Background
Local
Sample transects 7 and 8 were located in a recently cultivated field near Miami Manitoba
in late October. This field had a plow layer in the A horizon from cultivation. Transects 7 and 8
were composed of 3 sample pits; one on the upper slope, one on the mid slope, and one on the
toe of a slope (Figure 1). The A horizon was the thickest at the bottom of the slope. The soil
sampled in transects 7 and 8 had texture ranging from clay to loam, based on the feel method
performed on the soil from each horizon in each pit after the soil had been dried.
Regional
The South Tobacco Creek Watershed in Miami Manitoba is the area where soil was
sampled (Figure 1). This region is part of the Manitoba Escarpment created by retreating
glaciers (Lake Agassiz). The elevation of this watershed decreases from east to west and drains
into the Red River which flows into Lake Winnipeg (Red River Valley) (WEBs, 2012). It is part
of the Pembina Hills ecodistrict of the Prairie ecozone. This ecodistrict has a semi-arid climate
with mean annual precipitation of 540mm and a mean annual temperature of 2.5°C (Smith et al,
1998). This region has short growing seasons due to the short, warm summers and long, cold
winters (Smith et al, 1998).
The South Tobacco Creek Watershed is dominated by clay loam which includes the soil
types horose and dezwood. This soil is generally calcareous and forms from shale parent
material (Ellis and Shafer, 1981). Shale parent material was deposited as till by morainal glacial
activity (Ellis and Shafer, 1981). This deposition created a hummocky landscape with variable
drainage due to the presence of moderate slopes despite low precipitation (Ellis and Shafer,
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1981). The land of the South Tobacco Creek Watershed was once heavily forested but has
since been converted primarily for agricultural purposes which primarily consist of oil and cereal
crops (WEBs, 2012).
N
Step 7-1
Step 7-2
Step 8-3
Step 7-3
Step 8-2
Step 8-1
1cm=32.7 m
Figure 1: Bird’s-Eye-View. The image above shows the location of transects 7 and 8 within the field and the
location of steps 1, 2, and 3 within each transect. This field is located 10.7km south-west of Miami Manitoba.
The insert image in the bottom right corner shows the location of the sampled field in context to the South
Tobacco Creek Watershed. Located on the edge of the sampled field is trees.
Soil Formation and Landscape Processes
The soil from the sample site has undergone dramatic development from till deposition to
a forested area to its present state of an agricultural field.
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History
Shale is a type of fine-grained, clay rich sedimentary rock that is easily weathered. Even
though the soil may be relatively young due to glacial deposit, soil formation from this parent
material may be well developed as shale is fairly easily weathered (Brady and Weils, 2008,
pg.62). Soil presently seen in this area represents the features from this parent material. The soil
found throughout the sampled pits in transects 7 and 8 were determined to be primarily
clay/loam by the feel method. Clay has a high surface area and high total pore space. This
allows the soil to hold more water and nutrients. These pores are primarily micropores filled
with water, which is for the most unavailable to the plants. The high water content in the pore
space of clay leaves a small amount of space for air which creates high carbon dioxide levels
(Brady and Weils, 2008, pg.24). These conditions are unfavourable for higher plants. As the
soil type approaches loam, these conditions improve.
At the top and middle of the slope, the plants may have insufficient moisture. In
response, these plants will invest more in root growth over shoot growth, creating lower yields.
There may also be periods of high moisture content at the bottom of the slope from the
accumulation of water from higher elevations. This may cause leaching of essential nutrients
and basic OH- ions, decreasing available nutrients for plants and increasing soil acidity. Soils
high in clay content are also prone to compaction from traffic which increases the bulk density of
soil, thereby decreasing the penetrative ability of plant roots. This decreases the amount area
containing nutrient and water roots are able to exploit.
After conversion to agriculture, trees were still established at the toe of the slope where
steps 3 of both transects are found. These trees would have allowed an increase accumulation of
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organic matter (leaf litter) when compared to farming which removes most of the plant residue
from the field through harvesting. Organic matter is a major source of nutrients in the soil. The
soil structure and stability also decrease in transition from native deep rooted trees to annual
crops. Roots from these trees are capable of exploiting more soil area, thereby decreasing the
soil moisture content and increasing the shrinking of soils. This increases the aggregation and
structure of soil (Brady and Weils, 2008, pg. 466). The extent of soil area exploited in forested
areas is even greater due to the common presence of mycorrhizae fungi which increase water and
nutrient uptake for the tree in exchange for sugars (Brady and Weils, 2008, pg.472). The
removal of trees causes a decrease in soil organism abundance as a result of the decrease of root
area and decrease in organic matter which serves as food for soil organisms. Soil organisms
have such positive influences on soils and plants including decomposing organic matter and
releasing nutrients for plants.
