Materials Included In Kit

Additional Materials Required

Safety Precautions

Isopropyl alcohol is a moderate fire risk as it is a flammable liquid. It is also slightly toxic by ingestion and inhalation. Soil may contain eye, respiratory or skin irritants. Do not handle soil with bare hands. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron during each part of the activity. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Contents of the Petri dishes may be disposed of according to Flinn Suggested Disposal Method #26b. The soil in the sealed plastic bags may be disposed of according to Flinn Suggested Disposal Method #26a. Petri dishes, funnels, insect screening, and work surfaces should be treated with a 10% bleach solution according to Flinn Suggested Biological Disposal Method #1B.

Lab Hints

• Enough materials are provided in this kit for 30 students working in groups of three, or for 10 groups of students. Parts A and B of this laboratory activity can reasonably be completed in one 50-minute class period and Part C may be completed in an additional 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and questions may be completed the day after the lab.
• Good places to collect soil samples are anywhere damp humus is found—under trees and shrubs, compost piles, under or near rotting logs and mulched garden areas. If an area has several inches of mulch or leaves, brush off the top layer and collect the layer immediately above the soil. As long as good hygiene is practiced (wearing gloves and eye protection, washing hands afterward), students may bring soil and leaf litter from home in a sealed plastic bag. The samples should be placed in the Berlese–Tullgren apparatus as soon as possible.
• As the organisms move downward, some soil may be carried down with them and the funnel stem may become blocked. Carefully lift the funnel from the beaker and use a toothpick to dislodge the dirt, being careful to not spill the contents of the Berlese-Tullgren apparatus.
• If a low-wattage lamp is used, note the following recommendations.
  • Use a bulb that is compatible with the lamp receptacle, but no more than 40 watts.
  • The bulb should be no closer than 10 cm above the soil sample. If the soil gets too hot or dries out too fast, organisms may die in the soil before they drop into the beaker.
  • Do not leave the lit bulb unattended. Turn off the lamp overnight and then back on again upon arrival in the lab.
  • Two lamps should be sufficient for 10 student groups.
• The longer the soil samples are allowed to dry, the more organisms students are likely to find. Studies have shown that more organisms are extracted when the soil sample is allowed to dry at room temperature than with a 25- or 40-W bulb. However, the location of the soil sample is also a factor in the number of organisms extracted.
• With a lamp, a minimum of one day (with the lamp on only when someone is present in the room) or up to three days—depending on the moisture content of the soil—is recommended. Without a lamp, a minimum of three days or up to a week is recommended.
• Be sure to wipe down work surfaces with a freshly prepared 10% bleach solution (one part household bleach with nine parts water) after completion of the lab. This will help ensure no “stray critters” will occupy your lab! The Petri dishes, funnels, and insect screening should also be placed in a 10% bleach solution for 24 hours, rinsed with water, and then either thrown away or stored for future use.

Teacher Tips

• This lab is a great hands-on activity for a life science unit on invertebrates, ecosystems or habitats or an environmental unit on soil ecology.
• Place a Petri dish containing a variety of organisms under a digital microscope such as the Kena (Flinn Catalog No. MS1088) and allow students to view the organisms on a computer screen. The images may also be captured and magnified as a reference for future classes.
• Most stereoscopes have a reversible stage plate. Have students examine their specimens with a light background and then a dark background to highlight different organisms. Black and white paper may also be alternately placed under the Petri dish during examination of the organisms.
• Occasionally a few nematodes (roundworms) may be extracted with the Berlese-Tullgren apparatus. Usually these organisms dry out and die before descending through the funnel, whereas the arthropods’ exoskeleton provides more protection from desiccation.
• The original Berlese funnel was heated from below by a hot water bath. The improved Tullgren funnel used a light source from above.
• An identification key for soil organisms may be found at http://www.cals.ncsu.edu/course/ent591k/kwikey1.html (accessed September 2010).
• For additional soil studies, see Exploring the World of Soil Protozoa (Flinn Catalog No. FB1785) and Biology and Chemistry of Soil (Flinn Catalog No. FB1095).

Further Extensions

Alignment with AP^® Environmental Science Topics and Scoring Components

Topic: The Living World. Ecosystem Diversity (Biodiversity; natural selection; evolution; ecosystem services).
Scoring Component: 3-Living World, Ecosystem Diversity.

Answers to Prelab Questions

1. Describe a difference between insects and arachnids.

   Insects have three pairs of legs and arachnids have four pairs. Insects have three body segments and arachnids have one or two body segments.

