The Rottin' Truth

By Eve Pranis, April 21, 2005

Given that a compost pile is, after all, just a huge smorgasbord for billions of microorganisms, how to create compost makes sense. As with all living things, these creatures require: nutrients (carbon for energy and nitrogen to build proteins), water, and oxygen for respiration. Here's the short course in creating a sizzling outdoor pile.

From the Ground, Up

Compost piles can be freestanding or built in enclosures made from wire fencing, snow fence, wooden pallets, or lumber. If you're concerned about animal pests or odors, you can purchase a ready-made, enclosed compost system. However, properly aerated compost piles, free of animal products, shouldn't have those problems. The enclosure, and the pile, should occupy at least one cubic yard. This volume maintains enough heat in the center for rapid decomposition. In a two-pile system, one pile actively composts while the other serves as a holding area for new materials.

Decomposition happens ... that's a fact of life! But to help it happen quickly and create high-quality compost (as far as plants are concerned), your pile should contain a mixture of dry (high carbon) materials, fresh (high nitrogen) materials, soil, air, and water. Use about three parts dry to one part fresh materials. According to conventional wisdom, smaller materials, where more surface area is exposed to microbial action, will break down more rapidly. Consider challenging your sleuths to test this theory themselves. Other items often added to compost piles are layers of garden soil (to add more decomposers), lime (to lower acidity), and nitrogen fertilizer (if there's an abundance of dry materials).

Many gardeners build their piles by alternating layers of materials. Although layering isn't necessary, making a compost "cake" helps students measure the relative amounts of different ingredients. Water each layer, then routinely keep tabs on the pile's moisture, keeping it about as moist as a wrung-out sponge.

Because this standard composting method relies on aerobic decomposers that require oxygen, you'll need to keep the pile well aerated. Place a dry, coarse layer at the bottom, then periodically add layers of dry high-carbon materials. You can also routinely poke the entire pile with a rod, or insert a PVC pipe with holes into the center as you build it. Turning a pile also introduces air. If your pile smells, it may be receiving too little air and too much moisture.

The Operation Heats Up

As soon as you build a pile, invite students to keep their eyes--and hands--on it. The center of a well-functioning pile will heat quickly to 90° to 140° F. As one set of organisms consumes and breaks down the most readily degradable material, they produce heat. (Consider having students draw analogies to human food consumption.) At that point, the more heat-loving microbes take over and thrive. Although a pile will eventually compost if left alone, you can dramatically increase the rate at which materials decompose by turning it inside out. Turning enables all parts of the pile to benefit from the rapid decomposition taking place in the hot center. Turn the pile whenever it begins to feel cool to the touch. A pile with the right balance of materials and moisture, if turned every day, will compost completely in just a few weeks. A pile left to sit without turning could take months to decompose. Turning the pile even once or twice greatly decreases 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 exude an earthy smell.

Compost Inquiries

Challenge students to experiment with ways of hastening decomposition by devising ways to provide better aeration, decrease the size of materials, add more microbes, or change the pH. Or have them design or investigate unique decomposition systems. Some farmers in China, for example, dig parallel trenches and fill them with organic wastes and redworm cocoons. They then plant soybeans between the trenches to take advantage of the nutrients released by the worms. Other gardeners bury organic waste in holes, cover it with soil, then plant seeds or seedlings directly above the pit.

To Feed or Not to Feed

Use what you've got! Consider where you might find sources of locally available compost. Unique compost ingredients include seaweed, apple pumice from cider-making, Spanish moss, and rice hulls. We'd love to hear what you and your students discover in your region!

Compost these:

* dry matter (high carbon: hay, straw, dried leaves, and grasses)

* fresh matter (high nitrogen: lawn clippings, vegetable scraps, green garden debris, cow or horse manure)

Never compost these:

* greasy foods

* human, dog, or cat feces

* meat, bones

Avoid adding noxious weeds or diseased plants, although a well-heated pile should kill seeds or pathogens.

To Feed or Not to Feed

Use what you've got! Consider where you might find sources of locally available compost. Unique compost ingredients include seaweed, apple pumice from cider-making, Spanish moss, and rice hulls. We'd love to hear what you and your students discover in your region!

Compost these:

* dry matter (high carbon: hay, straw, dried leaves, and grasses)

* fresh matter (high nitrogen: lawn clippings, vegetable scraps, green garden debris, cow or horse manure)

Never compost these:

* greasy foods

* human, dog, or cat feces

* meat, bones

Avoid adding noxious weeds or diseased plants, although a well-heated pile should kill seeds or pathogens.

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