From the Comox Valley Echo, Tuesday 7 May 2013, Ask a Pro
Question: Do I need to add an activator to get my compost started?
Answer: No. If you have a reasonable balance of carbon and nitrogen materials in your compost, you do not need an activator to start the decomposition. The needed micro-organisms are plentiful from the ground under your composter and on the leaves and scraps that you add. Just keep it damp and airy. Inoculants are different in that they supply the bacteria and fungus needed, not nutrients. Tumbler composters usually come with inoculant as they are not connected to Earth. Again they can be had for free from old compost, like a sourdough culture, or soil from your yard. Store bought soil is less effective as it is often sterilized first. If you are starting with a brand new compost system and can’t wait for the bugs to get going, here are a few readily available activators: dog food, alfalfa meal, manure from grass eaters (e.g. horses or rabbits, NOT dogs or cats!), dandelions, yarrow, comfrey, blood meal, fish meal, beer and ammonia. But the best activator of all … is pee. You figure out the logistics on that one!