soil movement by tillage as affected by slope

Soil& Tillage Research, 17 (1990) 255-264 Elsevier Science Publishers B.V., Amsterdam

255

Soil movement by tillage as affected by slope

M.J. Lindstrom l, W.W. Nelson 2, T.E. Schumacher 3 and G.D. Lemme 3 J USDA-ARS, North Central Soil Conservation Research Laboratory, Morris, MN 56267 (U,S.,4.)

2Southwestern Experiment Station, Lamberton, MN 56152 (U.S.A.) 3South Dakota State University, Department of Plant Science, Brookings, SD 57006 (U.S.A.)

(Accepted for publication 27 February 1990)

ABSTRACT

Lindstrom, M.J., Nelson, W.W., Schumacher, T.E. and Lemme, G.D., 1990. Soil movement by tillage as affected by slope. Soil Tillage Res., 17: 255-264.

Exposure of subsoil material on ridge tops and adjacent sideslopes indicates soil movement away from these positions, i.e. soil erosion. A study was conducted on the University of Minnesota South- western Experiment Station to determine if soil movement by tillage could be a contributing factor to the apparent soil erosion present on many ridge tops. Numbered soil movement detection units ( 11- mm steel hexagonal nuts) were buried 10-cm deep in a grid network in 16 individual plots, on a sideslope with slopes ranging from 1 to 8%. Plots were moldboard plowed and disked in June, and again in August. The direction of tillage was either across the sideslope or up- and downslope. The soil movement detection units were then located with a metal detector, excavated and identified, and distance moved was measured in relation to movement perpendicular and parallel to the direction of tillage. Soil movement was directly related to slope. Movement perpendicular off the moldboard on an 8% downsiope direction was approximately twice the movement upslope. Movement parallel to the direction of tillage was greater than movement perpendicular off the moldboard. Calculations on the angle of movement in relation to tillage direction showed movement toward the downslope position. Results from this study suggest that soil movement by tillage can contribute to soil movement off ridge tops and adjacent sideslopes.

I N T R O D U C T I O N

Lighter colored soils are commonly observed on ridge tops and adjacent sideslopes in the northwestern Corn Belt (western Minnesota and eastern South Dakota) of the United States. It is generally perceived that soil erosion is the cause of these lighter colored soils. As topsoil is removed from these areas, more subsoil material is exposed at the surface, hence the change in color.

The presence of apparent eroded areas in the northwestern Corn Belt does not necessarily correlate with high potential water erosion areas. Generally, the apparent eroded areas are found on ridge tops and immediately adjacent sideslopes, particularly on convex slopes. Young and Mutchler (1969a, b)

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256 M.J. LINDSTROM ET AL.

measured changes in surface elevations and soil loss by water from concave, linear and convex slopes, and found less change at the upper portion of convex slopes and correspondingly lower soil erosion rates. They concluded that soil loss from a slope is dependent on the steepness of the short segment of slope immediately above the point of measurement. On convex slopes, the slope gradient continually decreases to the apex of the slope.

Young and Wiersma (1973 ) concluded that raindrop impact energy was primarily responsible for soil detachment, but transport of detached particles was accomplished by rill flow. On convex slopes, rill formation is predomi- nant on the lower portions of the slope. Moore and Burch (1986) predicted lower soil erosion rates in the upper reaches of complex topographic terrain, with the highest erosion rates occurring along main drainage lines and at the midslope positions. Again, convex slope topography approaches maximum erosion at a greater distance from the ridge apex than linear or concave slopes.

Wind erosion potentials do increase for ridge tops and windward slopes (Woodruff and Siddoway, 1965 ), and may explain the presence of the apparent soil erosion. Until recently, wind erosion in the northwestern Corn Belt was not considered a serious problem. However, the latest National Re- sources Inventory (USDA/SCS, 1987 ) reports that wind erosion may poten- tially be a more serious problem than water erosion in the western Corn Belt. Exposed ridge tops subject to freeze-drying are speculated to be vulnerable to wind erosion during the winter season. Unfortunately, data to support this hypothesis are not available. In fact, validation data on wind erosion in the northwestern Corn Belt are limited.

