What Happens in a Compost Heap?

The Process

Compost is an organic material that results from the action of micro-organisms on plant waste. A handful of compost is crumbly and dark, and smells like damp, sweet soil. Gardeners add compost to their soil because it contains some nutrients and also lightens the soil, so that the resulting mixture will drain more easily. Gardeners make compost by mixing organic materials together in a compost heap or bin, watering it and turning the materials over occasionally to aerate the mass. Billions of microorganisms in the compost heap then digest the organic matter and produce compost as a byproduct. Under optimal conditions, this process will produce good compost in as little as 4 weeks.


The materials in the compost heap must contain two elements in the right proportions for the process to work at top efficiency. Carbon is an element that microorganisms feed on, while they use nitrogen to build the proteins that make up their bodies. Any organic material (meaning anything that was once alive) already contains carbon and nitrogen, though the ratios vary from one organism to the next. The Cornell Waste Management Institute (CWMI) notes that sawdust, for instance, has a carbon to nitrogen ratio of 500 to 1, while the ratio for grass clippings is 20 to 1. The ideal carbon to nitrogen ratio for a successful compost heap is 30 to 1. CWMI recommends combining two parts of fresh lawn clippings with one part of dry, fallen leaves, which will result in what's called a "hot" compost heap, one that produces compost quickly. CWMI says that a ratio of 50 to 1 will result in cooler, slower composting action.

Garbage In, Compost Out

You can also put vegetable waste left over from cooking into your compost heap to supplement the yard waste. CWMI cautions you not to add fatty foods like cooking oil or salad dressing, meat or bones, and Riverside County Waste Management Department in California advises that you can only manure produced by animals that don't eat meat.

Size Matters

For best results, the bits of material you put into your compost heap should be small, particularly if they're woody items like tree limbs. Chopping the material up will give the microorganisms the largest possible surface area to work on and will speed the process. Your compost heap should be at least 3 cubic feet in volume, to hold in the heat that the microbes generate. Keeping the heat in encourages the biological processes to continue. If the heap is smaller, it won't get warm enough to produce compost; if it's bigger than 5 cubic feet, the center of the heap won't get enough air to keep the process moving along.

Water and Air

Once you've put all your materials into your compost heap, you have to dampen it to get the microbial action going. CWMI advises that you should keep your compost heap as "moist as a wrung-out sponge." The compost heap also needs regularly aeration to encourage the microorganisms to do their work, so you'll have to turn the materials over and stir them up at least once a week. Use a long-handled pitchfork or a rake, and check while you're turning it to make sure that all of the composting matter is damp.

Keywords: compost ratios, turning compost, size of compost heap

About this Author

Cheyenne Cartwright has worked in publishing for more than 25 years. She has served as an editor for several large nonprofit institutions, and her writing has appeared in a variety of publications, including "Professional Bull Rider Magazine." She holds a Bachelor of Arts in English from Oklahoma Christian University and a Master of Arts in English from the University of Tulsa.