The soil you use affects rooting, feeding, watering and transpiration; it is where half your tree lives so this is our second biggest consideration in maintaining your bonsai. While the needs of individual species vary greatly a good rule of thumb is 30% grit, 70% humus for deciduous trees, and 70% grit, 30% humus for evergreen needled plants, but these are just general guidelines.
Hemlocks, for instance, are an evergreen needled tree that likes 70% humus and 30% grit, but the pines like even more grit than the 70%; purists in Japan grow pines in 100% sand. Find a soil that fits your watering schedule and your tree. While I have used commercial bonsai soils, the best tool purchase I ever made was a set of soil sieving screens. I now make a custom soil for each of my plants as I repot, adjusting additions of exploded clay, marble sand (these are my grits), peat moss, pine bark, cocoa mulch and compost (my humus).
The smallest screen allows me to screen and dispose (in the compost) all the fines in any soil components, the largest takes out all the debris too big for soil, and the remaining screen sorts my soil into a larger grade for the bottom of a big pot, and a smaller grade for the top of larger pots or for the entire depth of smaller pots (they dry out quicker and less porosity means slower drying. The shallower depth of small pots also means gas exchange is not a big a deal). The same basics apply for the tropicals, with a 70/30 humus/grit mix (with some of the more water hungry tropicals like ficus I substitute pine bark for grit and go for a 50/50 mix). Again, much like the watering process, watch your plants carefully and determine their individual needs for yourself.