Excellent post Tom.
drzaiusx11 wrote: My question is this: should these trees (a 5 year old conifer, and two 2 year old deciduous trees) be in "bonsai" soil (ie the arguably "standard" equal parts N-P-K with course/mostly inorganic mix)?
To answer your question- IMO Absolutely
WRT N-P-K Which are components of fertilizer. Not
a structural component of the soil. These are not something you add by the handful to your soils composition so to speak [Such as IE: grit], But instead they are added via a fert routine that you continually repeat.
Usually when purchased at a nursery these pre-bonsai plants/trees/shrubs/etc are in "potting soil" that has more nitrogen in it than anything else.
Is this just a regular old nursery? It is only natural that they would be in regular potting soil. It does not mean it's the best thing for bonsai though.
From what I've heard this helps promote growth which might be good for pre-bonsai plants--but not so much for bonsais already at their desired sizes. Is this true?
I don't know where you got that but ...No, If your aimed at bonsai.
IMO [particularly for your conifers] A looser [well aerated] more inorganic soil structure is more resistant to compaction & more conducive to promoting a lot of those little white feeder roots that are so important in having your trees root system be as efficient as possible within the confines of a small pot later in life [and generally for that matter].
Should I keep my trees in nitrogen rich organic soil? They seem to be quite happy in it...
[and they would survive with good care] but IMO- No.
Personally in my bonsai pots [Or training pots also] I don't use a speck of -IE: 'dirt','regular garden soil','potting soil' whatever you want to call it. [Technically on occasion there is some in things I have not had a chance to repot yet.
Nitrogen promotes foliage growth but excessive nitrogen can give you long internode lengths that are inappropriate for bonsai. Besides the issue of excessive water retention [Which you must be cautious of w/ an overly organic mix]. Any nutrients can be provided through a dedicated fert regimen.
EDIT: Oops! I accidentally erased this last bit from my post.
I ask because it seems that the bonsai soil sticky thread seems to contain a wealth of info on soils for bonsai's that are already in their appropriate bonsai containers, but not for trees in pre-training stages.
I will have to re-read [And perhaps edit] that as it is emphatically not
intended to seem that way. Thank you for the heads up on that.
[url=https://www.helpfulgardener.com/forum/viewtopic.php?t=3530&highlight=soil]Here[/url]is another thread that addresses soil.
[url=https://www.walter-pall.de/galerie.htm]Here[/url] is the photo gallery of the trees of a guy who uses entirely 100% inorganic soil all the time.
Glad your researching, Keep posting