A fruit scrap and a woody twig both contain carbon. Yet they do not offer the compost community the same food, or release it at the same pace. That difference helps us look more closely at two words we hear often in composting: carbon and nitrogen. We have explored who does the work in compost and how those organisms interact. Now we can ask what they need from the materials they consume. Microorganisms need both energy and materials to grow. Carbon-containing compounds, including sugars and other carbohydrates, supply energy and carbon for building cells. Nitrogen helps them make proteins, build DNA, and form other essential cell components. With usable carbon, enough nitrogen, and suitable conditions to include moisture and air, microorganisms can grow and reproduce, expanding the community and stimulate decomposition. Plenty of carbon-rich food alone cannot support their growth if available nitrogen is limited. As microorganisms use the food, they release some of its energy as heat. Greater activity can generate more heat, although whether the pile warms also depends on how readily that heat escapes. Leaves, grass clippings, and food scraps contain both carbon and nitrogen. Calling them “carbon materials” or “nitrogen materials” is shorthand for the balance between those elements. That balance is expressed as a carbon-to-nitrogen ratio, written C. For example, 30:1 means thirty parts carbon for every one part nitrogen by weight. It does not mean thirty buckets of browns to one bucket of greens. A higher ratio means more carbon relative to nitrogen; a lower ratio means more nitrogen relative to carbon. This example explains how to read the ratio rather than prescribing one mixture for every compost system. The ratio helps us understand what materials contribute, but availability matters too. Some carbon compounds, such as sugars in fruit scraps, are relatively easy for microorganisms to use. Carbon held in woody material generally becomes available more slowly because of the material’s chemical structure. A pile can contain plenty of carbon without offering a large supply that organisms can use quickly.