Soil Temperature Protocol

Purpose

To measure near-surface soil temperature, to detect diurnal changes in soil temperature and to learn about the insulating capabilities of the soil.

Overview

Soil Temperatures at 5 and 10 cm will be measured using a probe thermometer. Soil temperature is a function of climate, soil, soil moisture, depth and geographic setting. This protocol collects data to begin to explore these interactions.

Time Required

10-15 minutes per measurement set (eg. 6 probe measurements).

Skill Level

Measurement Frequency

Seasonal variations in soil temperature will be monitored weekly with triplicate measurements at 5 and 10 cm depth.
Daily variations in soil temperature will be measured seasonally every 2 to 3 hours during the daytime at 5 and 10 cm depth. Diurnal readings should be made on consecutive days to check repeatability and changing soil conditions.

Key Concepts

The basic idea is that soils are an insulating layer that reduce climatic extremes and provide a stable environment for plants and animals to live and grow. There are many cycles to consider, from diurnal to seasonal, with many interacting factors (sun strength, depth, soil type and moisture condition) contributing to the observed temperature.

Skills

Reading dial scales
Field sampling
Observing related phenomena
Graphing temperature cycles

Materials and Tools

Dial or Digital probe thermometer
- 12 cm (5") or longer stem
- Accuracy: +/- 1% over 50 to 100 C range (typically -20 to 50 C)
- Examples: Taylor 6074-1 dial OR Taylor 9841 digital
12 cm (5") finishing nail and hammer
A wooden block with 6 mm (1/4") hole along the vertical axis (see below)
Calibration thermometer (use the same one used to calibrate max-min thermometer).

Preperation

Take the class outside and have them find the hottest and coldest spots. Ask them how these will change with time-of-day and season. Have them draw and explain a picture or graph of air and soil temperature plotted for 24 hours.

Site Selection and Timing

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 Introduction to Part 2 Protocols which illustrate acceptable sampling loactions.

Select a relatively flat area.
Try to find an area with uniform characteristics across an area having a diameter of 5 m.
The ground should not be compacted but can be covered with litter or grass.
Make a note on the data work sheet if it has rained in the past 24 hours.

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 September, December, and March.

Preparing for the Field

Assume that your thermometer is most sensitive to temperature changes 2 cm from the tip (this is due to the length of the sensor used inside the probe). Thus it will require a 7 and 12 cm deep hole to measure the temperature at 5 and 10 cm, respectively.
Cut and drill a wooden block, such that 7 cm of your probe extends beyond the bottom of the block when it is fully inserted. This will help you maintain a uniform shallow-layer probe depth.
Cut or select you nail punch to be the same length as your thermometer probe.
Practice using the probe - testing insertion technique and observing local temperature variations.
Thermometer calibration: You should check the accuracy of your probe on your seasonal measurement schedule. This is particularly important if you are using more than one thermometer as relatively large differences or biases between two thermometers will make your data impossible to interpret. Follow this calibration procedure:
1. Use a laboratory grade (+/- 1 C) thermometer as a calibration standard.
2. Place your thermometers in a cold water bath; read and record after 2 min.
3. Place your thermometers in a hot water bath; read and record after 2 min
4. The temperature of the water baths should be different by more than 20 C.
5. There should be less than 2 C difference between your thermometer readings and the calibration thermometer.
6. Follow the manufacture's directions to reset dial-type thermometers, if your differences are greater than this.

What to do and how to do it

Make a pilot hole to 5 cm. If the ground is hard, use the nail to make a pilot hole to your target measurement depth. Insert the nail through your wood block and push it to 2 cm above the top of the block. If the ground is so hard you have to use a hammer, then complete the pilot hole to its full depth. Remove the nail using a twisting motion. If the ground cracks and bulges up as you remove the pilot nail, offset 25 cm and try again. Try to minimize the amount you disturb the soil.
Insert the thermometer to 7 cm. Insert the thermometer through your block. Gently push and twist the thermometer until the head is resting on the block. Do not force it as this will damage your instrument.
Read the soil temperature at 5 cm. Wait at least 2 minutes, Read the thermometer, Wait another minute and reread the thermometer. Do not remove the thermometer from the ground. Repeat until consecutive readings are within 0.5 C of each other. Record this value.
Remove the thermometer and the block. Use a twisting motion - try not to disturb the soil.
Repeat steps 1-4 without the wood block Gently push and twist your thermometer fully into the ground using the same hole as before. Instead of depths of 5 and 7 cm, use depths of 10 and 12 cm, respectively.

Weekly Measurements Make three sets of soil temperature measurements adjacent to your current soil moisture STAR pattern sampling location or next to your Atmospheric weather shelter at 5 and 10 cm depth. Complete these measurements within 1 hour of local solar noon and within a period of 20 minutes.
Diurnal/Seasonal Measurements Repeat every 2 to 3 hours for two consecutive days. Try to make at least 5 readings per day. One schedule might be 7:00, 10:00, 12:00, 14:00, 17:00. Another would be 7:00, 10:00, 13:00, 16:00, 19:00. Offset each new reading by at least 10 cm (see figure). The purpose of making readings on consecutive days is to test repeatability and the sensitivity of the soil to slightly different soil and weather conditions. Have your students record the air temperature for these measurement times in their field notebooks.

Data Analysis and Presentation

Construct a table in your field notebook similar to the one below for recording your results or use the "Soil Temperature data worksheet".
Plot the data using the figure below as a guide.

**Soil Temperatures, Tucson, AZ**
	2/12/97			2/13/97		ND=no data
Time	5 cm	10 cm	Time	5 cm	10 cm	Air Temp
8:00	5.0	7.2	8:30	5.1	7.7	ND
10:00	9.5	9.1	10:30	12.0	9.4	ND
12:30	17.9	12.7	12:30	19.4	13.8	ND
14:00	20.7	16.6	14:30	21.1	16.3	ND
17:00	16.9	16.4	17:00	16.7	16.3	ND
20:30	12.7	13.9	20:00	12.5	13.6	ND

Last updated: 5/21/97 Comments? globe@hwr.arizona.edu