We don't use chemical fertilizers because they contain "salts" that disturbs the soil ecosystem. The soil ecosystem is a living community of organisms in and above the soil. The organic matter in the soil supports bacteria, fungi and mycorrhizae which are used by large organisms like protozoa, nematodes, earthworms and arthropods. The cycle is completed by birds and other animals that eat the "bugs" and add more organic material to the soil.
We have been using GrowBetter organic fertilizer supplied to us from Jongs, a chicken farmer in Riverside, California. (yes its chicken poop). Read more about the Soil Food Web at www.jongs.com