If the charcoal was thoroughly burned, there is not likely to be any
starter residue. The starter material (fluid or solid) is generally
fairly volatile (which is what helps it start the fire), so it would
probably be gone at the beginning of the cooking sequence. After all,
you don't want the starter in the food you're cooking, so you have to
wait for the fire to get really hot. If the fire were put out
prematurely (before it got hot enough to cook anything) there may be
some residue. Look at the ash to see if there is a lot of unburned
charcoal in it.

Wood ash has a high pH, which is sometimes used for amending acid soils,
but charcoal ash generally has a pH a little lower (I think this is due
to the higher temperature that charcoal burns at). Plants need lots of
nutrients, so if there is some soil, compost, etc. mixed with the ash
(so that it's not predominantly charcoal ash), there shouldn't be a
problem growing things. The charcoal is basically made from wood, so
there shouldn't be any toxicity issues. Once again, the charcoal is used
to barbeque things, so you don't want toxic substances in it. Your
garden is one step removed from the cooking process, so the toxicity
problems will be lower there then in the barbequeing.


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?

2) Does the problem relate to toxicity for edibles, or simply that it
creates lousy growing conditions for ALL plants?