One interesting thing I've seen recently is called "microinjection."
It turns out that a tree only uses a tiny percentage of the nutrients that are available in the soil. Many go into the water table. Here is a link that shows how fertilizer/insecticides are injected directly into the trunk of landscape trees. A small hole is drilled in the trunk and a little plastic doohickey containing the fertilizer is hammered into the trunk.
http://www.treedoc.com/MAUGET.HTM
There is a Windows Media Format movie here:
http://www.treedoc.com/Videos/Micro...made%20easy.WMV
Those injectors are expensive, though and they definitely would be too much for a bonsai. Maybe pico-injectors (micro-micro).