The above YouTube video comes with an asterisk or "more to the story", which is that students and clients that fertilize like Andrew does are also otherwise doing horticulture like Andrew does and that these things go together. He is potting in certain soils, relying on training in watering & moisture monitoring practices, using pots that are correctly sized for a given tree's root system density/stage of development, waiting for generous extensions to signal strength for work suitability, etc etc. There is a magical horticulture sweet spot that dials in soil + pot dimensions + current root/canopy density + full sun w/ shade cloth adjustment + top dressing + person-always-on-site watering practices. Andrew is able to feed in a way that might cause trouble for a grower who has a young maple seedling in a 15 gallon fabric pot of organic potting soil, on a balcony, in zone 6, etc.
If you fertilize like Andrew does, then ideally you're set up in similar ways with techniques / materials / climate / etc. That is very similar to Walter Pall's situation with the "heavy feeding" comments: If you follow Walter's advice, you should probably be paying close attention to his other practices / climate factors / etc.
Consider that we are confronting an apparently-shocking idea. But then also consider that if you take a close look at what's going on with the successful growers with bushy/healthy trees, a heavy feeding scheme is often linked to some kind of horticultural constraint that gives us the "license" to do that. If their trees are healthy decade after decade, then they didn't over-feed, they heavy-fed a vigorous tree that was ready to take up a lot of fertilizer without any regrets.
An example might be that you hear from a hypothetical Japanese grower: "
I can cover this JBP with fertilizer bags because it is in this soil, this container, has this root system layout, this sun exposure, with this large unpruned leader tip, and can therefore handle heavy watering. Also I have seen this work for years". I think it'd be a mistake for the observer's takeaway from this grower to only be "
I can cover a JBP with fertilizer bags". Bjorn covered some pond basket JBPs in an ocean of Osmocote once, and when doing so he noted to the viewer that there were constraints that allowed this to be OK in that case, he saw it work in Japan, etc.
Rakuyo is a watering-heavy garden, with strong thirsty trees that have a lot of root mass with respect to container volume, are in soils that drain easily, are never lazy-watered ever (touch the top dressing of every tree to check moisture every time), and with surrounding work practices that allow shoots to form long bushy extensions. It's assumed water will not be sitting still, and if it is, it'll be nudged along and/or the tree will be marked and treated differently. For trees that are in-scope for the scheme of baseline + bonus-if-needed, the assumption is that trees are ready to gobble that fertilizer.
To bring it back to
@WNC Bonsai 's original question, the only way I was able to ground myself in terms of knowing "
what is actually meant by too much or too little fertilizer and how do I know if the grower is full of shit or not", really the only way, was to connect to growers that were aiming at similar results w/ similar species in my area and actually ask them how much they were using and when, and only ultimately trust growers in my "upstream": The ones that have vastly superior results to mine observed over the longest period of time. I've watched/helped with fertilizer doses get measured out and asked questions. For me this was the best way to get specific details. I could easily see my specific figures or advice screwing up the trees of someone in a growing season half the length of mine, in soils that are dramatically different, with a summer that isn't as wicking-dry as mine, etc.
