All

I am new to gardening and am about to construct a series of raised garden
beds. I have a good idea as to how to build the frame BUT am a little
stumped as to what to fill it with. I can't believe that I have to fill the
whole thing with topsoil as my calculations lead me to believe that a 30' by
4' by 24" area would require 8.9 yards or 12.4 tons of topsoil. I assume
that I have to have a drainage media such as gravel or sand, a middle layer
of some material and top level of higher quality soil. Am I correct. If so
what should the middle layer and drainage area consist of in order to
provide an excellent growth medium for a mix of plants ? what are the
depths of these layers and should I mix in fertilizers as I put it in?

Hope these are good questions and if anyone has suggestions as to web
resources or books I can read on this topic I would be most grateful


Thanks

MK