"My third graders had been exploring trees as part of a history unit when several began to wonder what happens to the leaves that fall in the autumn," reports Merion, PA, teacher Judith Linker. "They wondered why we aren't buried in old leaves!" Although some students had thoughts about what factors might contribute to decomposition, the class as a whole was unsure, says Judith. Using a setup made from 2-liter bottles as suggested in Bottle Biology, students worked in small groups to set up "decomposition columns," then brainstormed what factors in nature might promote decomposition.
"One group recalled seeing old leaves that were still intact after the winter, and inferred that perhaps heat is needed to help leaves decompose," Judith recounts. "So that group decided to put its decomposition column on a heating pad. Other students had discovered fungus growing in the darkness underneath leaves outside. They inferred that fungus might be important to decomposition and that darkness would promote fungus growth, so they left their decomposition chamber in the dark. Another group had noticed mold growing on old coffee grounds that had been put into the garden, so added coffee grounds to encourage mold growth in their column.
One group thought that organisms in soil might contribute, so deliberately added soil to their decomposition column. In addition to comparing experimental bottles, students also created a control bottle with leaves but no water or other additions.
Each month, students weighed their bottles, measured the height of the compost in each column, and described what they saw happening to the contents. "After a month, students saw a big drop in height, and began to see mold and fruit flies," says Judith. "They were intrigued by the dramatic changes that were taking place," she adds.
At the end of the year, each group dumped out the contents of its decomposition column on a desk. Students rotated among tables, observing the products, reviewing data, and rating each compost column. The piles that were most reduced and that looked the most like soil scored highest, notes Judith. "The group that had added moldy coffee grounds had the winning compost, while those in darkness and with added soil also fared quite well." Students had thought the heated pile would do better, she adds, but they realized that it may have gotten too hot, which prompted a discussion about how they would modify the setup another time.
"In addition to making good observations, using metric measurements, and compiling data, students came to appreciate another role of microorganisms," observes Judith. "It also inspired them to discuss how to use what they've learned about conditions for decomposition to create an effective school compost bin."