I have no knowledge of botany or horticulture but logic says that when a
tree or plant grows it draws carbon, oxygen, hygrogen and nitrogen from
the
air. But the air does not contain any of the other trace elements
needed.
So these must be drawn up from the soil.

When you have a bonfire you are returning the carbon, oxygen, hydrogen
and
possibly nitrogen to the air as carbon dioxide and water vapour, but not
the
trace elements. So your ash consists of some unburnt carbon (as
charcoal)
but the rest is all the trace elements that came from the soil. So
really
it must be returned, otherwise you are depleting the soil.

Is that right?
____________________________
Quite correct though you have over estimated the gases.
Best Wishes Brian.

What happens to the nitrogen Brian. You sound as if you know about this.
Plants contain nitrogen and although you can supply it in a fertiliser, in
the wild it must be absorbed from the air (I presume), but what happens when
you burn it?

On a slightly different slant, there was a survey done at the Massachusetts
Institute of Technology (the cream of american scientists) a few years ago
and only about 20% of the students were aware the the vast bulk of a tree
came from the air. Most thought that it was all sucked out of the soil
through the roots.

Peter