People write college theses on compost, but don't let that worry you—all the science you need to know is in the next twelve pages. Essentially, the perfect compost pile needs the right mix of ingredients, plenty of water (but not too much), warmth, and lots of air.
Ultimately, animals and microorganisms that turn plant material into compost need to eat the same things that you do. They need energy, most conveniently supplied by carbohydrates. In your case, this means starch and sugars from bread, potatoes, rice, and fruit. The main carbohydrate in plants is cellulose—you can't break this down, but compost microorganisms can. They also need nitrogen and phosphorus, to make proteins and other vital molecules.
Carbohydrates contain carbon, which provides energy and is the main structural element of living organisms. In practice, anything that contains plenty of nitrogen usually also has lots of phosphorus and other essential elements. A useful, shorthand way to describe compost ingredients is therefore by referring to their carbon:nitrogen, or C:N, ratio.
To understand why this ratio needs to be correct, a useful analogy is your own diet. Without even thinking about it, you aim to eat a balance of these two crucial elements: meat sauce and pasta; fish and chips; cheese and bread; burger and fries; roast beef and mashed potatoes.
You know that the perfect sandwich contains more bread than ham or peanut butter and, in a similar fashion, your compost pile needs more carbon than nitrogen.
This means, for example, that grass cuttings are relatively nitrogen-rich, whereas wood is mostly carbon.
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