Soil formation processes may also be a contributing factor to apparent soil erosion on ridge tops and adjacent sideslopes. Due to the location on the landscape, soil development and/or topsoil formation has proceeded at a slower rate because of lower effective moisture regimes (Ruhe, 1969). Therefore topsoil loss, for whatever cause, would be more conspicuous due to a shal- lower depth of topsoil formation.

Mechanical or tillage erosion is another factor that should be considered. Mech and Free (1942) reported high amounts of movement downslope because of tillage. Veseth (1986) reported that tillage erosion over time can be responsible for a significant reduction in topsoil depth and for subsoil exposure on ridge tops. Veseth (1986) and Powell and Herndon (1987) concluded that soil movement from tillage thrust is greater when the thrust is downslope. The objective of this study is to determine if soil movement by tillage could be a contributing factor to the apparent soil erosion on many ridge tops and to quantify soil movement as influenced by slope gradient.

M E T H O D S A N D M A T E R I A L S

The study was conducted during the 1988 cropping season on a sideslope, slope ranging from < l to 8%, located on the University of Minnesota South-

SOIL MOVEMENT BY TILLAGE AS AFFECTED BY SLOPE 2 5 7

western Experiment Station near Lamberton, Minnesota. The soil series was a Ves loam (fine-loamy, mixed, mesic, Udic Haplustoll) formed from loamy glacial till. The entire plot area was initially seeded to oats (Arena sativa L. ) in mid-April.

Sixteen individual plots, 3 × 3 m, were established in mid-May. The sideslope, ~ 70-m wide, was bordered on each side with permanent grass waterways. Permanent markers were placed in the grass waterways for determining exact plot location. Soil movement detection units (SMDUs; l l -mm steel hexagonal nuts), were inserted into the soil profile at a depth of l0 cm by removing a soil core, inserting the SMDU and replacing the soil core. SMDUs were placed in a grid network, 9 rows and 9 columns, giving a total of 81 units per plot, on a spacing of 38 cm. The SMDUs were individually numbered by row and column for location identification.

A detailed topographic survey was made prior to tillage on 1.5-m spacing extending 1.5 m beyond individual plot boundaries, a total of 25 points per plot, for accurate slope determinations. Based on measured relative elevations, an average percent slope value was determined for each plot for the slope perpendicular to the direction of tillage (ASLOPE) and in the direction of tillage (BSLOPE). Uphill slopes, in relation to movement perpendicular and /o r parallel to the direction of tillage, are designated as positive slopes. Downhill slopes are designated as negative slopes. Plots were located to obtain a range in ASLOPE with tillage across the sideslope for analysis of movement perpendicular to the direction of tillage travel (A direction ). A similar series of plots was established with tillage in the up- or downhill direction for analysis of forward movement (B direction). Individual plot locations and the direction oftiUage are shown in Fig. I.

Plots were moldboard plowed with a five-bottom (46-cm plow shears) on- land hitch plow at a depth of 24 cm in the first week of June when the oats began elongating. Soil moisture within the plow layer was low, resulting in a rough cloddy surface. Plots were then disked twice at a depth of 7.5 cm with a tandem disk equipped with 56-cm concave disk blades with 18-cm spacing. Two diskings were required to obtain a suitable seedbed. The direction of disking was the same as moldboard plowing for all plots. A constant wheel speed of 2.1 m s- L was maintained for the plowing and disking operations. The entire plot area was then immediately reseeded to oats. The seeding direction was across the sideslope. Plots were moldboard plowed and disked again in mid-August after the oats had emerged. Only one disking was required to obtain a suitable working surface. The delay between tillage operations was intentional to allow time for the tilled layer to consolidate and provide a more standard soil condition for moldboard plowing.

Plot boundaries were re-established and the SMDUs were located with a Model 6000/Di White's Electronic's 1 metal detector. After locating the posi-


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W

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IE] F-I 3.o J F l 2O ~

2 , 0 ~ " - " ' - - - ~ ~o

" - - / I 0 I I I ~ 1 . 0 " ~

0 10 20 30 4 0 50 6

M E T E R S

Fig. 1. Plot establishment on the sideslope showing relative elevations on a 0.5-m interval. Ar- rows indicate the direction of tillage.

tion of the SMDUs, they were excavated with hand garden trowels. The depth within the tilled zone and position was measured in the A and B directions.

