"Doug Kanter" wrote:

Important: I'm asking about this NOT because I WANT to use charcoal ashes in
my garden, but because as late as last September, the previous owner dumped
them in a couple of spots which would've been perfect for plants, either
ornamental or edible. I have other location options, but I'm still curious:

1) Is the problem something innate in the ashes themselves, or is it because
of the possibility that starter fluid residue may still exist?

Kingsford briquettes contain limestone dust, which may cause pH problems
for the plants.

Fluid won't be a problem, unless it was poured on the soil.