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SWEEP Report #38

Management of Farm Field Variability. I. Quantification of
Soil Loss in Complex Topography. II. Soil Erosion
Processes on Shoulder Slope Landscape Positions

Researchers: 
R.G. Kachanoski, M.H. Miller, R.D. Protz and D.A. Lobb, Department of Land Resource Science, University of Guelph, Guelph, Ont., and E.G. Gregorich, Agriculture Canada, Ottawa, Ont.

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

View / Download Final Report  [1959 KB pdf]  (includes appendices)

Associated SWEEP/LSP Research

 

 

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Completed: January, 1992

Key Words:

soil erosion, soil loss, topography, shoulder slope, slope position, soil management, Cesium 137

Executive Summary

This report documents the rationale, objectives, and general methodology of the SWEEP/TED Management of Farm Field Variability Project. In addition, two specific aspects of that project are also documented.

In the first study the movement of soil in a complex three dimensional landscape typical of southern Ontario is examined using 137Cs as a soil tracer. The study made use of a previous study by L.S. Crosson and R.D. Protz in the Department of Land Resource Science, University of Guelph. They sampled 180 soil landscape positions in a 10 m by 10 m grid in 1972 and stored the soil cores. The site was resampled in 1987 and through the SWEEP/TED program the soil samples were analyzed for 137Cs. The 137Cs in soil comes from atmospheric deposition (fall-out) from above ground nuclear testing in early 1950's and 1960's. The 137Cs does not naturally occur in soil, but when added to soil it binds tightly and is not leached or taken up by plants to a significant extent. The 137Cs deposited by the atmospheric deposition was subsequently mixed evenly in the plow layer by tillage. Thus, the movement (losses or gains) of 137Cs in a landscape is caused by the movement of soil.

The 137Cs landscape study indicated that soil losses on shoulder and crest slope landscape positions were greater than 100 t/ha/yr. This confirms earlier observations by L.A. Battiston and M.H. Miller, Department of Land Resource Science, University of Guelph, that these are areas of severe erosion. Total field scale soil loss was negligible with concave lower slope areas gaining all of the soil lost on the convex upper slope areas. The major conclusion is that water erosion is not likely the major process responsible for the soil loss. This suggests that the assumption that crop productivity losses from erosion and the environmental effects of erosion are linked, is not valid. Significant soil redistribution is occurring within the complex topography of Ontario, which is not related to the off field transport of sediment, phosphorus, and other chemicals.

In the second study summarized in this report, the processes responsible for the severe erosion on upper slope landscape positions, specifically shoulder slopes, are investigated. The study was carried out as part of the M.Sc. thesis research of D.A. Lobb, Department of Land Resource Science, University of Guelph. The hypothesis was that the soil being lost on these slope positions is the result of the mechanical action of tillage equipment.

Tillage erosion, the net downslope translocation of soil by tillage, was measured on eight shoulder slope landscape positions in two topographically complex field sites typical of the upland regions of southwestern Ontario. Cesium-137 was utilized as a labelling element to generate a tracer-pulse for the measurement of soil translocation. Soil translocation was calculated using the synthetic step response distribution synthesized from a succession of convoluted 137Cs pulse response distributions. Paired plots were utilized to compare soil translocation by upslope and downslope tillage.

A single sequence of conventional tillage operations, consisting of fall moldboard plow, and spring tandem disc (double pass) and C-tine cultivator (single pass), translocated upslope approximately 80 kg of soil per meter slope width when tillage was conducted upslope, and translocated downslope 120 kg/m slope width when tillage was conducted downslope. Therefore, the net downslope soil translocation of two tillage sequences, one upslope and one downslope, was approximately 40 kg/m slope width. Assuming tillage operations are conducted upslope and downslope equally as often, the rate of net downslope soil translocation would be approximately 20 kg/m slope width per tillage sequence. Assuming one sequence of tillage operations occurs per year, the rate of net downslope soil translocation would be approximately 20 kg/m slope width per year. The convex slope length between the crest and the position of the plots was approximately 3 m. Assuming this distance was the source length for the net downslope soil translocation, 20 kg/m slope width per year represents the total annual soil loss from the 3 m slope length. The average annual soil loss over this convex slope length would be 6.7 kg/m2/yr, or 67 t/ha/yr, when tillage operations occur upslope equally as often as downslope. Tillage erosion is a major cause of the severe soil loss observed on shoulder slope landscape positions in the complex topography typical of the upland regions of southwestern Ontario.

The significance of the conclusion that soil loss by tillage translocation is an order of magnitude higher than soil loss by water cannot be overstated. The implications are discussed in detail in this report. Briefly, soil loss by tillage will be restricted to the upland regions of Ontario, but will occur to some degree on all convex slope positions in all regions with complex topography. The validity of all soil loss and erosion estimates using any form of soil tracer such as 137Cs are questionable unless the changes are known throughout the landscape. Predictive soil loss and crop productivity models that do not include the process of soil loss by tillage will not represent reality for cultivated land in complex topography and will have little predictive value. Preventative and remedial managements which do not control tillage soil loss will have little value in maintaining or enhancing soil quality.