2. Briefly explain how a Berlese–Tullgren apparatus is used to collect soil organisms. Include a sketch if desired.

   A screen is placed inside a funnel and a soil sample is placed on top of the screen. A lamp above the soil is turned on, which causes organisms in the soil to move downward, away from the light and heat. The organisms eventually drop into a container of alcohol placed under the funnel.

3. Identify the potential safety hazards in this activity and list the precautions necessary to avoid those hazards.

   Isopropyl alcohol is a moderate fire risk as it is a flammable liquid. It is also slightly toxic by ingestion and inhalation. Soil may contain eye, respiratory or skin irritants. Do not handle soil with bare hands. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron during each part of the activity. Wash hands thoroughly with soap and water before leaving the laboratory.

Sample Data

Answers to Questions

1. Describe in detail the location and conditions around the soil sample obtained by your lab group.

   Soil was taken from under a rotting log at the bottom of a woodpile in a residential yard. Outdoor temperature was about 72 °F. A maple tree was next to the woodpile, so the area was shady. No other plants were growing nearby. Ground was fairly dry.

2. How many total organisms were extracted from your soil sample? Which type of organism was the most abundant?

   Twenty-eight organisms were extracted from the sample. Mites were the most abundant, with 11 oribatid and three predator mites documented.

3. List factors that might prevent an accurate count of all the organisms that were originally in your soil sample.

   Some organisms might have died before descending all the way through the funnel. Winged insects might have flown away.

4. Compare and contrast the type and number of organisms found in your lab group’s soil sample to other groups’ samples. Which soil sample seems to have the most biodiversity? Give reasons for your answer.

   Accept all reasonable answers.

References

Soil Biology, USDA Natural Resources Conservation Service [Online] http://soils.usda.gov/sqi/concepts/soil_biology/sbinfo.html (accessed October 2010)

Introduction

Hundreds of species of organisms may be living right under your feet! According to the U.S. Department of Agriculture, a single spade-full of rich, garden soil contains more species of organisms than can be found above ground in the entire Amazon rain forest. Discover what types of organisms are living in the local soil.

Concepts

• Ecosystem
• Biodiversity
• Arthropods

Background

Ecosystems include all the living organisms in a certain area and the environmental factors or conditions that impact them. The greater the variety of organisms found in a particular ecosystem, the greater the biodiversity. The upper layer of soil containing decaying plant matter is known as humus. The humus layer comprises an ecosystem that supports various forms of animal life, particularly invertebrates. Many of the invertebrates found in humus are arthropods. The name comes from the Greek words arthro—joint and poda—feet. Arthropods have jointed appendages, an exoskeleton and segmented bodies. Soil arthropods are often considered microarthropods with a body size of only 0.1–5.0 mm.

One method for extracting soil arthropods from their habitat is by use of a Berlese–Tullgren apparatus (see Figure 3 in the Procedure). A sample of humus or leaf litter is placed on top of a screen inside a funnel. The screen prevents most soil debris from falling through the funnel while allowing organisms to move through. A lamp is placed over the funnel and a container of alcohol is placed under the funnel stem. Since soil arthropods live in a moist, cool, and relatively dark habitat, the light and heat from the lamp cause the invertebrates to migrate further down into the soil sample. As the soil continues to dry out from the heat of the lamp, the arthropods eventually pass through the holes in the screen and drop into the alcohol below.

While the biodiversity of a particular soil ecosystem varies depending on environmental factors, several types of arthropods are commonly captured through the Berlese–Tullgren extraction method. See the Common Soil Arthropods handout for sample illustrations.

Insects
Insects have three distinct body parts and three pairs of legs. The most common insects found in soil are small species of ants and beetles as well as insect larvae.

Collembolans and Diplurans
Similar to insects, collembolans or springtails and diplurans have three pairs of legs. Most springtails have a forked appendage that folds beneath the body, giving springtails the ability to quickly jump great lengths compared to their body size when threatened. Diplurans (diplo—two and uros—tail) have a pair of rear appendages that may be long and slender or short and pincer-like. They also have a pair of very long antennae made up of bead-like segments.

Arachnids
Arachnids, such as spiders and mites, have four pairs of legs. Spiders have two major body segments, the cephalothorax and abdomen. Mites do not have a segmented body, but a single fused unit. Oribatid mites are the world’s most numerous soil arthropods and contribute greatly to the soil ecosystem as decomposers. Other groups of mites are predators. A very interesting soil arachnid is the pseudoscorpion, a predator of soil mites.

