Soil is an Ecosystem

Soil is one of our most important renewable resources. It is the basis for all plant life and agriculture on the entire planet! We tend to regard soil as lifeless dirt and not as the valuable resource that it is. In fact, soil is a delicate ecosystem where abiotic factors and biotic species interact. The formation of soil takes thousands of years and requires physical and chemical weathering of rocks and minerals, decaying organic matter, air and water and living organisms. It’s makeup includes solid minerals andorganic matter, liquid and gases as well as an array of biological and microbiological organisms. It has the ability to support life, anchor plant matter and store energy. It lives and it breathes!

Biological activity is a description of the rates of metabolism, respiration and other biotic functions of an organism and an ecosystem collectively. The term “organically grown” indicates that the plants were grown in soil rich in biological activity which promotes plant growth naturally. How can you measure the biological activity within the soil ecosystem? A quick and easy way to determine “activity” of a soil sample is to use a simple closed system soil respirometer. A respirometer measures the respiration of the soil, simply put.

The purpose of this investigation is to measure the output of Carbon Dioxide (CO2) within a soil sample, which is a by-product of aerobic respiration. The volume of CO2 will indicate the relative activity and ability of the soil to support life or to support its own ecosystem.

Making a Homemade Respirometer

Materials needed:

  • 1-½ Liter Plastic Bottle with ½” hole drilled in cap
  • Teflon connector with a flanged male–male fitting ½” to ¼” conversion (Figure 1)
  • 6-12” flexible tygon tubing (@ ¼” outer diameter)
  • Teflon tape
  • Universal tube holder with ¼” connector (Figure 2)
  • Fine-line permanent marker
  • 500-L micropipettor or 1-mL pipette

Step 1: Drill a ½” hole in the center of a plastic drink bottle cap. If you do not have a drill available, use a wood screw and a hammer to make a hole.

Step 2: Insert the ½” end of the teflon connector into the bottle cap. Use teflon tape around the connector if there are any gaps. Take the ¼” flexible hosing and connect it to the ¼” end of the teflon connector. (Figure 1)

Step 3: Calibrate the universal tube holder. First cap off the top of the tube and invert . Add water in 500-L increments using a micropipettor or a 1-mL pipette. Mark each increment with a fine-line permanent marker. Once the tube has been calibrated, empty the water.

Step 4: Connect the bottom of the ¼” universal tube holder to the flexible tubing. Now, when you put the bottle cap onto a plastic bottle, you will have a closed system soil respirometer.

The Experiment

Materials Needed:

  • Sample of soil
  • Ring stand
  • Test tube clamp
  • Sun lamp
  • Wide mouth funnel
  • Thermometer
  • Food coloring
  • Vernier Labquest with CO2 and Temperature probes (optional)

Step 1: Collect representative samples of soil from different areas around the school. You will need enough soil to cover the bottom of the plastic bottle when it is on it’s side, but will not block the mouth of the bottle. Describe the soil sample in your lab book.

Step 2: Using a wide-mouth funnel, add the soil sample to the plastic bottle. Make sure the bottle is clean and dry before adding the sample.

Step 3: Place the bottle cap tightly onto the bottle. If there are any gaps between the cap and the bottle, use teflon tape to line the ridges of the bottle top.

Step 4: Cap off the top of the universal tube holder. Open the bottom of the tube holder and invert. Add enough water to completely fill the tube. Add 1 drop of food coloring so that it will be easy to read the volume of gas generated by the respirometer. Replace the bottom of the tube holder. Be careful not to let any of the water out. When you place the tube right side up, it should now be full of water. If it is not, make note of where the water line is. Why doesn’t the water leak into the flexible tubing?

*Optional Step 4 using Venier Labquest: Place the CO2 probe into the top of the tube holder and use putty to insure an air-tight seal. Begin collecting data in a continuous run.

Step 5: Secure the tube holder in an upright position using a test tube clamp and a ring stand. (Figure 3) Place the plastic bottle under a sun-lamp or ordinary incandescent lamp. Use the thermometer to record the ambient temperature of the bottle. Copy the data table below in your lab notebook and record your observations every 5 minutes. Calculate the quantity of CO2 produced by your sample. (Remember those Ideal Gas Laws and Avogadro’s Principle!) If time permits, repeat the experiment at different constant temperatures.

Time (min) / Temperature (oC to K) / Gas Volume (mL)
(CO2 concentration) / Quantity of CO2 (mol)