Contribution agreements were supported to continue two ongoing research
projects which were deemed extremely important to understanding soil structure
and erosion modelling.
Contributions
- The Relationship between Landscape Position, Tillage Practices, and
Soil Loss: Model Development - University
of Guelph - Dr. R. G. Kachanoski - $76,528.00
View/Download report
[136 KB pdf]
- Methodologies for Assessing Soil Structure and for Predicting Crop
Response to Changes in Soil Quality - University of Guelph - Dr. B. D. Kay
- $83,472.00
View / Download Report [1001 KB pdf]
The Relationship
between Landscape Position, Tillage Practices,
and Soil Loss: Model Development
D.A. Lobb, Dr. R.G. Kachanoski,
Centre for Land and Water Stewardship
Department of Land Resources Science
University of Guelph, Guelph, ON N1G 2W1
September, 1993
EXECUTIVE SUMMARY
In 1987 the University of Guelph
initiated a soil erosion study, Management of Farm Field Variability II. Soil
Erosion Processes on Shoulder Slope Landscape Positions (SWEEP/TED),
at two field sites in southwestern Ontario, one in Brant County and the second
in Middlesex County. The study measured tillage translocation and tillage
erosion on convex upper slope landscape positions. The estimated rate of soil
loss resulting from net downslope translocation was in excess of 6.5 kg m-2
yr-1 at the Brant Co. field site and in excess of 4.5 kg m-2
yr-1 at the Middlesex Co. field site. Subsequent examination of
that data recognized that tillage erosion was responsible for at least 70
% of the total soil lost on the upper slope landscape positions based on estimates
of total soil loss using resident 137Cs.
A second study, Soil Loss by
Tillage Erosion: The Effects of Tillage Implement, Slope Gradient, and Tillage
Direction on Soil Translocation by Tillage (SWEEP/TED), by the University
of Guelph from 1990 to 1991 at two field sites in Huron County was conducted
to determine the effect of tillage implement type on the magnitude of tillage
translocation and tillage erosion under a range of slope gradients in topographically
complex landscapes. All four tillage implements, the chisel plough, mouldboard
plough, tandem disc and field cultivator, were found to be erosive, causing
soil loss on upper slope landscape positions and soil accumulation in lower
slope landscape positions.
Tillage erosivity, the potential
for tillage events to erode soil within a landscape, was recognized to be
a function of several physical and human parameters, including: tillage tool
shape and arrangement within a tillage implement, tillage implement length
and width, tractor-implement match, tillage depth and tillage ground speed,
and tillage operator response to varying landscape conditions. The tillage
parameters are controlled by selection and varied by the operator. Landscape
erodibility, the potential for the soil within the landscape to be eroded
by tillage events, was recognized to be a function of the topographic and
soil parameters, including: slope gradient and curvature, and field soil bulk
density, soil moisture content, and the ability of the soil to resist displacement
and translocation (internal friction due to cohesion and adhesion). The landscape
parameters evolve through erosion.
The objective of this, the
third study conducted by the University of Guelph, was to define the relationship
between tillage erosion and landscape position in the form of a model based
on the data collected in the Huron County study.
In the proposed model, tillage
erosion was calculated as the net translocation at specified points in the
landscape, the difference between the soil translocated into a point and the
soil translocated out from that point during a single tillage operation. Tillage
translocation was related to slope gradient and slope curvature by a simple
linear function. The translocation in to and out from a point was calculated
from forward and backward differences in topographic conditions. Therefore,
the model predicted soil redistribution from forward tillage translocation
along two-dimensional landscape profiles.
The proposed tillage erosion
model was calibrated using experimental data from the Huron Co. study Soil
Loss by Tillage Erosion: The Effects of Tillage Implement, Slope Gradient,
and Tillage Direction on Soil Translocation by Tillage (SWEEP/TED).
The proposed tillage erosion
model was validated using data collected during two preceding studies, Management
of Farm Field Variability I. Quantification of Soil Loss in Complex Topography
(SWEEP/TED) conducted in Brant County and Soil Loss by Tillage Erosion: The
Effects of Tillage Implement, Slope Gradient, and Tillage Direction on Soil
Translocation by Tillage (SWEEP/TED) conducted in Huron County. Resident
137Cs radioactivity was used to estimate soil redistribution within
the landscapes of the field sites. These estimates of soil loss and accumulation
were compared to those predicted by the tillage erosion model based on the
topography of the field sites.
The proposed tillage erosion
model provided a reasonably accurate prediction of soil redistribution at
the Brant County field site when compared to that estimated using resident
137Cs radioactivity. The tillage erosion model provided a relatively
poor prediction of soil redistribution at the Huron County field site when
compared to that estimated using resident 137Cs radioactivity.
