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Effect of erosion on morphology and classification of soils in the North Central United States.

Erosion has been occurring in the world for centuries and in the north central United States for over 150 years. Detection of the extent of erosion continues to be a problem for land managers and researchers alike. Erosion not only reduces productivity, it also slowly alters the soil properties. Unfortunately, tillage also makes the amount of erosion difficult to determine.

Soil erosion classes are based on the amount of the original A horizon that is remaining (Soil Survey Staff 1993). In areas that are or have been largely cultivated, it is usually difficult to find areas of uneroded soils for comparison. Mixing of surface soils with subsoils generally produces Ap horizons with less organic matter (lighter colors) and greater clay contents (Lewis and Witte 1980; Frye et al. 1982; Nizeyimana and Olson 1988; Jones et al. 1989; Mokma and Sietz 1992). Solum, mollic epipedon and A horizon thicknesses and rooting depth have been found to decrease as a result of erosion (Lewis and Witte 1980; White et al. 1985; Nizeyimana and Olson 1988; Jones et al. 1989).

Subsurface properties, rather than surface properties, have been used to estimate soil loss. Methods of determining soil loss that rely on topsoil thickness or depth to the top of either the clay maximum or argillic horizon underestimated [TABULAR DATA FOR TABLE 1 OMITTED] soil loss from erosion of a Mollisol and an Alfisol (Olson and Beavers 1987). The change in depths to the lower boundary of the argillic horizon was a good indicator of the amount of soil removed from an eroded pedon when compared with a slightly eroded pedon formed on the same or similar landscape segment.

Rhoton et al. (1990, 1991) used the depth to a fragipan (Bx) in loess to estimate the relative amount of erosion. However, Olson et al. (1988) found that depth to the lower boundary of a fragipan in loess was a better indicator of soil loss from erosion than depth to the upper boundary because the lower boundary is less vulnerable to removal or tillage than the upper boundary. These morphological methods permit a field soil scientist to estimate soil loss from erosion in the field. Methods which require detailed analyses have also been used to quantify soil loss; for example, fly ash (Jones and Olson 1990), extent of sediment in a basin or delivered to a stream (Kreznor et al. 1990), and organic matter redistribution (Kreznor et al. 1992).

Regional research project NC-174 was initiated to assess the effect of erosion-modified soil physical properties on potential productivity of selected soils under rainfed conditions. Several other studies report on other aspects of the project including review of literature (Olson et al. 1994a, 1994b), productivity (Lindstrom et al. 1992; Schumacher et al. 1994), physical properties (Lowery et al. 1994) and chemical properties (Cihacek and Swan 1994). The objective of this study was to determine the effects of erosion on soil morphological properties and soil classification.
Table 2. Slope gradient and position and natural drainage class of
soils in the study

Erosion              Slope                Natural
phase        Gradient      Position       drainage
               %

Alfisols

Dubuque

Slight         14           side          well
Moderate       12           side          well
Severe         14           side          well

Grantsburg

Moderate        9           back          moderately well
Severe         10           back          moderately well

Hoyleton

Moderate        3           back          moderately well
Severe          4           back          moderately well

Marlette

Slight          3           back          well
Moderate        4           back          well
Severe          4           back          well

Rozetta

Moderate       11           back          moderately well
Severe         12           back          moderately well

Mollisols

Beadle

Slight          2           back          well
Moderate        7           back          well
Severe          9           back          well

Clarence

Moderate        5           back          somewhat poorly
Severe          6           back          somewhat poorly

Clarion

Slight          2           back          well
Moderate        2           back          well
Severe          4           back          well

Egan

Slight          1           foot          well
Moderate        4           back          well
Severe          6           back          well

Otley

Slight          6           back          moderately well
Moderate        7           back          moderately well
Severe          7           back          moderately well

Sharpsburg

Slight          3           side          well
Moderate        3           side          well
Severe          3           side          well

Tama

Moderate        9           back          moderately well
Severe         11           back          moderately well

