Soil Temperature (Thermometers)

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Why is Soil Temperature Important?

Visual description of soil temperature ranges and effects on organisms.

Soils have many important roles towards the maintenance of plant life: they provide structural support, as well as water and nutrients. Their ability to serve these roles are largely influenced by temperature. Soils function as heat sinks during the day, and heat sources during the night and greatly influence rates of gas diffusion in and out of soil. On a larger temporal scale, soils absorb energy throughout warm spring and summer seasons and release this energy into the atmosphere during colder fall and winter months. When soils are depleted of oxygen and also subjected to high temperatures, they let out a net amount of methane into the atmosphere which has implications for climate change mitigation. Put simply, soil temperature is a measurement of a soil's warmth.

Being an important factor that drives seed germination, soil temperature directly affects plant growth. Furthermore, most soil organisms perform best within particular temperature ranges, and so rates of nitrification, moisture content and nutrient availability, the rate of organic matter accumulation as well as aeration are all affected by soil temperature.

Different Soil Materials Transmit Heat Differently

Different soil materials have varying heat capacity, thermal conductivity and thermal diffusivity characteristics. Heat capacity refers to the amount of heat required to be given to a certain mass of material to produce a unit change in its temperature. Thermal conductivity is a measure of a material's ability to conduct heat, which thermal diffusivity describes a material's rate of transfer of heat from the hot end to the cold end. The materials which a soil is made up of will therefore have an influence of its ability to hold, give off and transfer thermal energy.

How is Soil Temperature Measured?

Soil thermometers are the easiest and more straight-forward method of measuring temperature. Although some farmers and soil scientists rely on special soil temperature gauges, standard digital thermometers work well for general soil health assessments. As a general guideline, the best months for temperature measurements indicative of soil health are during the growing season: June through August.

Data points are usually ancillary, unless your investigation is more specific. Ex: How a layer of mulch affects the soil temperature beneath it.

Types of thermometers that can be used for soil temperature measurement

1. Glass Thermometers

This type of mercury-in-glass thermometer is commonly used for measuring soil temperatures at depths of 20 cm or less. Insert the thermometer below the lowest graduation and data can be read in situ. For soil depths greater than 20 cm, thermometers mounted on wooden, glass or plastic tubes, with their bulbs embedded in wax or metallic paint, are suggested. This thermometer-in-tube instrument can then be suspended inside a thin-walled metal or plastic pipe that has been sunk into the soil at the required depth.

If thermometers are to be left in the ground and snow cover is a possibility, a lightweight bridge can be constructed parallel to the thermometers such that the observer may approach them without disturbing the snow cover. The bridge can be constructed in such a way that it can be removed between readings without affecting snow cover.

A simple glass thermometer for measuring soil temperature

2. Digital Thermometers

Digital thermometers can also be useful at measuring soil temperatures. You must ensure, however, that your thermometer can provide temperature readings within a certain range. Some thermometers (ie. meat thermometers) cannot read temperatures below a certain range. A good rule of thumb is to make sure your thermometer can read temperatures before 3 degrees Celsius.

3. Infrared Thermometers

A handheld infrared thermometer can be used to instantaneously measure both leaf and surface soil temperatures.

An infrared thermometer can measure temperatures of vegetation and the surface of soils.

4. Data Loggers and Sensors

Data loggers are often attached to and used in conjunction with sensors that are left in the ground for extended periods of time to log data points over time. An example of this is the Hoskin Air/Water/Soil Temperature (20' cable) Sensor, which when connected to an appropriate data logger can detect temperatures between -40° and 50°C. Another example of a sensor is Campbell Scientific's CS230 Temperature Profiler. Its construction enables it to be used in roadbeds, soils, and water (snow and ice). Applications where the CS230 is used include spring-load adjustment, frost and permafrost monitoring, and soil  and water temperature profiling.

General Measurement Guidelines

Standard depths

Depending on your experiment some standard depths of soil temperature are 5, 10, 20, 50 and 100 cm below the surface. Additional depths can be included depending on your measurement objectives.

Sampling Location

It is important to remain consistent with measurement locations (i.e.: same area, soil type, and weather) depending on the objectives of your soil experiment. Level plots of bare ground (about 75 cm2) representative of the surrounding area's soil should be used. If the surface has been found to not be representative of the general surrounding, the plot's extent should not be less than 100 m2.

If the ground is covered If the ground is covered by snow, the snow cover's temperature can be measured as well. If snow is not common to the area, it can be removed prior to taking the readings and replaced.

Additional Information to Record

To provide context for your soil temperature readings in relation with the site/experiment, some additional information to include in your measurements are:

  • Soil moisture content
  • Soil texture
  • Air temperature
  • Time of day
  • Vegetation or canopy cover
  • Soil cover
    • Depth of LFH layer
    • Depth of mulch or organic matter
  • Soil bulk density
  • Thermal conductivity
  • Depth to water table (if within 5 m of the surface)
  • Soil structure

References

  1. Soil. (2020, August 11). Retrieved August 11, 2020, from https://en.wikipedia.org/wiki/Soil
  2. United States Department of Agriculture. (n.d.). Measuring Soil Health: Soil Temperature. Retrieved August 11, 2020, from https://webcache.googleusercontent.com/search?q=cache:ZeO0YU-Kax0J:https://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download%3Fcid%3Dnrcseprd1356820%26ext%3Dpdf+&cd=2&hl=en&ct=clnk&gl=ca
  3. World Meterological Organization. (2008). CHAPTER 2. MEASUREMENT OF TEMPERATURE. Retrieved August 11, 2020, from https://library.wmo.int/doc_num.php?explnum_id=3149