Recommendations are also discussed and include reducing the frequency and intensities of tillage. In addition, the size of tillage implements should be reduced. The possibility of varying tillage patterns within fields is also discussed. Further research, on the amount of tillage soil loss by secondary tillage implements and the amount of loss under different topographic shapes and equipment speeds needs to be carried out.

 

Evaluation Summary

(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)

The intent of the study was to examine the relationships between soil loss, landscape configuration and slope position on Ontario croplands. Of question to the investigators was the tenet that most soil loss on complex landscapes in Ontario was caused by soil erosion by water and that the process was directly linked to surface water contamination.

The study was divided into two parts:

  1. to quantify in situ losses of soil from complex landscapes;

  2. to examine the process of soil loss from these areas.

In the first study, Cesium 137 was used as a radioactive isotope tracer to correlate with soil loss following calibration procedures. The field site was first sampled on a grid pattern in 1971 for detailed soil and landscape features. The soil material was stored and subsequently analyzed for Cesium 137 in 1986. The procedure was repeated in 1986. Changes in the surface content of Cesium 137 were compared to the 1971 data. On the silty soils found in the field site, marked drops in 137Cs content on crest and shoulder slope positions were observed. Considerable increases were observed in lower slope positions and depressional areas. Using calibration procedures and mathematical models the rate of soil loss and gain was estimated. The rates were up to 100 t/ha/yr from convex slope positions. This far exceeds predictions from other models such as the Universal Soil Loss Equation (USLE).

The process of soil erosion on complex topographies with erodible materials was examined in the second study. First, the processes of soil erosion were explained and evaluated for the likelihood of explaining the relationships observed in the first experiment. From the literature, this study and previous work in Ontario, tillage erosion was identified as having a potential causal relationship with extremely high erosion rates on complex topography. The planing and displacement effects of tillage implements were examined. Cesium 137 was sampled before and after a variety of tillage passes on metre-wide strips along the contours of slope positions. Upslope and downslope operations were examined. If, on average, an operator plows up and down slope the same number of times over any landscape position, the net loss could be up to the equivalent of 67 t/ha/yr on shoulder slope positions.

The investigators concluded that wind and water erosion processes could not explain soil losses of these extremes (USLE predictions would be 10 to 30 t/ha/yr). The investigators concluded that excessive tillage is directly related to the loss of soil and its productive capacity.

Comments:

A combination of research vision, understanding of geomorphological processes and mathematical modelling make this study among the most comprehensive and controversial studies conducted within the TAP sub-program of SWEEP. This information could be particularly useful for the research community as a hypotheses-generating document for further work. It could also shed light on program planning and extension work in tillage and cropping systems on upland soil- landscape conditions in Ontario. For programming, perhaps water quality and soil erosion are not necessarily linked - soil quality and productivity are also valid rationales for financial and technical assistance.

For extension work, excessive tillage is directly and not just indirectly related to soil productivity and financial losses. Conservation tillage and no-till may be appropriate technologies for yet another reason.

However, it should be clear that this is a well done case study. It is difficult to extrapolate to all upland or complex topographies without a more comprehensive process study.

Associated SWEEP/LSP Research:

  • SWEEP Report #45 - Management of Farm Field Variability. III. Effect of Tillage Systems on Soil and Phosphorus Loss
  • SWEEP Report #46 - Management of Farm Field Variability. IV. Crop Yield, Tillage System, and Soil Landform Relationships
  • SWEEP Report #49A - Land Reshaping of Lowland Clay Soils. I. Field Study
  • SWEEP Report #55 - Soil Loss by Tillage Erosion: The Effects of Tillage Implement, Slope Gradient, and Tillage Direction on Soil Translocation by Tillage
  • SWEEP Report #60 - The Effect of Conservation Tillage Practices on the Losses of Phosphorus and Herbicides in Surface and Subsurface Drainage Waters
  • SWEEP Report #66 - Volume V. Economic Assessment of the Technology Evaluation and Development (TED) Program

Future Research: ( ) indicates reviewers suggestion for priority, A - high, C - low.

(C) An integrated research program is needed to examine erosion and other degradation processes and the impact of current and remedial practices on soil quality on Ontario cropland. From this, knowledge could be gained about the relative impact of tillage, water and wind erosion on a variety of representative soil and landscape conditions.

(A) Further, the impact of tillage and cropping practices could be examined. And finally, changes to farming systems as well as rehabilitative work could be tested to determine the effectiveness of conservation measures. This study could complete the much needed work left undone by the successful T-2000 program.

 

 

 

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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 03:04:01 PM