Fungus gnats will be a problem only if the containers don't drain well. They have only been a problem for me indoors, fwiw, they don't thrive where the air circulation is good. So you may find the reservoir in the bottom of your containers is a problem, if so the answer is to drain them better, and treat with BTi, that is Mosquito Dunks, Mosquito Bits or similar product which kills the gnat in larval stage. I just soak a dunk in a gallon of water and pour that on. I believe you can keep soaking it until used up. You can also crumble the bits or dunks onto the surface or lightly mixed in, and it will continue to release whenever you water. The BT israelensis products are specific to larvae of flies and don't harm any beneficials (unless they are flies!).
People who change out container mix regularly are mainly using a soil-less mix and liquid ferts. The rationale I believe is that the fibrous particles in the mix break down and no longer give the desired texture. An organic mix, well you have to keep adding organic matter, make sure it is well aerated and well drained enough, and being outdoors it should have no more pests or soil problems than any other soil/plants in the area.
Worth posted some interesting discussion of a method without drainage in the container that seems to work in the dry and hot climate of where he's living. There is a drain installed so in case of heavy rain flooding the container, it can be siphoned off. This could be a lot of work though if it rains often. I also suspect that waterlogging encourages rots and unhelpful microbes instead of the good ones.
@Goodloe, what are you doing with coffee grounds? Looking for ways to tweak my mix.
