Wasteful Lessons

"It was a terrific example of an accident that turned into a teachable moment," reports Minneapolis, MN, teacher Joanne Taft. "That's how my third and fourth graders learned about composting." When Joanne's students returned from a long winter break and discovered that many of their unwatered indoor plants had died, they dumped the moist soil mix and plant remains into a clear plastic bag to discard. But then they began to wonder what might happen to the materials over time so they made predictions and placed the bag in the warmth and light of a windowsill to observe. The steaming, changing contents inspired students to imagine what other types of things would (or wouldn't) change, so in went samples of lunchbox leftovers, aluminum pop tops, wood, and other items.

"By spring," says Joanne, "the bag contained a rich, dark soil. We combined it with potting mix to grow herbs and other seeds. Noticing which things didn't break down inspired a discussion of what happens to other types of garbage. We're now doing a lot more school recycling, and some of the kids have even started compost piles at home."

We've heard from teachers around the country who have used decomposition and composting as a springboard for a wide range of lessons. Their activities range from observing bread mold in the classroom to schoolwide worm-composting projects. Some classes focus on observing and measuring the physical and chemical changes that occur during decomposition. Many tie composting into studies of food webs and nutrient cycling. For others, composting is part of broader recycling projects, often tying in with local and state waste-reduction mandates.

"These kids are being bombarded with pleas to recycle and reduce waste," notes San Jose, CA, parent Michelle Steever. "But the concept of recycling things like bottles and cans versus the recycling that happens when things return to the earth can be confusing for kids. Seeing, feeling, and creating compost really helps them grasp the concept."

Exploring Decomposition

Decomposers, the final links in food chains, use dead plants and animals as food, breaking them down into smaller particles. Among the decomposers are fungi, which include the familiar molds and mushrooms. Other decomposers -- called bacteria -- are so small that a mere teaspoon of soil could contain billions of them.

Composting happens when humans promote this natural process of decomposition and nutrient cycling by creating an environment in which particular decomposers thrive. As the decomposers use the organic matter for energy and maintenance, they break it down into simpler molecules that can be used again as nutrients for plants, and the cycle begins all over again. This process also give off heat, which in turn speeds up decomposition. While microorganisms accomplish most of the chemical decomposition in a compost pile, small invertebrates such as sowbugs and earthworms are responsible for much of the physical breakdown of materials.

Although your students can't actually see many of the decomposers, they can explore their behavior up close. Whether you plan to build an outdoor compost pile or not, you can lay the groundwork with some exploratory activities. Consider the following:

  • Fill a plastic bag with some "once living" materials (e.g., cut fruit, grass clippings, moist bread) and hang the bag on the bulletin board with a sign reading "What do you think is happening in this bag?" Encourage students to observe and to make and explain predictions.
  • Have students generate a list of things that they think will and will not decompose. To test predictions, create mini-decomposition chambers (e.g., sealed plastic bags or clear plastic shoeboxes) to leave in the classroom or bury outside. Students may want to experiment by providing air holes, blowing in air, or adding soil to some containers. Have them observe containers regularly, or dig them up after a month and examine the contents. (See the activity "FungusAmongus" on pages 210 - 213 in GrowLab: Activities for Growing Minds for further ideas.)
  • Challenge students to work outdoors in small groups to identify examples of decomposition in action. Have them describe what they observe that leads them to think decomposition is occurring.

Cultivating Compost

When constructing and maintaining an outdoor compost pile, consider organizing cooperative student groups to rotate through the necessary tasks. As your students decide how to construct and care for your compost pile, they should begin to understand that they are providing the basic conditions that all living things-humans and decomposers-need to thrive: water, air, and nutrients.

Building the Pile. You can either build a freestanding pile or create an aerated enclosure using chicken wire, snow fence, wooden pallets, or lumber. To maintain enough heat in the center for rapid decomposition, make the pile at least one cubic meter. Some people build two piles, one for active compost and the other as a holding area for new materials. If you're concerned about animal pests or odors, you can purchase a ready-made, enclosed compost system, but properly aerated compost piles free of meat scraps and other animal products shouldn't have those problems.

To create an environment that encourages efficient decomposition, your compost pile should contain a mixture of dry (high carbon) materials, fresh (high nitrogen) materials, soil, air, and water. Microorganisms digest carbon as an energy source and take in nitrogen to make proteins. The smaller the materials, the more surface area is exposed and the more rapidly they will break down. Examples of high carbon and high nitrogen materials follow.

Never include greasy foods; human, dog, or cat feces; meat; bones; or toxic materials. Also avoid adding noxious weeds or diseased plants (although a well-heated compost pile will kill many diseases and weeds).

Other additives sometimes recommended for compost piles are: layers of garden soil (to add additional decomposers), nitrogen fertilizer (if the pile has an abundance of dry materials), and compost inoculants or activators. (Your students might want to experiment with some of these compost pile additions.)

Compost is most efficiently made by "aerobic" decomposers that require oxygen. If your pile is too dense or wet, thus poorly aerated, "anaerobic" decomposers will create a foul-smelling mixture. To keep a pile well aerated, place a dry, airy layer at the bottom, and periodically mix in coarse materials like hay. But be sure to water it occasionally: a compost pile should be kept about as moist as a wrung-out sponge.

Maintaining the Pile. During the first few weeks of composting, the center of an active pile may reach about 140° F. The heat produced as decomposition occurs further speeds up the process. Students may be surprised after feeling and measuring the temperature in the center of the pile. Ask them what analogies they can draw to human food consumption.

Although a pile will eventually compost if left alone, you can drastically increase the rate at which materials compost by turning the pile inside out. By turning the pile, you aerate and mix the materials so they all benefit from the rapid decomposition that takes place in the hot center. A pile with the right balance of materials and moisture, if turned every day, can completely compost in just a few weeks. A pile left to sit without turning could take many months. Turning the pile even once or twice will greatly decrease the time it takes to finish.

Compost is finished when it cools off and decreases to about a third of its original volume. It should be dark brown, soil-like, and have an earthy smell. You can use it in your outdoor garden or flower beds, where it will attract earthworms, help improve soil structure, and slowly release nutrients. You can also add compost to your indoor potting mix. Try experimenting with different proportions. (More than one third by volume of compost in potting mix may be too strong for tender seedlings.)

Composting Help

Are you inspired to try a composting project with your students, on a small scale or a schoolwide basis, but need further written materials, technical advice, or other support? With increased national attention to recycling, there are more and more resources available locally. Start by checking with your local Cooperative Extension Service or statewide recycling networks. In some areas, Cooperative Extension have Master Composter programs to train community volunteers to help others.

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