Ves

Slight          3           side          well
Moderate        6           side          well
Severe          6           shoulder      well

Wymore

Slight          3           side          moderately well
Moderate        5           side          moderately well
Severe          5           side          moderately well


Study methods

We studied 14 soils (five Alfisols and nine Mollisols) from the north central United States (Table 1). At each location three or four replications of each of two or three erosion phases (Soil Survey Staff 1993) of the same soil series were identified in cultivated fields receiving the same management. Experimental units were larger than 0.01 ha (0.02 ac). Corn (Zea mays L.) was used at all locations in this study. None of the locations were converted from forestland or grassland just prior to this study. Slope gradient and position and natural drainage class of each erosion phase are given in Table 2.

We described and sampled pedons within the area of each erosion phase or a representative pedon for each erosion phase. Horizon thickness, color and texture were recorded. Soil samples were collected from each Ap horizon for particle size distribution by the pipette method (Day 1965) and also from A horizons for organic C by wet combustion (Nelson and Sommers 1982).

Results and discussion

Horizon thickness. Horizon thickness tended to decrease with increasing amounts of erosion with the exception of Ap horizons (Table 3). Means of most horizon thicknesses of slight and moderate erosion classes were significantly different at P = 0.05 - 0.2, whereas those of slight and severe erosion classes were significantly different at P [less than! 0.05. Tillage maintains the thickness of the Ap horizons. Horizon thickness in the till - derived Clarion soil did not follow the trend. Nine of the slightly or moderately eroded soils had transitional horizons (E/B, B/E, BE, EB, AB or BA horizons) however, only two of the associated severely eroded soils had transitional horizons. This suggests that the transitional horizons have been incorporated into the plow layer through cultivation. As a result, using depth to the upper boundary of a B horizon or argillic horizon to estimate the amount of erosion will produce an underestimation of soil loss. Several map unit descriptions in soil survey reports from the north central United States indicate the thickness of the surface layer has been reduced by erosion (e.g., Grantham and Indorante 1988; Martin 1991) but not that the thickness of the subsoil horizons have decreased (e.g., Anonymous 1979; Gundlach et al. 1991; Minger 1991). Map unit descriptions of eroded soils indicated transitional horizons were present (Anonymous 1979; Gundlach et al. 1991; Minger 1991).

More severely eroded soils had thinner solums than the less severely eroded soils except for the Clarion soil. In the Dubuque soil that consists of loess overlying clayey residuum, the depth to the lithologic discontinuity decreased from 84 cm in the slightly eroded phase to 38 cm in the severely eroded phase (means were significantly different at P [less than] 0.001). The depth to the lower boundary of a fragipan (Bx) in the Grantsburg soil decreased from 149 cm in the moderately eroded phase to 118 cm in the severely eroded phase (means were significantly different at P = 0.01). These results support the hypothesis that horizonation throughout the soil is modified proportional to the amount of erosion.

The clay content of the Ap horizons of [TABULAR DATA FOR TABLE 3 OMITTED! all severely eroded soils with Bt horizons (Alfisols and Argiudolls), except Tama, was greater than that in the associated slightly or moderately eroded soils (Table 4). The mean increase in clay content was 5 percent with a standard error of 1. Where the color of the Ap horizon differed with erosion phase it tended to have a higher value and/or chorma as the degree of erosion increased. The means of color value were significantly different at P [less than] 0.001, except the Dubuque soil which was different at P [less than] 0.2. Means of chroma were significantly at P [less than] 0.001 for the Hoyleton and Clarence soils. The descriptions of most Ap horizons did not include colors of distinct subsoil material indicating cultivation has thoroughly mixed the subsoil material into the Ap horizon. This makes determining the erosion class using the current criteria (Soil Survey Staff 1993) more difficult. The Ap horizons of four of the ten severely eroded Mollisols did not meet the color requirements for the mollic epipedon. In every soil except Dubuque, the organic C contents of the Ap horizon of the moderately and severely eroded phases were less than that of the slightly eroded phases. Means of the slight and severe erosion classes were significantly different at P [less than] 0.2. These trends also support the premise that increased erosion results in the real loss of A horizon, loss of organic matter, clay, etc. and tillage incorporates subsurface horizon material into the plow layer. Additional chemical and physical properties of these soils have been described by Cihacek et al. (1994) and Lowery et al. (1994), respectively.

