Theme: Gardening
for a Sustainable Future
Rottin' Lessons: Cultivating
Compost
"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 degrees 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 has even started Master Composter programs
to train community volunteers to help others.
Author: Eve Pranis
© 2008
National Gardening Association