Soil Temperature

Objective

In this activity, students measure and record the soil temperature at their field site over time. By measuring and recording the soil temperature over a period of time, they can discover how it varies at their field site.

Making the connection:

Activity

Hypothesize

What will the soil temperature be at your field site?

Plan it!

  • If you haven't already done so, choose a field site and map it in your field journal.
  • If you are doing this activity along with the Compaction, Percolation, and Texture tests, Air Temperature, or Cloud Cover, you should do all of the activities at the same time and place. If you haven't yet done so, decide how often to take soil temperature readings. If you are working with a group or as a class, you should decide together. As you decide, think about these questions:
    • How might your results be affected if you take daily readings?
    • How might your results be affected if you take monthly readings?
    • How might your results be affected if you take annual readings?

Most scientists would agree that taking readings at least once a week will give you the most accurate record of changes in temperature over time.
Taking the temperature more often will provide more data, but it is important to be consistent in the frequency of the measurements and in the time of day at which they are taken. Help students create a plan that will work well with their schedule.

Do it!

  1. Choose a spot in your field area in which to measure the soil temperature. If you are also measuring the air temperature, you should take air and soil temperature readings at the same place and time. You will need to take your soil temperature readings in the same spot and at the same time of day each time you do it.
    Taking the readings at the same time of day and at the same place is very important. Students should understand that time of day and location are variables which must be controlled in order to get useful data. To help your students understand the importance of variables, ask them to list all the things that might make the soil temperature change. Ask them how, with so many choices, they will decide which of these variables actually caused any soil temperature changes they see. If they don't come up with it themselves, suggest that you could intentionally control some of those variables. You can have different pairs or groups of students take the temperature at various places around your field site. Just be certain that each group returns to the same place each day.
  2. On your soil thermometer, measure 2.5 inches from the tip and mark the spot with the permanent marker or tape.
  3. Gently insert the soil thermometer in the soil until your mark is just touching the ground. You should try to take readings at this same depth each time.
  4. Hold the thermometer in place for two to three minutes in order to get an accurate reading.
    If students get an unexpected reading or think they took the reading incorrectly, have them take the temperature again. If they continue to get an unexpected reading, have them try again with a different thermometer.
  5. Remove your soil thermometer from the ground and store it in a dry place.

Record it!

  • Record the soil temperature on the Soil Properties Data worksheet . Be sure to mark the date.
  • If the ground is frozen, write "frozen" on your worksheet.
  • On the field site map you drew in your field journal, mark the spot where you took the temperature reading.
    Note that after the first time, students should not need to mark their spot on the map again, as they will return to the same spot each time. It may also be a good idea to have students make note of a particular landmark that will help them find the place again. If you have groups of students going to different places around the field site, you may want to note where each group is for your own reference and/or check each student's map to be sure the location is marked accurately and with enough detail to be found again.

Remember, be sci-wise!

  • Do not allow the direct heat of the sun to warm up the thermometer. This could affect the temperature reading.
  • Store the thermometer in a cardboard tube when it is not in use, and take the temperature reading right after pulling it out of the tube.
    You may also elect to keep all of the thermometers in a box or case and distribute them to students at the field site.
  • If your results seem unusual, double-check them by taking another reading. If your reading is still unusual, try using a different thermometer.
    Some common mistakes that lead to an incorrect reading are placing the thermometer in direct sunlight or using improperly calibrated thermometers. If the reading continues to be unusual after the second attempt, the student should take another reading with a different thermometer.
  • Do not force the thermometer into the ground. If the ground is too hard or dry to insert the soil thermometer, try carving out a small hole with a pencil before inserting the thermometer. If the ground becomes crumbly, move a few inches away and try again.
  • Do not leave your soil thermometer outside for long periods of time. Soil thermometers should be stored in a dry place out of direct sunlight when not in use.

Assessment and Reflections

  • How did the soil temperature at your site change over time? This will vary depending on where you are and the duration of your study.
    Students should find that the soil temperature changes slowly, without large fluctuations.
  • Based on the data you collected, would you revise your hypothesis before testing again? Why or why not?
    Answers will vary.
  • How did the soil temperature compare with the air temperature?
    Students should find that air temperatures will fluctuate more and change faster while soil temperatures remain more constant over time.
  • What were some of the possible variables that could affect the data?
    There are a number of possible answers here, including sunlight, shade, time of day, type of soil, and the accuracy of the thermometer.
  • Which of these variables did you control? How?
    Students controlled sunlight and shade, time of day, and soil type variables by always checking the soil temperature in the same place at the same time.
  • What patterns or links did you notice between the season and the soil temperature?
    Depending on where you are, soil temperature may change slightly with the season.
  • What patterns or links did you notice between the weather and the soil temperature?
    Students should find that the weather does not usually have an immediate effect on the soil temperature.
  • If you continued taking the soil temperature for a whole year, what differences would you expect to find?
    This will vary depending on the location of your field site.
  • Looking at the bigger picture, how might this data help you to answer your research question?
    Answers will vary, but students should begin to connect this data with the data from other activities to answer the research question.
  • Fill in this section of the Soil Properties Overview worksheet .

Extension Activities

  • Have students take the soil temperature in different locations in the field site, such as under trees, bushes, leaves, or bricks. Do the readings vary? Do students expect them to? Why or why not?
  • Have the students take the soil temperature at different depths—for example, at 2 inches down and then at 6 inches down. Do the readings vary? Do they expect the soil to be warmer or cooler further down in the ground? Why or why not?

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