Professionally, for me, if I can the get the customer or client to mulch
with composted wood chips and leaves at my specs, I consider myself lucky.
If then we desire to go further, for mature trees such as cucumber magnolia,
we would have to recommend a soil application of micro-elements. I would
prefer to fertilize with 0 nitrogen. I call the micro and not minor because
they are anything but minor. if there was hemlocks and the client desired
fertilizing I would recommend testing and using my results as a guide. Like
I said, more research is needed. How you would fertilize a young trees is
very different than how you would want to fertilize a mature trees. I do
not believe a mature tree requires to grow bigger and faster as adding a
10-10-10. Nitrogen in the form it is in 10-10-10 was explained to me as
follows. Imagine the biggest grand finale on the 4th of July (in USA) you
have ever seen and multiply that by 10. Chemistry speaking that is what it
is like when you take a pinch of nitrogen in that form and toss it on the
soil. I am absolutely not the last word on fertilizing.

Sincerely,
John A. Keslick, Jr.
Arborist
http://home.ccil.org/~treeman
and www.treedictionary.com
Beware of so-called tree experts who do not understand tree biology.
Storms, fires, floods, earthquakes, and volcanic eruptions keep reminding us
that we are not the boss.