Talk It Over

What's in soil? It's part soil particles, part water, and part air. Healthy soils that support good plant growth have ample air spaces in them. Water moves through them easily. Soils that get too packed down don't drain well. The compression of soil particles is called compaction. Heavy equipment and construction work compress soils. Soils that have a lot of clay in them are also very compact. You can measure the compaction of soils in your area. Perhaps you can relate what you measure to how well—or how poorly—plants grow in the places you study.

Get

• Metal knitting needle, size 7 or smaller
• Small spool
• Ruler (metric)
• Access to a photocopier
• Scissors
• Transparent tape
• Permission to test soils in the sites you choose

Go

1. Make sure the knitting needle fits through the spool, like this:

   

2. Make a photocopy of the ruler (centimeter side). Cut out the thin strip of marks and numbers.
3. Set the spool on a table. Put the knitting needle, point side down, in the spool. Tape the ruler strip to the knitting needle, with the 0 at the top edge of the spool. Tape the strip very securely so that it will hold when you push the needle into the ground.

   

4. To measure compaction, place the spool and knitting needle on the soil surface. Push down hard. The top of the spool will show you how far the needle went into the soil. The farther it goes, the less compaction.
5. Take compaction measurements at different sites near your home and school. (Be sure to ask permission.) Observe the plants that grow in each site you measure. Can you relate compaction to plant growth?

Stay Safe

Be careful with the point of the knitting needle. It is not sharp, but it could poke you or someone else.

Go Easy

The "Go" procedure will work for you.

Go Far

Water has a big effect on how far soils compact when pressed—as they are, for example, when a tractor or mower runs over them. Devise a way to measure the effects of water on compaction. Use your experimental design to compare soil samples taken from sites in your area.

Surface compaction and subsoil compaction are different. Find out about them. Then design and carry out experiments that will let you compare and contrast their mechanisms and effects.

Show Your Results

You can keep track of your measurements and observations using a table like this:

Make bar graphs to compare compaction at the different sites. (Remember, the greater the measurement, the less compact the soil—and the better for plant life.) You might want to take photographs or draw pictures of your test sites to relate plant growth to compaction.

For "Go Far," make data tables and graphs to match the experiments you design and conduct. Include in your display information about the different forms of compaction and how water and mechanical action affect them.

Tips and Tricks

• If you test many soils that are very wet or very compact, your measuring strip may pull loose. Take tape and extra photocopied strips into the field with you so you can make repairs at your test sites if you need to.
• If you live in an agricultural area, ask a farmer about the impact of soil compaction on crop yields. Find out what is done to reduce or prevent it.

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