Soil Temperature Protocol

Site Selection and Set-up

Make measurements adjacent to your Soil Moisture Study Site, or if this is not possible, within 10 m of your Atmosphere Study Site. Study the figures of the star or transect sampling patterns described in the Sampling Strategies and Site Layout sections which illustrate acceptable sampling locations. If you are making these measurements at your Atmosphere Study Site, follow the sampling pattern and site layout for the Star Pattern.

1. Select a relatively flat sunny area.
2. Try to find an area with uniform characteristics across an area having a diameter of 5m.
3. The ground should not be compacted but can be covered with litter or grass.
4. Check the box on the Data Work Sheet if it has rained in the past 24 hours or if the soil is frozen.

When making measurements on consecutive days, try to make your readings on days with similar weather conditions and for soil conditions that are typical for the week you are making them. Try to make diurnal readings around the middle of March, June, September, and December.

Instrument Construction

Your thermometer should be most sensitive to temperature changes about 2 cm from the tip because of the length of the temperature sensor inside the probe. To take measurements at 5 and 10 cm depths, the thermometer will have to be pushed 7 and 12 cm into the ground.

Drill a hole in a wooden block so that when the soil thermometer is pushed all the way into this hole, 7 cm of your probe extends beyond the bottom of the block. This will help students maintain a uniform depth for each measurement.

Get a nail that is the same length and diameter as your thermometer probe or cut a nail to this length. Many hardware stores carry "gutter spikes" that work for this purpose .

Teacher Support

Key Concepts

• Soil is an insulating layer.
• Soil temperature varies with depth, soil moisture, and air temperature.
• Soil temperature varies less than air temperature.

Skills

• Reading dial scales
• Field sampling
• Observing related phenomena
• Graphing temperature cycles

Processes

• Solar heating

Additional Materials

• Soil Temperature Data Worksheet

Prerequisites

Select a site

Teacher Preparation

Consider using the learning activities: Land, Water and Air; A Field View of Soil - Digging Around

Student Preparation

Do the activity, Introducing Soil Temperature.

Helpful Hints

• Depth Jig: Use a wooden block to help maintain proper probe depth. See Instrument Construction.
• Pilot Holes: Make a pilot hole if you can not easily push the temperature probe into the ground. Hammer the pilot nail in 7 or 12 cm. Remove the nail using a slight twisting motion. Try to minimize the amount you disturb the soil. If the hole caves in, offset about 25 cm and try again.
• If the soil is dry and sandy/rocky, it will be very difficult to keep your pilot hole open. You might have to wait for a natural ground wetting event to begin measurements again.
• The temperature probe is delicate and will bend if forced. Easy does it.

Key Definitions

• Diurnal: Occuring each day; daily; Used here to indicate daily temperature cycles.
• Insulator: A material that insulates - to slow or prevent the transfer of heat.

Student Support

Scientific Justification

Animals burrow into the ground to escape extremes of hot and cold. Many seeds will not germinate until the soil has warmed sufficiently. A warming soil can also trigger metamorphosis in insect larvae buried in the soil. Scientists are interested in the surface temperature in order to understand the Earth's surface energy balance - or how much heat is radiated to space.

Synthesis Questions

• When is the soil at depth warmest?
• When would you expect deeper soil temperatures to be warmer than shallow soil temperatures?
• Why is there a difference between the daily maximum air and soil temperatures?

Calibration and Instrument Maintenance

Task:

Monthly calibration check of field and standard thermometers

What you need:

• Dial or Digital probe thermometer
• Calibration thermometer
  

How to Make Monthly Calibration Checks:

Check the accuracy of your probe every month. This is particularly important if you are using more than one thermometer, as differences or biases between two thermometers will make your data impossible to interpret. Follow this calibration procedure:

1. Use the calibration thermometer from the Atmosphere Investigation as a calibration standard.
2. Place your thermometers in water at room temperature; record their temperature readings after 2 minutes.
3. There should be less than 2° C difference between your thermometer readings and the calibration thermometer.
4. Follow the manufacturer's directions to reset dial-type thermometers, if your differences are greater than this.

Protocol

Task:

Weekly soil temperature observations at 5 and 10 cm depth.

What you need:

• Dial or Digital probe thermometer
• Guide block, 12 cm spike and hammer
  

How to Measure Weekly Soil Temperature

Take three sets of soil temperature measurements adjacent to your current soil moisture star pattern sampling location or within 10 m of your Atmospheric weather shelter at 5 and 10 cm depths. Complete these measurements within 1 hour of local solar noon and within a period of 20 minutes.

1. Expose a soil surface. Brush away grass and litter until you see the soil surface.
2. Make a pilot hole to 5-7 cm. If needed.
3. Insert the thermometer to 7 cm. Insert the thermometer through your block. Gently push in the thermometer until the tip is 7 cm below the soil surface.
4. Read the soil temperature at 5 cm. Wait at least 2 minutes; then read the thermometer every minute until consecutive readings are within 0.5-1.0° C of each other. Record this value and the current time on the Soil Temperature Data Worksheet.
5. Remove the thermometer.
6. Repeat steps 1-4 in the same hole. Instead of depths of 5 and 7 cm, use depths of 10 and 12 cm, respectively.
7. Report your measurements to the GLOBE Student Data Server on the Soil Temperature Data Entry Sheet.

How to Measure Diurnal Soil Temperature

Take diurnal temperature measurements every three months, preferably during March, June, September, and December (if the soil is not frozen). Use the procedure given for Weekly Soil Temperature Observations but:

1. Repeat the measurements every 2 to 3 hours for two consecutive days.
2. Do not collect triplicate observations. Try to take at least 5 sets of readings per day. Offset each new reading by at least 10 cm. See Figure SOIL-P-18.
3. Read the current temperature at your Atmosphere Investigation Instrument Shelter and record it on the Soil Temperature Worksheet each time you measure soil temperature.

Figure SOIL-P-18: Soil Temperature: Layout of Diurnal Observations

Data Analysis and Presentation

Construct a table in your GLOBE Student Notebook similar to the one below for recording your results or use the Soil Temperature Data Work Sheet. Plot the data using Figure SOIL-P-20 as a guide.

Diurnal Soil Temperature

Figure SOIL-P-19: Table of Diurnal Soil Temperatures Tucson, AZ, USA

Figure SOIL-P-20: Plot of Diurnal Soil Temperatures