Analysis for movement in the A direction, as influenced by ASLOPE, is based on data points obtained from tillage across the sideslope; ASLOPE values ranged from 7.3 to - 7.7% slope. Additional data points for movement in the A direction could be obtained from up- and downslope tillage direction, but we chose not to include these data points because they were clustered in the + 2% ASLOPE range. Inclusion of these points would result in a bias toward the lower ASLOPE values. Analysis for movement in the B direction is based on data points obtained from tillage up- and downslope; BSLOPE values ranged from 7.7 to - 6 . 8 % slope.

For presentation of data, distances measured in the A and B direction were divided by the number of tillage operations (moldboard plowing plus disking) to convert the data to a single tillage operation.

RESULTS AND DISCUSSION

Movements of the SMDUs, as affected by slope in the A and B directions, are shown in Figs. 2 and 3, respectively. Data points shown are mean distance

~Names of products are included for the benefit of the reader and do not imply endorsement or preferential treatment by USDA, University of Minnesota, or South Dakota State University.

SOIL MOVEMENT BY TILLAGE AS AFFECTED BY SLOPE 2 5 9

"7 fJ

l - - z u.i : i LU

0 :E

I 0 0

8 0

6 0

4 0

2 0

0 - I 0

• I r i I T I "

A = 4 4 . 1 - 1 . 8 ( A S L O P E )

R 2 = 0 . 8 4 . "

I I J I I 1 I I I

- 8 - 6 - 4 - 2 0 2 4 6 8 I 0

A S L O P E (%)

Fig. 2. Movement of SMDUs in the A direction (movement perpendicular to the tillage direction ) as influenced by the slope in the thrust direction (ASLOPE). Upslope gradient is positive slope; downslope gradient is negative.

"7 I - -

z LU :E I, i l

0 : i

I 0 0

8 0

60

4 0

20

0 - I 0

B = 6 3 . 6 - 2 . 4 ( B S L O P E )

R 2 ~--- 0 , 7 7

I ~ t I I I I l I

-8 - - 4 - 2 0 2 4 6 8 i 0

B S L O P E (%)

Fig. 3. Movement of SMDUs in the B direction (movement in the direction of tillage) as influenced by the slope in the direction of tillage (BSLOPE). Upslope gradient is positive slope; downslope gradient is negative.

moved within the individual plots based on located SMDUs and regression equations were developed from these points. A large standard deviation between individual SMDUs was observed for all plots with an average standard deviation o f ~ 40% of the mean value in both the A and B directions. This was expected because movement offthe moldboard is strongly dependent on where the SMDUs strike the moldboard. For example, an SMDU that is struck by the front face of the moldboard would be expected to move a greater distance perpendicular to the tillage direction (A direction) and forward (B direc- t ion) than an SMDU that comes in contact with the back edge of the moldboard. Two tillage operations did not eliminate this large variation in move-


ment. The 38 × 38-cm grid network was established to insure that the SMDUs would strike all portions of the moldboard.

Statistical analysis to test if the slope of the two regression equations was significant was by analysis of all located SMDUs for respective ASLOPEs or BSLOPEs. Results showed the slopes of the two regression equations for movement in the A and B directions to be significantly different from zero at the 1% level. This analysis did reduce the R 2 value considerably because of the large standard deviation in movement by individual SMDUs. However, both the slope and intercept of the two regression equations were essentially the same when determined from mean values within a plot or from individual analysis of all located SMDUs, indicating that the individual SMDUs were normally distributed around the mean value.

Assuming that soil movement is comparable with movement of the SMDUs, then results from these data suggest that soil movement by tillage, particularly when using the moldboard plow, needs to be considered and that the slope is an important consideration. For example, movement on an 8% slope in the A direction is approximately twice the distance downslope as upslope (Fig. 2 ). Similar differences in movement are indicated in the B direction (Fig. 3 ).

Actual movement is the resultant distance moved (C), determined by distance moved in the A direction as influenced by ASLOPE and distance moved in the B direction as influenced by BSLOPE. This distance was calculated, based on the regression equations shown in Figs. 2 and 3, as the hypotenuse of movement in the A and B directions as a function of ASLOPE and BSLOPE (Fig. 4).

