Sparking Curiosity about Decomposition

If this is your student gardeners' first experience with decomposition concepts, here are some suggestions for stimulating curiosity and inquiry.

  • 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.
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  • 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) 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.
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  • 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.

Does Soil Need Plants?

Without plants, the land would be a desert: a barren surface, unprotected from the effects of sun, wind, and rain. Besides protecting soil from the elements, when they die, plants supply worms, insects, and microbes with the raw material to build humus, rich in nutrients that support a robust soil food web. (Recently, researchers discovered that living plants secrete excess carbohydrates through their roots to encourage growth of microbes!)

To find out what your students know or assume about this relationship, ask, Plants need soil, but does soil need plants? Record their speculations and supporting reasoning, then suggest they explore outside to make observations. They might break up into teams to explore soil in areas with different types of vegetation (e.g., garden, lawn, weedy patch, woods), or those with different degrees of plant cover (e.g., a compacted area along the edge of the driveway or sidewalk vs. a less-traveled area of the playground).

Each team can record its observations and report them back to the class: "There was a thick layer of rotting leaves under the trees," "There was a gully in the steep bank," or "Grass roots are hard to dig through!" Have them use other senses, too: record the texture and moistness of the soil, and even the odor (if they're game). Also, did they find insects and worms, or evidence of them? Do their discoveries support or contradict their initial assumptions? Is there anything that inspires them to dig deeper into the mutually beneficial relationship between plants and soil? Consider asking small groups to create presentations (e.g., slide show, PowerPoint project, play, poster) describing their findings and any inferences or conclusions they've drawn.

What About Weeds?

We call a plant a weed if it's growing where we don't want it to grow. The fact is, what we often call weeds are just plants doing exactly what they've evolved to do: take advantage of available growing space. (And they're very, very good at it, which is why we don't want them in our gardens!) In the process, many prevent erosion and weathering of the soil, and if left to grow, will actually improve texture and nutrient content, making it possible for other kinds of plants to grow and thrive. (Fallowing -- allowing a cultivated field to lie undisturbed for a period of years -- takes advantage of the soil-building ability of colonizing plants.) In general, undisturbed weed patches will yield to another kind of vegetation as ecological succession marches on.

Ask your class if they think weeds have a role in nature, and if so, what it might be. They might want to test the value of weeds as a soil cover by allowing them to take over a small corner of a garden bed. Let them grow (though we advise removing seed heads before they drop their bounty) and during the growing season, compare the weed patch soil to that of the cultivated garden. Note the amount of soil surface covered with plants, soil texture, number and type of soil creatures, and so on. They might brainstorm other tests, e.g., comparing a mulched plot, a weeded plot, and a plot where weeds rule.

Cooking Up Math Connections

A "hot" compost pile is a dynamic environment ripe for measurement, tracking, and graphing. Here are some ways to apply math to compost.

  • Constructing the pile in the appropriate dimensions (at least three cubic feet), and keeping track of the volume and/or density of the pile as it changes during "cooking."
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  • Tracking temperatures through heating and cooling cycles.
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  • Counting the number of units (e.g., bushels, bags, wheelbarrow-loads) of "browns" (carbon-rich materials) and "greens" (nitrogen-rich materials) used. A pile will heat up most quickly and cook most completely when the ratio of carbon to nitrogen is around 30 to 1. Challenge older students to estimate the carbon-nitrogen ratio of their finished pile based on the C/N ratio of the materials used. You'll find a list of C/N ratios for common materials, along with helpful formulas for doing calculations, here: Carbon/Nitrogen Ratio
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  • Measuring the amount of compostable waste produced by the school cafeteria in the course of a week, and projecting how much is produced during the school year. Students might also calculate how much waste -- in total and percent, or projected over a multi-year period -- a school composting project might divert from the landfill, and how that translates into money such a project could save the school garden.
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Last updated on 11/25/2014
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