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.
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