C = S Q R (A2+B 2) (1)

where C = total distance moved, SQR = square root, A = distance moved perpendicular to tillage direction (A direction ) and B = distance moved parallel to tillage direction (B direction).

The angle of movement in relation to tillage direction (ANGLE) was also observed to be dependent on both ASLOPE and BSLOPE. This angle was calculated, again based on the regression equations shown in Figs. 2 and 3 (Fig. 5 ).

ANGLE = A T N (A/B) 57.32 (2)

where ATN = arc tangent and 57.32 is the factor to convert radians to degrees. The change in ANGLE as a function of ASLOPE and BSLOPE shows that soil movement by tillage will be directed toward lower slope positions.

A two-dimensional presentation of calculated A, B and C movement plus ANGLE is shown in Fig. 6 for an 8% slope with a 2% cross-slope. The direction of tillage is shown for both upslope and downslope directions, and both directions across the sideslope. Resultant movement (C), as a function of A and B movement plus ANGLE, shows how random tillage direction or op-

S O I L M O V E M E N T B Y T I L L A G E AS A F F E C T E D B Y S L O P E 2 61

/ __ t '~° 120

150 / 90

~ 9 0 I,- z

IE ,,, £° • 6 0 O

,Vo/ 3 0

u - 1 0

ASLOPE (%1 - 5

Fig. 4. Calculated movement in the C direction calculated as a function of ASLOPE and BSLOPE from the hypotenuse of movement in the A and B directions, based on the regression equations shown in Figs. 2 and 3.

6 0 1

~ 50

F- Z 4 0 Ul

UJ > 3 O O :o ~ 20

_1

o i i / o z 1 0

o J. e 1 0

- 5 - 1 0 ASLOPE (16)

Fig. 5. Calculated angle of movement (ANGLE) in relation to the tillage direction calculated as a function of ASLOPE and BSLOPE from the movement in the A and B directions, based on the regression equations shown in Figs. 2 and 3.

2 6 2 M.J. LINDSTROM ET AL.

8 % UPSLOPE

2 % 2 % DOWNSLOPE ~PSLOPE

8 % DOWNSLOPE

Fig. 6. Calculated soil movement by tillage (moldboard plow plus disk) in the A, B and C directions and ANGLE on an 8% slope with a 2% sideslope. Tillage directions represented are both up- and downslope, and both directions across the sideslope. The inner circle represents a radius of 60 cm, the outer circle 90 cm.

posing tillage direction in alternate years will in t ime result in net soil movement downslope, and effectively move soil away from ridge tops and adjacent sideslopes.

Some problems were encountered in this study, which need to be considered in the interpretat ion of these data. Only ~ 55% of the SMDUs were located. The mean depth at which SMDUs were found was 10 _+ 5 cm. In effect, SMDUs located in the lower port ion of the tilled zone were not found. The metal detector was not capable of locating these SMDUs. Therefore our analysis may only apply to soil movemen t o f the upper 15 cm of soil and not to the entire plow layer. Tillage depth in this study was 24 cm.

We assumed that only slight movemen t would occur with disking. However the data indicate that more than expected movement occurred, and in some cases excessive movemen t probably resulted from disking, particularly in the B direction. Therefore the total effect of soil movement by tillage for this study is the additive result o f moldboard plowing and disking. The purpose of disking with the first tillage operation was to provide an adequate seedbed for oats. Two disking operations were required to break down the coarse clods resulting from moldboard plowing in a dry soil condition. The disking depth

SOIL MOVEMENT BY TILLAGE AS AFFECTED BY SLOPE 263

was kept shallow (7.5 cm) to stop soil build-up in front of the disk blades. However soil tended to build up in front of the blades adjacent to the tractor wheel tracks. Disking after the second moldboard plowing was done to keep the tillage operations uniform and to provide a smooth working surface. Only one disking was required. Again, soil build-up was observed occasionally in the area adjacent to the tractor wheel track. Casual analysis of forward movement showed an increase in movement from SMDUs located near the soil surface, indicating that the disk was causing forward movement . The magnitude of movement could not be determined; however we can assume that if full penetration of the disk had been allowed, the magnitude of forward movement would have been greater. In extreme cases, forward movement was observed to be/> 3 m. SMDUs that were judged to have excessive movement were eliminated from analysis. As a guide, SMDUs were eliminated from analysis when movement exceeded 2.5 times the mean value. Approxi- mately 5% of the located SMDUs fell into this category.

