Compost is the most important material in organic horticulture, and the qualities of the compost will vary depending on the method used to create it and the raw materials. There are two main types of composting: hot and cold. The materials added need to have a good ratio of carbon to nitrogen, as well as a good balance of other plant nutrients.
Hot composting is driven by thermophyllic (heat-loving) bacteria, which are active in an oxygen-rich environment. This system is preferred for larger operations due both to its speed and its ability to kill weed seeds and disease spores (since these the piles get hot enough to pasteurize the material). The US national standards for windrow composting in organic agriculture require that a pile maintain a temperature between 131°F and 170°F for a minimum of 15 days, and that the pile be turned at least 5 times during that period, to ensure pasteurization of the entire pile.
Hot composting can be very difficult in smaller operations, or in general anywhere where machinery is not at hand to do the turning. The main advantage is that it allows the horticulturist to compost in a "single stream", rather than needing to sort materials for cold systems.
Cold composting does create heat, but as a rule only the core of a cold system will reach the thermophyllic stage and thus care needs to be taken to exclude weed seeds, rootstocks of persistent weeds, and any diseased materials from the pile. Materials not suited for the cold pile should either be burned or disposed of off-site. Most cold piles contain earthworms, producing a very high quality compost.