I suppose earthy could work (here), but I want to know if there's one particular word in the English language that specifically pertains to the unique smell of leaves in the fall. The best answer on this Yahoo Answers page also proposed dank or musty but those don't quite fit either. Any help?
I suggest peaty, which refers to the partially decayed vegetation called peat.
It's not easy to find a good definition for peaty, and many a whisky drinker might confuse this with smoky because some whiskies that taste of peat were also prepared with smoke (sometimes from burning peat). So, by the way, smoky would be entirely different.
From a whisky drinker's perspective, I'll offer this explanation, given by Matt2 at whiskymag.com:
I remember someone saying peat is more of the damp vegetation, fallen leafs in Autumn, sweetish, try smelling compost or other decomposing plant matter. Got a book somewhere that explained it quite well, will try and dig it out for you..
Reading other posts there might give you further insight into the nuances of these kinds of aromas, but they might also drive you to drink.
(By the way, it looks like Matt2 never came back with his book.)
Woodsy might possibly be used:
of, or characteristic or suggestive of, the woods: a woodsy fragrance.
Or a word that might suggest the smell of leaves in the autumn is "autumnal" meaning "belonging to or suggestive of autumn; produced or gathered in autumn." E.g., from An End to Autumn by Iain Crichton Smith:
She liked a number of teachers in the school, especially the remote competent Miss Hales, whose asceptic perfume seemed to combine with it a nostalgic autumnal smell as of leaves.
Mingled with the autumnal smell of leaves, the gravestones and faded flowers breathed forgiveness, melancholy and peace.
Since the word 'humusy' doesn't exist (and is unwieldy), I would venture to use
humusque or humusk
humus - The organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms.