How Do Compost Piles Work?
If a compost pile starts to get stinky, it may be a sign that something's wrong.
Credit: Lianem | Dreamstime

It may seem almost like a magic trick: leftover veggies from last night's dinner and yard debris go into a pile, and nutrient-rich fertilizer comes out.

But that's what happens in an outdoor compost pile. Microbes, worms, snails, insects and fungi decompose organic material aerobically, which means they use oxygen as they breakdown the materials in the pile.

How it works

Bacteria are the powerhouse of a compost pile. They break down plant matter and create carbon dioxide and heat. Run-of-the-mill microbes usually start off the process, but as their consumption of the compost materials raises the temperature of the pile, heat-loving microorganisms take over. Compost can get up to 100 to 140 degrees Fahrenheit (40-60 degrees Celsius) as it brews.

Larger critters such as worms, slugs and insects also digest the decomposing matter, pooping out finished compost as they munch their way through. Their secretions improve compost's texture, binding small particles into larger crumbly bits.

What to put in a compost pile

When adding raw materials to a pile, it's important to balance nitrogen-rich plant scraps (called greens) and carbon-rich leaves, woodchips, coffee grounds and even bits of paper (called browns).

If there is too little nitrogen, the microbes may not be able to raise the pile's temperature enough to kill most pathogens and dangerous bacteria in finished compost. But too much nitrogen and the tiny composters will not be able to handle it all, releasing odorous ammonia gas.

A lack of oxygen is the likely culprit for other bad smells. Without air, the bacteria will switch from doing aerobic decomposition to anaerobic decomposition, which produces methane and other gases that can give off the odor of rotten eggs. Turning compost over with a shovel or pitchfork and adding dry, bulky material can allow oxygen back in and reduce smelliness.

What not to put in a compost pile

Anything that was once alive can be composted, according to the Cornell University Waste Management Institute. But for normal outdoor composting, they advise against adding meat, bones or especially fatty foods . These, if not heated sufficiently, can leave harmful bacteria in the compost.

The fertilizer from a compost pile is ready for use when it no longer resembles its raw materials and the microbes and worms have digested all there is to digest. This usually takes a few months or longer.

The New York Compost Project recommends a "plastic bag test" to measure whether compost's decomposition is complete. To conduct the test, put a handful of compost into a re-sealable plastic bag and leave it sealed for a week or so. If, upon opening the bag, the smell of ammonia or a sour odor is detected, then the microorganisms are still at work.

The finished compost, sometimes called "black gold," is a great fertilizer. It can be mixed into garden soil or laid around newly bedded plants.

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