Theo,

The answer is not simple. However... I do not grow tropicals, only temperate
climate hardy plants in zone 6.5 If one uses a soiless mix (e.g. 80% Turface
or similar), Micronutrients are important although more so for some species
than others. Micro nutrients are not essential throughout the year. Bonsai
use different things at different times of their annual cycle. I use a
commercial product called Micromax once per year, in the early spring just as
the buds are opening. 1/2 level teaspoon per gallon of soil in the pot gives
near miraculous results in the color, quality, and substance of leaves on the
bonsai. My observation is taht the trace elements provided are most needed
when the new leaves are forming but are of very limited use after the leaves
have hardened off. At that point regular fertilizers seem to have enough stuff
in them to keep the juices flowing properly. Regular use of micro
nutrientscould actually be toxic to some plants.

Julian Adams
Adams' Bonsai
Lynchburg, VA