geomorphic surfaces can be sampled for and documented. The importance of hydrology and stratigraphy to soil variability within this study area indicates the need for deep sampling and analyses of Fe and Mn variables in soil survey updates for northern Missouri.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Methods
 Results
 Discussion
 REFERENCES
 
Contribution from the Missouri Agric. Exper. Stn. Journal Ser. no. 12514.

1 Mention of a specific trade or product name does not necessarily mean the endorsement by the University of Missouri or the exclusion of other products. Back

Received for publication September 2, 1996.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results
 Discussion
 REFERENCES
 




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