Compost is the breakdown product of organic wastes, containing large proportions of humus which is a long-acting soil fertility booster. Any organic material (not talking about certified organic designation, just the carbon-based once-living kind of organic) can be composted, eventually becoming soil again as it once was before being integrated into an organism.
“Composts offer a practical means of maintaining the soil fertility which is the most important factor in the successful operation of a subsistence farm….There are available on practically all farms and gardens many materials which are useful for composts, although the farmer or gardener often fails to appreciate their value. Some of the common materials which are often wasted are leaves, straw, muck, vegetable tops, grass clippings, and garbage material which is inedible for chickens or pigs.”
~C.C. Fletcher, 1935 USDA Yearbook of Agriculture