There is some indication that the poor prediction for the Huron site was due
in part to the model's simplicity (not able to predict the effect of curvature
asymmetry on tillage erosion - a problem which would be greater at this site
than the Brant site because of smaller scale of the ridge). Soil losses, based
on the 137Cs data, were situated on the convex upper slope landscape
positions, but they were greater in severity on the shoulder slope position
of the steeper of the ridge's two slope faces. Although the model correctly
predicted the general pattern of soil losses and accumulations, the model
underpredicted the magnitude, or severity, of soil losses at both field sites.
Too few data of soil accumulation estimates were available to make a similar
inference about soil accumulation. Several possible reasons for this underprediction
of soil loss were identified:
-
the tillage implements and
the tillage sequence used to predict the soil redistribution may have been
less intensive than those responsible;
-
inaccuracies associated with
the use of resident 137Cs may have caused overestimation of soil
redistribution (the problem associated with point measurements resulting
in apparent losses on backslope positions, as well, the current level resident
137Cs for a non-eroded site may be much less than the assumed
2500 Bq m-2 in Huron County);
-
wind and water erosion may
have caused soil redistribution in addition to that caused by tillage erosion
(the redistribution pattern is inconsistent with that of soil erosion by
overland water flow).
For a first attempt at modelling
tillage erosion in complex landscapes the performance of the proposed model
was considered very good. Clearly, there are limitations to the complexity
and consequently predictive capabilities of the model due to the lack of experimental
data for calibration procedures, particularly tillage depths. At the time
the study was initiated the number of parameters involved and the complexity
of the relationships was not fully appreciated. This was exploratory research,
and therefore presuming that a model could be developed on such a data set
was very ambitious.
The fact that the proposed
tillage erosion model predicts greater rates of soil loss on convex upper
slope landscape positions where severe soil loss occurs, and soil accumulation
in concave lower slope landscape positions where soil accumulation is observed,
indicates that this model is more appropriate than water erosion models for
predicting soil erosion in topographically complex landscapes. Consequently,
it can be presumed that the proposed tillage erosion model is more appropriate
than water erosion models for basing soil conservation decisions relating
to soil degradation and soil productivity. Comprehensive soil erosion models
including submodels for erosion by wind, water and tillage may provide the
best prediction of soil redistribution in topographically complex landscapes.
Methodologies
for Assessing Soil Structure and for Predicting
Crop Response to Changes in Soil Quality
University of Guelph
B. D. Kay, A. da Silva, K. Denholm, N. Eshraghi, E. Perfect and V. Rasiah
EXECUTIVE SUMMARY
The objectives of this study were:
-
to identify a method(s) for measuring soil structural
changes which may be related to soil management systems and which can be
shown to be useful for characterizing changes in soil quality across a range
of soil conditions and;
-
to evaluate existing crop productivity in terms of their
suitability for predicting crop response to changes in soil quality.
The budget associated with the contract was directed to
field and laboratory studies related to objective (a) and the collection of
field data to be used in the evaluation of crop productivity models [obj.
(b)]. The research related to objective (b) has been part of the work plan
of an Agriculture Canada Research Branch staff person and the salary expenditures
associated with this part of the project have not been charged to the contract.
The field studies for the project were located on the farm
of Mr. Don Lobb, Huron County. This site was one of the
T-2000 sites investigated during the Ontario
Land Stewardship program and is one of the longest running field scale side-by-side
comparisons of zero and conventional tillage in Ontario. The comparison is
maintained as a strip about 0.5 km in length which traverses soils with clay
contents ranging from 7 to 35%. The site was maintained in corn production
in 1991 and 1992. (The study has been extended to 1993 and supported by funds
from alternative sources). Thirty-six locations (soils) were identified on
each transect (tillage treatment) for detailed studies on soil structure.
Soil structure can be defined in terms of structural form
and structural stability. Structural form relates to the arrangement or "architecture"
of solid and void spaces whereas structural stability refers to the resistance
of structural form to deformation (including fragmentation) when stress is
applied. Structural form can progressively change subsequent to a change in
soil or crop management practices through changes in the level of stress applied
to a soil or by changing the population of soil organisms (e.g. earthworms).
Structural form will also change if the stress remains constant but stability
changes. Management practices can cause changes in stability by causing changes
in the level of stabilizing materials (primarily organic in origin) in soils.
Methodologies to assess both structural stability and structural form were
assessed in this study. Pedotransfer functions were developed, where possible,
in order to describe the contribution of inherent soil properties to the magnitude
of the different parameters that were measured.
Parameters which were used to describe structural stability
related to the resistance of soil to deformation by two types of stress: moving
water and mechanical stress causing fragmentation. Stability parameters related
to moving water were assessed at two different scales: that of aggregates
> 0.25 mm, and that of clay-sized particles (< 0.002 mm). The resistance to
mechanical stress was assessed using tensile strength and the distribution
of aggregate sizes created by tillage.
