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Buried Treasure

Objectives:  Students will learn about the role soil micro- and macroorganisms play in the decomposition process.

Grade Level:  3-6

Groupings:  Small groups

Materials:  Soil sample; wire mesh screen with 1/2 inch holes; funnel; empty coffee cans; light source; shallow plastic containers; cottage cheese; spoons; plastic wrap; rubber bands; stakes or markers; plastic bags without holes; bags with large holes such as net onion bags; bags with very small holes such as nylon net or pantyhose; selection of vegetable scraps and other trash; scales; trowels or shovels; rubber gloves.

Time Allotment:  30 minutes for setup of introductory experiments, 30 minutes for buried treasure bags; 20 minutes at various intervals for observations.


Directions:

1. Explain that soil is full of macro and microorganisms that are important in the decomposition process. Divide the class into small groups and have them set up two simple introductory experiments to learn more about these organisms.

2. Have each group collect in a plastic bag a sample of soil and leaf litter from a depth of 4-6 inches underground. Ask them to be certain that the sample is moist as there isn't much life in dry soil and leaves. Have them place a piece of wire mesh (1/2-inch holes) inside a funnel and rest the funnel inside an empty coffee can. Direct them to place their soil sample on top of the wire mesh then hang a light bulb over the whole setup, shining it directly on the soil. Explain that the heat from the lamp will drive soil critters deeper as they look for moisture. The critters will land in the bottom of the can where the students can take a good look at them. Have them try to identify some of the critters and speculate on their role in the decomposition process (See Soil Critter Chart).

3. Explain that in addition to these macroorganisms, the students will perform an experiment that will allow them a look at some microorganisms. Give each group two shallow plastic containers. Have them add 1-2 tablespoons of cottage cheese to the bottom of each. Ask them to sprinkle some soil over the cottage cheese in one of the containers. Then have them seal the containers with plastic wrap, secure tightly with a rubber band and set aside. After two or three days, have them observe the containers. Can they see anything growing? Is there a difference between the two containers? (There should be more growth in the container with the soil. Under good conditions, whole colonies of microorganisms can flourish, making them visible to the naked eye. Fuzzy looking colonies are often types of mold. Soft, slimy, yellow, orange or white colonies are yeast. Tiny cream, yellow or red colonies that look moist and shiny might be bacteria.)

students burying bags 4. Distribute one of each of the three types of bags to each group. Explain to the students that they will be burying decomposable materials in these bags in the school yard. In light of the results of the introductory experiments, ask what effect they think the holes or lack of them will have on the decomposition of the materials in the bags.

5. Have the students select an assortment of trash to add to each of the bags. They will need to gather three of each item selected, as the contents of each bag must be the same.

6. Ask the students to list the items to be placed in the bags and make predictions as to what they will look like after being buried for two weeks, four weeks, six weeks and eight weeks. Will the objects look different in the three bags over time? Why or why not? Have them weigh the bags and record their weights.

7. Have each group dig a hole large enough to hold all three bags. Direct them to place the bags in the hole at the same depth. Remind them to cover the bags completely with soil and mark them with a stake.

students examining contents of bags 8. At the biweekly intervals, have the students dig up the bags. Outdoors, have them put on rubber gloves, weigh the bags and examine the contents, noting any physical changes in the selected items. Did the weight of the bags change? Did decomposition rates differ among the bags? How did the actual results compare with the predictions?

9. Have the groups compare and discuss their results. What conclusions can the students draw about the role of oxygen and the presence of macro and microorganisms on decomposition?


Extensions:

a.  Conduct a similar experiment varying the soil depth at which each group of three bags is buried. For example, have one group of students leave their three bags above the ground, a second group buries their bag six inches below the surface, and a third group buries their bag one foot deep.

b. Read DEEP DOWN UNDERGROUND by Oliver Dunrea (Macmillan, 1993). Count and identify the soil critters. Have the students create their own counting books about critters in their garden soils.


A ROTTING LIFE

It's a rotting life down under the soil. A whole web of decomposers is hard at work turning dead organic matter back into nutrients that help plants grow. First come the bacteria and fungi. They grow on and into the leaves, secreting enzymes that break them down. Next come small soil critters such as springtails and mites. The fungal or bacterial coating makes these decaying leaves more appealing than fresh ones, a bit like toast with the addition of butter and jam! Then larger soil critters, like earthworms, millipedes, and isopods get into the act, and they too consume these decaying plants. Before you know it, dead matter is reduced to rich brown humus, and the nutrients released enter growing plants, starting the cycle all over again. So the next time you bite into a juicy apple or drink a glass of orange juice, remember you are eating the goodness of the rotting life beneath the soil!

Soil Critter Chart

Soil Critter Chart

* This material has been used with permission from Shelburne Farms, Copyright © 1995.

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