Composting is an extension of the natural life process of decay and nutrient recycling. The litter layer in grassland and forest is analogous to a compost heap. In that litter layer, as in the compost heap, decomposers, such as bacteria and fungi, aided by earthworms and small insects, break down the remains of plants and animals, obtaining energy for their own life processes and liberating nutrients in a form available to plant roots.
During early decomposition, some of the organic debris is quickly converted to water, carbon dioxide, methane, and ammonia, but more resistant materials decompose slowly, gradually becoming humus. Humus is a homogeneous, amorphous, dark-colored, and practically odorless material that is a valuable component of rich soil. It makes soil soft and easily penetrated by plant roots. It binds strongly to water, helping to retain soil moisture. Humus also acts as an ion exchanger, holding plant nutrients and slowly releasing them to the soil and plant roots. Nutrients bound to humus are not readily leached away by rain as are the nutrients in chemical fertilizers.
Humus is slowly broken down by soil bacteria, releasing its component nutrients. When crops or lawn clippings are removed from the land where they grew, plant nutrients are also removed with the organic matter, while soil humus gradually decomposes away without being replenished. The soil loses texture and water holding ability as well as nutrients.
Composting provides favorable conditions where both resistant materials, such as leaves, and esthetically unpleasant materials, such as garbage, will quickly break down into humus. Properly done the process is free of odors and flies.
Nearly any natural, organic, material can be composted, but some, such as human body waste, require special care to prevent disease spread, and others, such as sawdust, contain so little nitrogen that they decompose very slowly unless mixed with nitrogen-rich materials.
Common school-site materials suitable for composting include:
Grass clippings, weeds and leaves.
Uneaten food, fruit peels, etc, from snacks, lunches or parties. (Mixed-in bits of paper such as napkin or towel are not a problem, but plastic and metal should be avoided.)
Remnants from many projects. (Use common sense. Natural materials, like cotton, silk, and leather, compost readily; synthetic materials, like polyester and plastic, usually do not.)
As demonstrated in past Science Fairs, the bio-degradability of diverse materials can readily be determined by leaving them in a compost heap for a time. Simply enclose each different material inside a separate, labeled container made from hardware cloth or plastic mesh and bury them in the heap. (Labels are also subject to decomposition; a reliable one can be scratched into a strip of thin aluminum.) After a time interval, dig them up and see what's left. As always in science, the more replicas, the surer you are that the results didn't just happen by chance.