I am a strong believer in high-frequency fertigation for growing trees in a well-draining mix. I should preface this by saying that I have grown a lot of fast growing plants in coco coir, with a frequency of 3 waterings daily. With the right set-up, this ensures nutrients are constantly available to the plant. The plants were under strong grow lights, with adequate air, and constant temps... A controlled environment is essential when watering at that frequency. I have, not once, had issues with root rot doing this, but admittedly, this is not for novice growers. For trees, the frequency is typically much more sparce, of course.
Anyway, I use a diluted nutrient solution, with an EC around .5 to .8 (depending on growing conditions) with every watering for my established, deciduous trees. I grow indoors, so I cannot stress enough, a well controlled environment matters if doing this indoors. If, for instance, the tree is not receiving enough light, it will not be able to use the nutrients provided on the same level. I try to maintain my grow lights with a PPFD of about 600 - 700 for trees getting 14 to 16 hours of light a day. Under the LEDs, this comes out to a LUX of roughly 33,000 - 36,000. A timer is useful, of course. Also, about once every three to four weeks, I water with a super low amount of minerals (like tap water, etc) to ensure the flushing of any salt build-up. Using chelates can make the 'flush' more efficient, as the chelates grab salts, bind them, and help them move through the medium. If your bonsai is in a soiless mix, like Akadama, calcinated clay, pumice, etc. it should drain well. A heavy watering will help push out salt build-up and allow for healthy gas exchange to ensure the roots get adequate oxygen (standard knowledge, I know). I have never burnt leaves or caused root rot with this method. I recommend anyone using a nute mixture to get an EC, PPM, or TDS meter, and if growing acid or alkaline loving species, especially if your water source may vary, to get a PH pen as well. One caveat- organic fertilizer will not read accurately on a pen... simply different rules. But if using a salt-based formula, one can easily get away with a diluted solution every watering during grow season.