I don't know what your soil chemistry is, but around here one of the better natural fertilizers is Cotton Burr Compost, which you can also use as a mulch (what I do). It is more pricey then pine bark mulch but adds nutrients to the soil instead of depleting them. A lot of people were using mushroom compost, but the product they sell has already had the nutrients leached out of it by the companies who were growing mushrooms in it. It may sound good, but isn't as good as Cotton Burr Compost.....Just a suggestion to start with.............Maryl - (see link below)

Cotton Burr Compost