Soil classification. Eleven severely eroded pedons and eight moderately eroded pedons failed to meet one or more criteria for the taxonomic placement of the associated uneroded pedons (Table 5). Two of the five severely eroded Alfisols changed classifications. One because the epipedon was too light for a mollic intergrade and the other because it lacked inter fingering of albic materials in the argillic horizon. The only moderately eroded Alfisol that changed classification had an Ap horizon that was too light for a mollic intergrade. One of the four pedons of the moderately eroded Marlette soil had a B/E horizon that was too thin to meet the requirement for interfingering of albic materials and this pedon did not classify in the same subgroup as the other three moderately eroded pedons.

Seven of the nine severely eroded Mollisols failed to meet the color and or thickness criteria of a mollic epipedon. Two of these severely eroded Mollisols (Beadle and Wymore) also lacked an argillic horizon below the Ap horizon that was present in the associated uneroded pedon. The classification of the severely eroded Egan soil was changed because the soil was calcareous from the surface to the calcic horizon. The slightly and moderately eroded Egan soils had 60 cm and 48 cm, respectively, of non-calcareous materials above the calcic horizon.

Seven of the nine moderately eroded Mollisols did not classify in the same taxonomic placement as the associated slightly eroded or uneroded soil. Five of these moderately eroded Mollisols failed to meet the color or thickness criteria for the mollic epipedon. One lacked an argillic horizon below the Ap horizon. The other soil (Clarion) changed family texture placement which is thought to be not related to erosion but to parent material differences.
Table 4. Clay contents, colors, and organic C contents in Ap
horizons of erosion phases of several Alfisols and Mollisols

Erosion                     Munsell color           Organic C
phase             Clay    Value      Chroma       Ap        A(*)
                   %                                    %

Alfisols

Dubuque

Slight            13(1)   3(0)       5(1)       1.5(0.2)     -
Moderate          16(1)   5(1)       4(1)       1.7(0.1)     -
Severe            17(1)   4(1)       4(1)       2.1(0.2)     -

Grantsburg

Moderate          16(1)   4(0)       3(0)       1.3(0.1)     -
Severe            25(1)   4(0)       3(0)       1.1(0.1)     -

Hoyleton

Moderate          22(2)   4(0)       2(0)       1.3(0.1)     -
Severe            30(1)   4(0)       3(0)       1.2(0.0)     -

Marlette

Slight             8(1)   3(0)       2.5(0.3)   1.1(0.1)     -
Moderate          14(1)   3.5(0.3)   2.5(0.3)   0.9(0.1)     -
Severe            16(1)   4(0)       2.5(0.3)   0.7(0.2)     -

Rozetta

Moderate          24(2)   4(0)       3(0)       1.2(0.1)     -
Severe            27(1)   4(0)       3(0)       1.1(0.1)     -

Mollisols

Beadle(***)

Slight            36      2          1          2.0(0.1)     -
Moderate          40      2          1          1.8(0.1)     -
Severe            31      2          1          1.6(0.1)     -

Clarence

Moderate          43(2)   3(0)       2(0)       1.2(0.2)   0.9(0.1)
Severe            48(1)   5(0)       3(0)       1.0(0.0)     -

Clarion(***)

Slight            18      3          1          2.4        2.2
Moderate          16      2          2          1.1        1.0
Severe            15      3          2          1.3        0.8

Egan (***)

Slight            35      2          1          1.6(0.1)     -
Moderate          35      2          1          1.4(0.1)     -
Severe            40      3          2          1.3(0.1)     -

