Many different materials are suitable for composting organisms. Composters often refer to "C:N" requirements; some materials contain high amounts of carbon in the form of cellulose which the bacteria need for their energy. Other materials contain nitrogen in the form of protein, which provide nutrients for the energy exchanges. It would however be an over-simplification to describe composting as about carbon and nitrogen, as is often portrayed in popular literature. Elemental carbon - such as charcoal - is not compostable nor is a pure form of nitrogen, even in combination with carbon. Not only this, but a great variety of man-made, carbon-containing products, including many textiles and polyethylene, are not compostable - hence the push for biodegradable plastics.

Suitable ingredients with relatively high carbon content include: Dry, straw-type material, such as cereal straws Autumn leaves Sawdust and wood chips paper and cardboard (such as corrugated cardboard or newsprint with soy-based inks) Ingredients with relatively high nitrogen content include: Green plant material (fresh or wilted) such as crop residues, hay, grass clippings, weeds Manure of poultry and herbivorous animals such as horses, cows and llamas Fruit and vegetable trimmings

The most efficient composting occurs by seeking to obtain an initial C:N mix of 25-30 to 1 by dry chemical weight. Grass clippings have an average ratio of 10-19 to 1 and dry autumn leaves from 55-100 to 1. Mixing equal parts by volume approximates the ideal range. Poultry manure provides much nitrogen but with a ratio to carbon that is imbalanced. If composted alone, this results in excessive N-loss in the form of ammonia - and some odor. Horse manure provides a good mix of both, although in modern stables, so much bedding may be used as to make the mix too carbonaceous. For home-scale composting, mixing the materials as they are added increases the rate of decomposition, but it can be easier to place the materials in alternating layers, approximately 15 cm (6 in) thick, to help estimate the quantities. Keeping carbon and nitrogen sources separated in the pile can slow down the process, but decomposition will still occur. Some people put special materials and activators into their compost.

A light dusting of agricultural lime (not on animal manure layers) can curb excessive acidity, especially with food waste. Seaweed meal provides a ready source of trace elements. Finely pulverized rock (rock flour or rock dust) can also provide minerals, while clay and leached rock dust are poor in trace minerals. Composting in the form of bioremediation can break down petroleum hydrocarbons, TNT and a variety of toxic compounds. It is the bacterial and in some cases fungal content of the compost that possess the enzymatic properties to de-polymerize the complex man-made molecules. In other words, there is nothing about the composting process per se that adds or detracts from this, unless as noted above, by warming, to increase the metabolic rate of the constituent organisms.

Some materials are best left to a high-rate thermophilic composting system, as they decompose slower, attract vermin and require higher temperatures to kill pathogens than backyard composting provides. These materials include meat, dairy products, eggs, restaurant grease, cooking oil, manure and bedding of non-herbivores, and residuals from the treatment of wastewater and drinking water. Meat and dairy products can be recycled using bokashi, a fermentation method. Human waste can be composted by industrial, high-heat methods and also composting toilets, even though most composting toilets do not allow for the thermophilic decomposition believed to be necessary for a rapid kill of pathogens, such as Salmonella.

This is not a problem, however, since composting toilets also incorporate the essential element of time required to reduce the available substrate on which pathogens can feed, while increasing the growth of competing microbes. If high temperatures are reached, the resulting compost can be safely used as a fertilizer for food crops and even directly edible crops provided it is not illegal in the regions where the humanure is applied. Careful filtration of the compost also prevents contamination. Humanure fertilizer is, however, used throughout the less developed world and is becoming more accepted as a garden amendment in the developed world (see humanure).