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