The key to a successful compost pile is in the diversity of ingredients you put into it – the greater the variety the better. A large pile of leaves may be an enticing destination for leaping children, but it does not a compost pile make. The goal is to give all those microorganisms the ideal environment for their feeding frenzy, and that means four key ingredients – carbon, nitrogen, oxygen and moisture. Carbon is the energy source, nitrogen the protein source.
Carbon materials sustain the pile, while nitrogen materials are considered activators, because they heat up the pile. The ratio of each is important, too. Scientists have determined that the ratio of carbon to nitrogen should be 30 to 1 (30:1, or just “30”). In other words, the amount of carbon should be about 30 times the amount of nitrogen in the pile. Too much carbon matter and the pile will only smolder slowly, seemingly taking for ever to decompose. However, too much nitrogen will heat up the pile, but it will smell bad. This is because an over abundance of nitrogen will cause the pile to release ammonia gas. Think of carbon as the food for the pile, and the nitrogen is the digestive enzymes. The nitrogen is used to break down the carbon. This is why it is important to decompose your yard materials. If you were to mix a pile of high-carbon materials into the soil all the nitrogen in the soil would be absorbed by microorganisms as they try to digest all that carbon, and your plants would starve.
Now, what sorts of materials are meant by carbon or nitrogen? Typically, carbon is the dry, tough, fibrous plant materials like leaves, straw, sawdust, and cornstalks. Much of the bulky plant material is carbon. A piece of wood is very high in carbon and would take a long time to decompose, even with a lot of nitrogen mixed in. Nitrogen materials come from things like manure, grass clippings, green vegetation, blood meal, and kelp meal. Another way to think of it is that carbon materials tend to be yellow or brown, whereas nitrogen items are usually green.
Oxygen is Essential to Making Compost
There are microorganisms that work in the presence of air, called aerobes, and others that work in the absence of air, called anaerobes. Both can be involved in decomposition, but if oxygen is limited the aerobes die off. Anaerobes will still decompose your pile, but they are much slower than aerobes. This relates directly to the need for moisture. All living things need water. Too little water and the microorganisms will not be able to work effectively. Too much water will force out the oxygen and suffocate the aerobes. Your compost should be damp, but not drenched. It should feel like a wrung out washcloth.
When building your compost pile the best method is usually a layering approach. This means simply that you make layers of different kinds of ingredients, usually alternating higher carbon layers with layers of nitrogen-rich materials. More will be said about the different commonly used ingredients, but if you just follow the rule of layers of yellow/brown bulky materials alternated with layers of green stuff you should be OK. Usually the layers should be around 6 inches deep, and the total size of the pile should be at least 1 cubic yard (3 feet high, 3 feet wide and 3 feet deep). You can read more about this later in this article.