Isopods
Isopods are crustaceans. Soil isopods are commonly called pill bugs due to their ability to curl up into a ball. Although similar in appearance to pill bugs, isopods called sow bugs do not have this ability.

Centipedes and Millipedes
Centipedes have long, slender, segmented bodies with one pair of jointed legs per body segment and a pair of very long antennae. Most are carnivorous. Millipedes differ from centipedes with more body segments, two pairs of legs per segment and a slightly rounder body with a pair of short antennae. Some species of millipedes roll into a ball when disturbed.

Experiment Overview

The purpose of this experiment is to extract microarthropods from a local soil sample using a Berlese–Tullgren apparatus. Extracted organisms will be documented and counted and a comparison of the biodiversity among different soil samples will be made.

Materials

Prelab Questions

1. Describe a difference between insects and arachnids.
2. Briefly explain how a Berlese–Tullgren apparatus is used to collect soil organisms. Include a sketch if desired.
3. Identify the potential safety hazards in this activity and list the precautions necessary to avoid those hazards.

Safety Precautions

Isopropyl alcohol is a moderate fire risk as it is a flammable liquid. It is also slightly toxic by ingestion and inhalation. Soil may contain eye, respiratory or skin irritants. Do not handle soil with bare hands. Exercise caution when using a lamp as the bulb may be hot. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron during each part of the activity. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Part A. Obtaining a Soil Sample

1. Obtain a resealable plastic bag and a large spoon or trowel.
2. Place a 4-inch piece of masking tape on the plastic bag. Mark the bag with your class and group number, leaving room to record the location of the soil sample.
3. Locate a plot area for the soil sample according to your instructor’s directions. Record the location on the piece of tape.
4. Using a large spoon or trowel, scoop up about ⅓ cup (80 mL) of material within the plot area including any plant debris along with 1–2 cm of surface soil and place the soil sample in the plastic bag. Seal the bag.

Part B. Extraction with a Berlese–Tullgren Apparatus

5. Cover the work area surface with newspaper.
6. Obtain a piece of insect screening and a pair of scissors.
7. Fold the screening in half and cut three small (0.5-cm) slits in the center of the screen (see Figure 1). Note: The slits will allow larger organisms to crawl through the screen.
   {11053_Procedure_Figure_1}
8. Obtain a funnel and a 100-mL beaker. Label the beaker with your class and group number.
9. Measure 8 mL of 70% isopropyl alcohol in a graduated cylinder and pour the alcohol into the beaker.
10. Place the funnel in the beaker.
11. Fold the screening in half again, and then pull one side open to form a cone (see Figure 2).
    {11053_Procedure_Figure_2}
12. Place the screening in the funnel making sure the slits in the screening do not spread apart. Note: If the slits open up too much, an excess amount of dirt may fall through the screen and obstruct the funnel stem.
13. Open the plastic bag containing the soil sample and use a spoon to transfer the soil in small quantities into the screening “cup” in the funnel. If any soil spills onto the newspaper, scoop it up with the spoon and place it in the screening cup. Remember: Wear gloves and do not touch soil with bare hands. The soil and plant debris may mound up above the rim of the funnel; do not pack the soil down (see Figure 3).
    {11053_Procedure_Figure_3}
14. Carefully, so as not to disturb the soil sample or shake any of it into the beaker, set the Berlese–Tullgren apparatus aside or place under a lamp for at least 24 hours or up to one week, according to instructor’s direction.

Part C. Examining Organisms

15. After the allotted time, remove the funnel from the beaker and empty the dried soil sample along with the insect screening back into the plastic bag. Reseal the bag.
16. Obtain the bottom of a small Petri dish and a stereoscope.
17. Gently swirl the contents of the beaker to lift any collected material off the bottom.
18. Quickly and carefully pour the alcohol with the collected material into the Petri dish. Use a small amount (no more than 2 mL) of water from a wash bottle to rinse any remaining material from the beaker into the Petri dish.
19. Place the Petri dish on the stereoscope platform under low power.
20. Look for any organisms. A toothpick may be used to move or break apart any bits of soil in the dish.
21. When an organism is located, slowly move the Petri dish so the organism is in the center of the field of view and change to a higher magnification.
22. Sketch or describe the organism in the data table on the Discover Life in the Soil Worksheet. Record the type of organism, naming it as specifically as possible. Use the Common Soil Arthropods handout or online references for help with identification.
23. Change the magnification back to low power and repeat steps 20–22 for each different type of organism found.
24. Record the total number found of each different type of organism in the data table.
25. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

11053_Student.pdf