Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Soil Compaction and Its Effects on Erosion By Jarrod Ward
Background
According to the Department of Primary Industries (2015) compaction occurs when a force compresses the
soil and pushes air and water out of it so that it becomes denser. Such forces can come from agricultural
stock, farming equipment or recreational activities such as four wheel driving.
Compaction becomes more severe when the soil is wet and less able to withstand compression. The
amount that the soil compresses can also be dependent on its parent material. Sandy soils do not compact
well as they do not hold water and have more rounded particles, whereas clay soils can be highly
compacted due to their high moisture content and extremely fine particles.
Plant growth is hindered by compaction as the plants are unable to establish deep roots and reach valuable
nutrients. With a growing population comes a growing food demand and a rising need for suitable soils to
cultivate. This means that the negative effects of soil compaction is doubled due to not only the decreased
crop yield but also due to the following erosion. The effect will multiply over time as the soil that is left
becomes more and more heavily farmed.
Soils Matter (2015) states we lose about 1.7 billion tons of soil per year just from our cropland. Dr. David
Pimentel from Cornwell University takes this statistic even further in his report Population Growth and the
Environment, pointing out that it takes approximately 500 years to replace 25 millimetres of top soil lost to
erosion and the minimum depth for agricultural production is 150 millimetres. From this perspective
productive soil is a non-renewable, endangered ecosystem.
Erosion rates are increased by compaction due to water pooling on the surface and carrying away any
unconsolidated materials when it runs off. The University of Minnesota (2015) says from the standpoint of
crop production, the adverse effect of soil compaction on water flow and storage may be more serious
than the direct effect of soil compaction on root growth.
Aim
To investigate the effects of compaction on three different types of locally found soils.
Hypothesis
Compacted soils will experience an increase in erosion rates and compaction will cause a decrease in the
permeability of soils.
Equipment
Retort stand, sieve tray, burette, clamp, beaker, ruler, scrap wood, buckets, plastic tubing, tape, funnel,
stopwatch, brick, tub, netting, paper towel, local sourced sandy soil, locally sourced clay soil and locally
bought potting mix.
Risk Analysis
The risk associated with executing this experiment was extremely low but care still needed to be taken in
some areas. Care must be taken when handling any sort of glass material such as beakers and burettes.
Care should also be shown to avoid slipping in any spilled water.
Method
Experiment One: Permeability
Two 33cm long sections of PVC piping were cut, one was used for compacted soil and one was used for
uncompacted soil. A piece of scrap dowel wood with a mark approximately 10cm from the bottom was
used as a measuring stick and as a tool for compacting soil, pushing it down by hand. Each pipe had a piece
of netting taped underneath it to prevent any soil from falling out of the bottom. Both pipes were filled
with one type of soil, either a clay soil, sandy soil or potting mix. One of the pipes would be compacted by
pushing it down with the dowel while the other would be allowed to settle under the weight of the dowel
without being pushed down. Both were measured to ensure the soil was at the same height before being
clamped to retort stands and having 50ml of water poured into them using a burette, which was attached
to the retort stand above the pipes. The time taken for a single drop of water to make it through the soil
was recorded before the soil was removed from the pipes. The pipes were cleaned and dried using paper
towel before having the next soil type added to them and the process repeated. This was done for all three
soil types used and the results were recorded.
Experiment Two: Erosion
In this experiment a sieve tray was filled with one soil type that was compacted by placing a piece of flat
wood on top of the soil and having a person stand on top of it. The tray was then placed at a 45o angle
before approximately 500ml of water was poured onto it using a burette. The results were recorded by
taking pictures before the soil was removed from the tray. The same soil type was then placed in the tray
and the experiment repeated, this time with uncompacted soil. The results were again recorded before
moving on to the next soil type and repeating this process
Results
Experiment one
This experiment measured the amount of time taken for water to soak through compacted and
uncompacted soils.
Compacted Uncompacted Clay Never Penetrated 36 seconds Sand 2 minutes 5 seconds 1 minute 22 seconds
Potting Mix 12+ Hours 3 minutes 24 seconds
Experiment two
There was no clear way to draw any form of data from this experiment so the results must be drawn from
the photos taken and discussing what was observed during the experiment. The experiment aimed to
model erosion and its effects on compacted and uncompacted soils on a slope of approximately 45o.