Otley(***)

Moderate          31      3          2          2.0        0.7
Severe            34      3          3          1.5        0.5

Sharpsburg(***)

Slight            29      2          2          2.0        1.2
Moderate          32      3          2          1.6        0.9
Severe            37      4          2          1.4          -

Tama

Moderate          28(1)   3(0)       2(0)       1.3(0.1)     -
Severe            28(1)   4(0)       2(0)       1.0(0.0)     -

Ves (***)

Slight            23      2          1          2.1        1.3
Moderate          26      3          1          1.8          -
Severe            20      4          2          1.4          -

Wymore(***)

Slight            36      2          1          3.2        1.3
Moderate          41      3          1          1.4          -
Severe            41      3(**)      2          1.2          -

Standard errors of the mean are given in parentheses.

* A horizon below Ap horizon.

** Hue was 2.5Y rather than 10YR for slight and moderate erosion
phases.

*** Only representative pedon sampled and analyzed.


The failure of eroded soils to meet criteria for the taxonomic placement of the associated slightly eroded or uneroded Mollisols has also been reported by Lewis and Witte (1980), Jones et al. (1989) and Kreznor et al. (1989). There should be concern over how eroded Mollisols and Alfisols should be classified. Should soils be classified based solely on properties observable in a pit or on a landscape perspective? We believe a landscape perspective should be the preferred approach.

To minimize differences in management the erosion phases were located in the same fields. To achieve this goal, some small areas, too small to map, of the eroded phases were included in this study. Areas of these and similar eroded soils have been mapped in the north central United States (e.g. Anonymous, 1979; Grantham and Indoranate 1988; Lensch 1989; Gundlach et al. 1991; Martin 1991; Minger, 1991).

Relative yield reduction for severely eroded Alfisols that were classified differently was 20% with a standard error of 4, whereas that for Alfisols that were not classified differently was 10 percent with a standard error of 2. The differences in these means was significant at P = 0.2. This significance is low because of the small number of soils and the fact that three of the five severely eroded soils were compared with moderately eroded soils because slightly eroded soils were not included in those comparisons. Relative yield reduction for severely eroded Mollisols that were classified differently was 10% with a standard error of 4. Because all severely eroded soils were classified differently than the none or slightly eroded associated soils, we could not compare them with those that were not classified differently. Additional aspects of yield differences have been reported by Lindstrom et al. (1992) and Schumacher et al. (1994).

Conclusions

Increasing amounts of soil material had been eroded as erosion phase went from slight to moderate to severe erosion. This conclusion is supported by the general reduction in horizon thickness and solum thickness. Also, Ap horizons had greater clay contents when Bt horizons were present and became lighter in color. Estimates of erosion in these soils can best be made by considering solum thickness or depth to C horizon, lithologic discontinuity, or lower boundary of a Bt horizon or a fragipan. Eleven of the 14 severely eroded soils (all nine Mollisols) and eight of the 14 moderately eroded soils (seven Mollisols) failed to meet the criteria for the taxonomic placement of the associated uneroded soils. Severely eroded Alfisols that were classified differently had greater yield reductions than those that were not classified differently.

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D.L. Mokma is a professor, Department of Crop and Soil Sciences, Michigan State University, East Lansing, 48824. T.E. Fenton is a professor, Department of Agronomy, Iowa State University, Ames, 50011. K.R. Olson is an associate professor, Department of Agronomy, University of Illinois, Urbana, 61801. Supported in part by Cooperative Regional Research Funds NC-174, Soil Productivity and Erosion. The authors acknowledge A.J. Jones, University of Nebraska, M.J. Lindstrom, USDA-ARS, Morris, MN, B. Lowery, University of Wisconsin, and T.E. Schumacher, South Dakota State University for their assistance, including providing data, with the study.
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Author:Mokma, D.L.; Fenton, T.E.; Olson, K.R.
Publication:Journal of Soil and Water Conservation
Date:Mar 1, 1996
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