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Dirty Science: What Makes Soil Become Dense?

An earthy activity from Science Buddies

George Retseck

Key concepts

Have you ever noticed how much work it is to dig a hole in really hard soil? It's much easier to dig a hole in soft, loose soil. But why is that? Soil that is hard and dry is often compacted, which means that it has been packed down, making it denser and thereby difficult to penetrate. Soil that has become compacted is not only harder for you to dig a hole in, but it can also be much harder for a lot of other organisms, such as helpful earthworms, to survive in.

Just as it is difficult for you to dig in compacted soil, it is also difficult for soil-dwelling organisms, like bugs and worms, to tunnel in compacted soil. You won't usually find many organisms living in compacted soils because they cannot get the air, space and nutrients they need to survive. Also, compacted soil makes it difficult for plants with delicate root systems to thrive. Very compacted soil tends to support only the growth of weeds, which have thick tap roots that can penetrate deeply into compacted soil and out-compete other plants.

Some areas are more susceptible to soil compaction than others. For example, the number of people and other animals that walk in an area—in other words, the amount of "foot traffic"—can affect how compacted the soil is there. The quantity of sunlight and moisture can also affect how susceptible an area is to compaction.

•    Small spool of thread
•    Metal knitting needle (small enough to fit inside the spool hole)
•    Permanent marker
•    Patches of soil in different locations to test for compaction
•    Rubber band
•    Measuring tape or ruler
•    Sheet of paper and pencil or pen

•    Place the knitting needle into the small spool to make sure it fits.
•    With the needle inside the spool, place both (pointy side down) onto a table. Mark on the knitting needle the point where it sticks out of the top of the spool with your permanent marker.
•    Choose a variety of different locations to test the soil for compaction, such as areas that get different amounts of foot or animal traffic, places that receive different amounts of moisture or sunlight, and patches that have lots of plants versus those that have none.

•    Take your needle and spool apparatus, along with a rubber band outside to the first location you chose to test the soil for compaction.
•    Place the spool on the ground with the needle in it, pointy side down. Push hard on the needle until it stops penetrating the ground.
•    If the needle goes into the ground easily and could be pushed past the top of the spool, simply stop pushing when the needle reaches the spool top.
•    Tightly wrap the rubber band around the top of the knitting needle (the blunt end). Slide the rubber band down the shaft until the rubber band touches the top of the spool.
•    Pull the needle from the ground, making sure to keep the rubber band in the same place. Measure the distance between the rubber band and the marked line. How deep could you push the needle into the ground? Write this on your sheet of paper.
•    Go to the other test locations you chose and repeat this process to find out how compact the soil is. The more compact the soil is the harder it will be to push the needle into the ground.
•    Where was the soil most compacted? Where was the soil least compacted? What characteristics did you notice about the most compact soils? What characteristics did you notice about the least compact soils? Think about factors such as foot traffic, soil type, moisture, exposure to sunlight and plant covering.
•    Extra: Investigate the effect of walking on soil compaction by digging up some soil so that it is loose. First measure its compaction with your spool-and-needle apparatus, then walk over the site and measure the compaction again. How did the soil's density change? If you walk over the site more, will the compaction continue to change?
•    Extra: Test whether wet or dry soils become more compacted by adding different amounts of water to dry soil and compacting it with a tamper or roller. Compact a sample of dry soil in the same way. Measuring each sample with your spool-and-needle apparatus, which soils are the most compacted? What happens if you let them dry out and measure them again afterward?

Observations and results
Were some soils you tested more compacted than others? Did you find that the areas that received a lot of foot traffic were the more compacted? Were dry areas more compacted than moist areas? Did more compacted areas have fewer plants?

As soil is pressed down, such as by humans and other animals walking over it, it becomes compacted, or denser. This is why areas that receive a lot of foot traffic or areas that are driven over by vehicles are more susceptible to becoming compacted. When soil becomes too dense many organisms such as bugs, worms and some plants will be unable to live there. This is in part because it is harder for oxygen and water to penetrate into highly compacted soil. The lack of moisture can cause compacted soil to become dry, and you may have found that dry areas were denser than moist ones. What other qualities did you notice about soils that had different levels of compaction?

Clean the knitting needle with soap and water.

More to explore
Soil Compaction from Colorado State University
Recoverability and Vulnerability of Desert Ecosystems: Soil Compaction from the U.S. Geological Survey
Soil Compaction from Science Buddies

This activity brought to you in partnership with Science Buddies

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