Erosion
Erosion is a major factor influencing soil in the area being sampled. Even with the
incorporation of plant residues into the soil through tillage and the removal of the trees reducing
cover, water and wind erosion have minor implications in this particular landscape. Wind
erosion is minimal because of the trees surrounding the field and the hummocky nature of the
field obstructing wind. Water erosion is minimal because of the low annual precipitation of this
region and the slope occurring throughout transects 7 and 8 is not long or steep enough to
concentrate the flow of water to cause serious erosion of the soil.
The major source of erosion in the sampled field was tillage erosion. Tillage erosion
occurs from the disturbance of soil during cultivation. During the farming season, the field may
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be cultivated multiple times to mix in organic matter and alleviate compacted soils at the surface.
This is in to increase decomposition of plant residues, control weeds, aerate soil, and prepare the
field for seeding. This may have immediate benefits but the long term effects of intensive tillage
are negative. It creates compaction deep in the soil (plow pan) from the increase in traffic and
reduces organic matter and structure which prevents the penetration of roots (Brady and Weils,
2008, pg.154). Another factor of cultivation is the negative impact on soil organisms due to the
decrease in organic matter and disruption of habitat.
Tillage erosion from the mechanical movement of soil was the most evident form of
erosion in transects 7 and 8. The hummocky nature of the sample site resulted in the removal of
nutrient rich A horizon from the slope top to the slope bottom. Tillage down slope moves more
soil than tillage up slope due to the forces of gravity. This increases productivity at the toe of the
slope as this top soil is rich in many nutrients such as organic carbon, which is an essential
nutrient for all life (Xiaojun, 2013). With the topsoil relocated, less productive subsoil may be
exposed to the surface at the top of the slope. This subsoil is also more prone to erosion from
water and wind (Lobb, 2011).
This was evident in transects 7 and 8. As seen in Figure 2, the A horizons of step 3 in
transects 7 and 8 extended deeper than steps 2 and 1 in either transect located at the middle and
top of the slope. The similar depths of horizons A, B, and C between the middle and top steps (1
and 2) can be explained by tillage erosion and the lack of soil formation on slopes due to
decrease of water infiltration and plant establishment (Brady and Weils, 2008, pg.60). The deep
A horizon at the toe of the slope creates high plant productivity and crop yields, while the
shallow A horizons on the top and middle slope lead to decreased crop yields (Lobb, 2011). If
intensive tillage continues, the topsoil may be completely moved from the top of the slope and
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moved to the toe of the slope. Eventually subsoil may be translocated from the slope top to the
slope bottom, covering of the slope bottom with unproductive subsoil and causing widespread
declining crop yields (Lobb, 2011).
Figure 2: Transects in Profile View. The image above shows the soil profiles from each step in transects 7 and
8. The A horizons are greater in steps 3 of both transects 7 and 8 when compared to steps 1 and 2. The
estimated distance from the top to the bottom of the slope is indicted as 4m. The Legend shows the meaning of
the various symbols on the image.
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Remediation
There are several methods that can be implemented to slow and even reverse the impacts
of tillage erosion. Conservation tillage will minimize the displacement of soil while reducing
removal of crop residue on the field, decreasing wind and water erosion (Lobb, 2011). Some
conservation tillage practises such as using a chisel plow to leave plant residues will still cause
significant tillage erosion (Lobb, 2011). Even zero tillage practises result in some erosion during
seeding. In order to reverse the current tillage erosion, surface soil should be removed from the
area of topsoil accumulation at the toe of the slope and deposited in an area of topsoil depletion
at the top of the slope (Lobb, 2011). “This will decrease the soil variability, decreasing crop
variability, and increasing the overall productivity of the field” (Lobb, 2011). Conservation
tillage may then be implemented in order to retain the remediated site.
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References
Brady, Nyle C., and Ray R. Weil. The Nature and Properties of Soils. 14 ed. Pearson Prentice
Hall, 2008.
Ellis, J. H., and WM. H. Shafer. Report of Reconnaissance Soil Survey of South-Central
Manitoba. 1943. Reprint. Manitoba Department of Agriculture, 1981.
Lobb, D. A.. Understanding And Managing The Causes Of Soil Variability. Journal of Soil and
Water Conservation 66.6 (2011): 175-179.
Smith, R.E., H. Veldhuis, G.F. Mills, R.G. Eilers, W.R. Fraser, and G.W. Lelyk. Terrestrial
Ecozones, Ecoregions and Ecodistricts of Manitoba. Agriculture and Agri-Food Canada,
1998.
Watershed Evaluation of Beneficial Management Practices (WEBs): South Tobacco Creek.
Agriculture and Agri-Food Canada, 2012.
Xiaojun, Nie, Zhang Jianhui, and Su Zhengan. Dynamics of Soil Organic Carbon and Microbial
Biomass Carbon in Relation to Water Erosion and Tillage Erosion. Plos One 8.5 (2013):
1-7.
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