Sandy Soil
Potting Mix
Compacted Uncompacted
Clay
Compacted Uncompacted
Discussion
There was some intriguing results that came from the performed experiments. While the aim was achieved
and the hypothesis correct it was not expected that clay and potting mix soils would have such a large
decrease in permeability when compacted. Sand did not experience such a large decrease and was also
much harder to compact than the other two soils used, possibly explaining why compaction made such a
small difference. This shows that naturally occurring sandy soils are bad for plant material as it has a very
loose structure and a low water retention capacity. Clay soil can be highly compacted due to the small
particles and fine silt that it is made up of and as a result the compacted clay soil only absorbed about 50%
off the water poured onto it and that water never made it all of the way through the pipe. This shows how
compacted clay soils can be devastating during periods of high rainfall as it takes a long time for a small
amount of water to be absorbed, the water that is absorbed barely reaches a depth low enough for plants
to benefit from it and any unabsorbed water will pool on top and flow off carrying any material away that
may have become unconsolidated due to the high velocity of rain drops. This increases erosion rates as rills
begin to form and pollutes water storage areas. Potting mix showed a reliable water retention capacity and
absorption rate when uncompacted with 100% of the water being absorbed but taking 3 minutes and 24
seconds to drip through. This would be perfect for any plant that has grown in potting mix with the water
retaining long enough for the plant to absorb it. Compacted potting mix showed a very different result with
only 50% of the water being absorbed and it taking over 12 hours for a single drop on water to make it
through the soil. While the water retention is good for any plants that may be planted in compacted
potting mix the absorption rate will increase erosion and damage any nearby water storage areas when the
water runs off.
It was hard to draw results from the erosion test as there was no way to gather any sort of data and
pictures had to be used to display the outcomes. It was very evident that sandy soil does not compact very
well in this experiment with nearly identical results being achieved with only a few differences. The
compacted sand experienced rill similar to the uncompacted but these rill turned into more gully like
formations further down the sieve tray as the soil experienced undercutting. This created more sediments
while the uncompacted sand tended to create a single channel and follow it without undercutting
occurring thanks to the much more loose structure. The sandy soils eroded much quicker and much easier
than the clay and potting mix. Clay soil did not experience much erosion in either the compacted or
uncompacted tests. The compacted clay was much too dense to absorb the water as it ran over the top
and the only erosion visible is where the water hit the clay from the burette. This may not be the same
case outside of laboratory testing as water will be hitting all parts of the clay surface instead of just 1. The
uncompacted clay absorbed much of the water and did not show any visible sign of erosion other than
where the water hit. This may be due to the rocks that were present in the clay sample helping to keep the
soil stable. Potting mix performed well in the erosion test with minimal erosion present and a high
absorption rate being shown by both the compacted and uncompacted soil and a solid structure due to the
organic matter present. Slight rills began to form in both cases and there was a slight sediment deposit
area on the uncompacted sample.
National and state governments have introduced legislations to help improve the soil situation. The NSW
department of Land and Water conservation has legislative responsibility for the protection and
conservation of the states’ soil resources and the prevention of soil erosion. The policy aims to sustain and
improve New South Wales’ soil quality. Different strategies implemented to help prevent erosion occurring
includes designated and elevated paths for the public through national parks, Riparian strips, Avoiding
growing crops with slopes, crop rotation and leaving crops to avoid having a paddock fallowed.
There was many problems associated with this experiment and if it was to be done again in the future then
some simple improvements could be made. These improvements include pouring water evenly over the
erosion test instead of just 1 location and pouring it more slowly as in this test it was poured much quicker
than natural rain. Less water should also be used at once as it is highly unlikely that an area will experience
500ml of rain in under 2 minutes as was modelled. For the permeability test a pre-determined weight of
soil should be used instead of a measurement as 23cm of compacted soil could contain twice as much soil
as 23cm of uncompacted soil. A way to measure the density of the soils before adding water should also be
implemented to ensure all tests were consistent.
The independent variable in these experiments were the time taken for water to fully penetrate the soils
and the types of erosion experienced. The dependent variables are the soil type while some controlled
variables include the amount of water used in each experiment and the angle of the sieve trays.
Conclusion
The experiments performed successfully modelled the effects of compaction with multiple naturally
occurring soils. Compacted soils tended to experience a decrease in permeability, increased in water
retention rates and increased in erosion rates, supporting the stated hypothesis.
Bibliography
http://eusoils.jrc.ec.europa.eu/library/themes/compaction/
European soil research commission - Info
http://www.fewresources.org/soil-science-and-society-were-running-out-of-dirt.html
David Pimentel – PhD professor - Info
http://www.extension.umn.edu/agriculture/tillage/soil-compaction/
University of Minnesota - Info
https://www.agric.wa.gov.au/soil-compaction/soil-compaction-overview
Western Australia Agriculture - Info
http://www.dpi.nsw.gov.au/agriculture/resources/soils/structure/compaction
Department of Primary Industries Australia – Info and Picture 1
http://www.drakehomesiowa.com/guest-blogs/soil-compaction/
Drakes Homes Iowa – Picture 2