What's Happening Underground?
Taking the Soil's Temperature!

* Soil Thermometer
* Outside Thermometer
* What's Happening Underground? data sheet (below)

Overview: Students record air and soil temperatures in their Journey North garden each month through the winter and spring.

Teacher Background
Your bulbs are settled seven inches underground, the temperature outside (on most parts of the continent) is dropping, and we're in the darkest time of year. What's going on underground? Before you have students ponder that question, read this short teacher background on soil temperatures.

Laying the Groundwork
Invite students to ponder the conditions their bulbs might be experiencing.

  • How cold does it feel outside today? What is the air temperature? Use a school thermometer or newspaper to find out.
  • What do you think the temperature is underground where our bulbs are? (Or, Do you think the underground temperature is the same as the outside temperature? Do you think it's higher or lower?) Ask students to explain their thinking. How do you think we could find out?
  • Invite the class to set up an investigation in search of answers.


    Tip: If you live where the ground freezes solid in the winter, you can drive a long nail into the ground with a hammer. Or, if your garden is soft enough, make a pencil hole for your soil thermometer. But if you start this activity before the soil freezes, you can put a drinking straw into a hole in the soil to keep it open. Cover the hole with a coffee can so it will not be buried under the snow. 

    If you mulched your bulb garden or had an early snow cover, you may find that the soil is actually warm enough to work with — even in mid-winter.

  1. Introduce the soil thermometer; explain that it can take the temperature underground. Ask, What question do we want to answer? For instance, you might choose, Is the soil temperature the same as the air temperature? Discuss how you might answer the question.
    For Grades 4 and Up: Help older students plan their own science study. See Temperature Experiments: Starting with a Good question.

  2. Hand out copies of the "What's Happening Underground?" Data Sheet or create a chart for the entire class to use. Students will fill them in as they conduct the next few steps.

  3. Find the air temperature. Look at an outdoor thermometer to find the air temperature. Do this just before going out to measure the soil temperature. Ideally, you'll have a thermometer you can bring right to the garden so you can measure the air temperature just above where you'll measure the soil temperature.

  4. Predict the soil temperature and then find it. Have students predict the soil temperature in their garden on that day. If the ground in your garden is hard, make a hole with a pencil before inserting the thermometer. If the ground is frozen, hammer a long nail into the ground for the thermometer. Try to make the hole about the depth of your tulips; leave the thermometer in place for a few minutes. Record the temperature on the data sheets. (Note: Always bring the soil thermometer inside when you're through.)

  5. Repeat steps 3 and 4 each month from January through May or June. Try to take the measurements on about the same day at about the same time each month. Before students predict each month's soil temperatures, they should review and reflect on earlier data. Each time you return to class, ask students to share other observations about the garden site (e.g., presence or depth of snow, amount sunshine or rainfall) along with questions they have. Keep a dated class list of observations and questions.

Making Connections: Discussion and Journal Questions
As a class or individually, make a graph to represent the data. As students review the data and their observations, ask,

  • What patterns do you notice? (How do soil and air temperatures change from winter to spring?)
  • What does our data "tell" us about soil and air temperatures? Which are warmer?
  • How would you explain our findings? What questions do you still have?
  • What do you think soil temperatures (or air temperatures) have to do with tulips emerging and flowering? What other "signals" might affect tulip growth? (Think about other things, besides temperature, that change in the spring.)

Digging Deeper


  • Show students this photo of thermometers in snow and share this: One winter, Journey North staff dug through the snow and stuck in a soil thermometer. When the air temperature was 2 degrees F. below zero, the soil two inches down was 27 degrees F above zero! Do you think they made a mistake? How would you explain what happened?
Where to Purchase a Soil Thermometer
You can purchase inexpensive soil thermometers from many nurseries and stores with garden supplies, such as Home Depot. Kidsgardening.com has a nice 7 1/2-inch soil thermometer. It has a disk at the top that gives readings from 0 to 220 degrees F. for just $10.95. You can order it here or by calling 800-538-7476.