Preliminary studies using rainfall simulation techniques
indicated that the amount of runoff and the amount of sediment in the runoff
arising from a rainfall event were related to dispersible clay and time to
ponding; and that these parameters became more important as the extent of
surface cover by crop residues decreased. Time to ponding is related to infiltration
characteristics and was found to be strongly dependent on wet aggregate stability.
Stability parameters at the scale of aggregates and at the scale of dispersible
clay both appeared, therefore, to be important in describing runoff and sediment
load in the runoff. Studies were therefore initiated to assess both characteristics
in more detail.
A turbidimetric technique was developed to expedite characterization
of dispersible clay across the range of soils on the study site. The technique
involved developing a standard curve (turbidity as a function of concentration
of dispersible clay) which can be described as a function of inherent soil
characteristics (clay and organic matter content), and then characterizing
the dispersibility of clay. Variation in the characteristics of the standard
curve with soil properties appeared to be due to the concentration range in
which the standard curve was determined and the mean weight diameter of the
dispersed clay fraction. A single curvilinear standard curve was found to
be applicable to all of the soils on the study site since the curvilinear
representation incorporated the influence of both concentration and mean weight
diameter. The dispersible clay content was found to increase with increasing
clay content, increasing water content and decreasing organic matter content;
the variation in dispersible clay content with tillage appeared to be due
primarily to the influence of tillage in reducing the organic matter content.
Wet aggregate stability was found to increase with clay,
water and organic matter content. The reduction in stability with tillage
appeared to be related to the reduction in organic matter content with tillage.
The response of soil to mechanical stress was assessed
by considering tensile strength measurements and the dry aggregate size distribution
created in seedbed by tillage. Tensile strength increased with increasing
clay content, wet aggregate stability and decreasing organic matter content.
Aggregate size distributions were assessed using different approaches. A description
of the distribution by fractal theory was found to be most accurate. The analyses
indicated that the number of aggregates in the largest size fraction increased
with increasing clay content, wet aggregate stability and decreasing organic
matter content. A comparison of tensile strength and aggregate size distribution
characteristics showed a highly significant correlation indicating increasing
fragmentation with decreasing tensile strength. The analyses suggest that
one parameter could be predicted from the other and that, for a given application
of stress through tillage, either parameter could be predicted from inherent
soil characteristics.
Parameters that were used to describe structural form included
both static and dynamic parameters. Bulk densities and relative bulk densities
were measured. The concept of least limiting water range (LLWR) was used to
describe the combined effects of structural form on aeration, resistance to
penetration and available water and represented measurements under "static"
conditions. Structural form was characterized under dynamic conditions using
infiltration measurements. Once again the sensitivity of these parameters
to inherent soil properties and to management was determined.
Bulk density was found to vary with clay and organic matter
contents and was higher on the no till than the conventional till treatment.
The relative bulk densities were determined by dividing the observed bulk
density of each soil by the bulk density determined after compacting each
soil with a compressive stress of 200 kPa.The bulk density after compaction
was also found to vary with clay and organic matter content. The relative
bulk density was however constant across all soils for a given tillage treatment
and was 11% higher on the no till treatment. This type of analysis has not
been done before and obviously has important implications for all laboratory
studies in which bulk density and inherent soil properties are variables.
Values of LLWR were determined by establishing the functional
dependence of the water release curve (potential versus water content) and
the soil resistance curve (resistance to penetration versus water content)
on bulk density, clay and organic matter content. Limiting values were then
assigned, using generally accepted criteria in the literature, for aeration
(10% air filled porosity), field capacity (0.01 MPa), permanent wilting point
(1.5 MPa) and resistance to penetration (2.0 MPa) to these functions in order
to define the LLWR for each soil. Analyses showed a wide variation in LLWR
with clay and organic matter content for a given tillage treatment. Correlation
of LLWR with plant growth parameters indicated a strong correlation between
LLWR and plant population. Analyses are still underway relating soil water
content and LLWR to leaf extension during the growing seasons.
Infiltration was measured in the non-trafficked inter-rows
on all 36 locations under both tillage treatments. The field saturated hydraulic
conductivity, Kfs, was found to be higher under the no-till treatment
than under conventional till and may reflect greater continuity in macropores
in the no-till treatment. A statistically significant, but poor, correlation
existed between Kfs and inherent soil properties.
Data were collected that could be utilized in evaluating
plant growth models. Climatic records were obtained from a weather station
maintained on the site. Additional information on plant response parameters
(yields, root distributions) were also recorded.
Adaptation of current crop productivity models is being
undertaken by Mr. Ken Denholm, Agriculture Canada Research Branch, Guelph
as part of this project. This activity has not progressed as rapidly as originally
anticipated. However, once the models are developed a complete data set is
available to assess the models in terms of their ability to predict yield
response on soils of different structure and under the dramatically different
climatic conditions that existed in 1991 and 1992.
Created: 09-21-1996
Last revised:
Sunday, May 08, 2011 02:50:13 PM
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