The use of 1 l -mm nuts for SMDUs may also be questionable. The SMDUs have a higher density (7.8 Mg m - a ) than natural soil aggregates of similar size ( 1.45-1.65 Mg m - 3 ) and may not necessarily move the same as the tilled soil. Different metallic objects were tested for their ability to be located in the soil at varying depths, and it was determined that metallic objects similar in size to the 11-mm nuts or larger were required for detection at the desirable depths. In trying to locate the SMDUs after the two tillage operations, it be- came apparent that units at the lower depths were not being found. Our as- sumption was that the SMDUs would move with the soil mass provided they were part of the mass. The SMDUs were initially inserted into the tilled layer to insure that they would become part of the soil mass.

The speed of tillage could also have an effect on movement . A Model 4440 John Deere tractor, 180 kW, was used for the tillage operations. The tractor had adequate power to pull the plow and disk up- and downslope without loss of RPMs; constant wheel speed was 2.1 m s-1, but wheel slippage was not measured. Wheel slippage may have been present when plowing upslope, but would be comparable to actual field operations.

C O N C L U S I O N S

Random tillage or opposing tillage direction in alternate years by moldboard plow plus disking will result in a net downslope soil movement over time. Soil movement is strongly correlated to slope and soil will move a greater distance when the thrust is downslope rather than upslope. Also, the angle of movement from this combinat ion of tillage equipment is not constant and will direct soil movement toward the lower slope position. The results from this study, while not eliminating the effects of soil erosion by wind or water, do show that the effect of soil movement by tillage will contribute to the ap-


parent soil erosion observed on ridge tops and adjacent sideslopes. Since the moldboard plow moves the greatest volume of soil, we hypothesize that con- tinued moldboard plowing across these landscape positions with time will result in larger areas of exposed subsoil material at the soil surface.

ACKNOWLEDGMENTS

This paper is a joint contribution of the North Central Soil Conservation Research Laboratory, Agricultural Research Service, USDA, in cooperation with the Minnesota Agricultural Experiment Station, Scientific Journal Se- ries #17093, and South Dakota Experiment Station Scientific Journal Series ~2414.

REFERENCES

Mech, S.J. and Free, G.A., 1942. Movement of soil during tillage operations. Agric. Eng., 23: 379-382.

Moore, I.D. and Burch, G.J., 1986. Modeling erosion and deposition: topographic effects. Trans. Am. Soc. Agric. Eng., 29: 1624-1630, 1640.

Powell, G.M. and Herndon, L.P., 1987. Maintenance of field conservation structures. In: Opti- mum Erosion Control at Least Cost. Proceedings of a National Symposium on Conservation Systems. American Society of Agricultural Engineers, St. Joseph, MI, pp. 374-383.

Ruhe, R.V., 1969. Quaternary Landscapes in Iowa. Iowa State University Press, Ames, IA, 225 PP.

USDA/SCS, 1987. Basic Statistics 1982. National Resources Inventory. Stat. Bull. 756, Iowa State University, Ames, IA, 153 pp.

Veseth, R., 1986. Tillage erosion - changing landscapes and productivity. In: R. Veseth and D. Wysocki (Editors), STEEP Extension Conservation Farming Update, Fall 1986. University of Idaho, Moscow, ID, pp. 2-7.

Woodruff, N.P. and Siddoway, F.H., 1965. A wind erosion equation. Soil Sci. Soc. Am. Proc., 29: 602-608.

Young, R.A. and Mutchler, C.K., 1969a. Effect of slope shape on erosion and runoff. Trans. Am. Soc. Agric. Eng., 12: 231-233, 239.

Young, R.A. and Mutchler, C.K., 1969b. Soil movement and irregular slopes. Water Resour. Res., 5: 1084-1089.

Young, R.A. and Wiersma, J.L., 1973. The role of raindrop impact in soil detachment and transport. Water Resour. Res., 9: 1629-1636.

soil movement by tillage as affected by slope

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