Compaction Test

Objective

In this activity, students will measure and record the compaction of soil to determine how much space is available for air and water.

Making the connection:

Activity

Hypothesize

How compacted is the soil at your field site?

Plan it!

  • If you haven't already done so, choose a field site and map it in your field journal.
  • You also need to decide how many times you will repeat the test. As you plan, think about these questions:
    • How might your results be affected by the number of trials (how many times you repeat the test)?
    • Can you be more certain of your results if you do the test many times or just once?
  • Prepare your pencils before you begin. Make sure they have sharpened points, and one pencil should be new or almost new so that it is long enough to be pushed into the ground.
  • If you are working with a group or as a class, be sure to spread out over the study site. When you arrive at the field site, each student or group of students should set up a quadrant in which to work. If you are doing this activity along with the Soil Temperature, Percolation, and Texture tests, you should do all four activities in the same quadrant.
    Doing all four activities within the same quadrant allows students to put the results of all four tests together to form a more complete picture of the soil. By spreading out over the study site, students create an opportunity to compare results from different areas within their field site, to note differences and similarities in the soil properties, and to get a more representative sample of the entire field site.

Do it!

  1. Within your quadrant, push the pointy end of a pencil into the soil as far as you can, using normal force.
    It is important here that students not use excessive force, such as hammering or stomping the pencil into the soil.
  2. Use the other pencil to make a mark on the pencil at the soil level.
  3. Pull the pencil out of the soil. Using a ruler, measure the distance from the mark you made to the pencil point. This will tell you how far you were able to push the pencil into the soil.
  4. Use an eraser to remove the mark you made on the pencil, and then repeat this test several times in different places within your quadrant.

Record it!

  • On the Soil Properties Data worksheet , record how far your pencil went into the soil each time.
  • If the ground is frozen, write "frozen" on your worksheet.
  • Record the average of your measurements on your data sheet.
    If students need a review of averaging, this can be done in the classroom either before or after the field study.
  • Add the Soil Properties Data worksheet to your field journal.

Remember, be sci-wise!

  • Push hard enough to move the pencil into the soil, but not hard enough to break it. Do not try to force the pencil into the soil by stepping on it or hammering it. Be especially cautious if the soil may be frozen.
  • Try to use the same amount of force each time you do the test.
  • You'll need to prepare for dirt and mud that will be brought back on students' shoes. Consider having students bring in an old pair of sneakers or boots to use for your field activities. Students can leave their dirty shoes "at the door" and then shake or brush them off when the mud has dried.

Assessments and Reflections

  • What do your data tell you about the compaction of your soil? You can read about soil compaction and what it means at the Soil Properties Overview page.
    Students should understand that lightly compacted soil will allow the pencil to be pushed deeper into the soil. It will be more difficult to push the pencil into highly compacted soil.
  • Based on the data you collected, do you think your hypothesis was correct or incorrect? Why?
    Answers will vary.
  • How did your average soil compaction measurements compare with those of other students and other areas of the field site?
    Answers will vary.
  • Were the results from any of your trials very different from your other results? What might cause that to happen?
    Answers will vary. Some things that could cause unexpected results for one trial include running into a rock or using more or less force than on other trials.
  • How do your results compare to the Percolation Test results?
    Both tests measure soil compaction, and should therefore yield similar results.
  • How might the weather or season affect the data?
    If the ground is frozen or nearly frozen, that might make it more difficult to push the pencil into less compacted soil. Likewise, it might be easier to push the pencil into more compacted soil if the ground is extremely saturated.
  • What were some other variables that could affect the data? How?
    Some possible answers include soil temperature and soil saturation.
  • Now that you have measured the soil compaction at your site, would you expect to find many soil critters there or just a few? What kinds of critters would you expect to see?
    This answer will vary based on the site. In general, you would expect to find greater numbers and varieties of creatures in soil that is less compacted. This is because soil that is less compacted has more room for the air and water that organisms need to survive.
  • Based on the soil compaction at your field site, do you think this might be a good place to start a class garden? Why or why not?
    Again, this answer will vary. In general, soil that is less compacted is more hospitable for plants, especially garden plants, although various plants are adapted for growth in highly compacted soil.
  • Looking at the bigger picture, how might the data you collected in this activity help you to answer your research question?
    Answers will vary but students should begin to connect the data from this activity to data from other activities to help answer their research question.
  • Fill in this section of the Soil Properties Overview worksheet .

Extensions

  • Have students choose several spots around school or home that have different plant life. For example, they could choose an area that has a lot of healthy plants and an area that has no plants or has unhealthy ones. They should test the soil compaction at each area. How does the soil compaction compare? Can students draw a conclusion about how soil compaction affects plant life just from this test? Why or why not? What other variables beside soil compaction might affect the plant life?

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