A compost accelerator is anything which will speed the rate at which composting occurs. People add compost accelerators to get their compost piles working more quickly, generating usable compost for their gardens. A number of garden supply stores and nurseries sell commercial products for compost acceleration, and it's also possible to add a few things which can be found around the house and garden to a compost pile to get things cracking.
It is important to be aware that a compost accelerator is only as good as the underlying compost. If compost is poor, not handled properly, and held in a bad choice of container, even an accelerator will not help it break down more quickly. Furthermore, the rate of breakdown is always slower in cooler weather, so adding accelerators during the winter months is not really advisable. If a compost pile looks unhealthy, it may be necessary to start all over again, and it can help to consult an experienced composter to get advice on getting the pile whipped into shape.
Composting happens as the organic material in the compost is broken down by all sorts of organisms. Fungi are major players in the composting cycle, along with bacteria, but insects and other organisms can play a role. A compost accelerator increases the rate of breakdown by introducing more microorganisms to the compost, so that more creatures will be going to work on the material.
Commercial products typically include an assortment of microorganisms to blend into the compost; some are designed to be mixed with water which is poured over the compost. However, just adding healthy soil to compost can have the same effect, because good soil has all of the same organisms needed to get a compost pile going. A good source is the soil underneath a tree or in the woods. A few spadefuls can be worked into a compost pile and used as a compost accelerator.
Worms can also be an excellent compost accelerator. Vermicomposting, as it is known, is a popular form of composting which can be done in a wide variety of environments. Numerous companies sell packages of worms which people can add to their compost, and people who are actively vermicomposting may have some worms they can give away. Other additives include manure, which should be added in moderate amounts to avoid overwhelming the compost, along with unpasteurized beer, seaweed extract, nettles, and seaweed which has been rinsed to remove salt.