I think it is important to know what is in your charcoal and what type of soil you have before you add it to your garden/plants. Charcoal ashes are really basic and can really screw up the pH of the soil, kind of like dumping a big pile of lime on the garden.
Also alot of charcoal has lots of semi-toxic substances in it that you might not want in your garden, especially sulfur compounds. These are similar to the stuff found when coal is burned and that contribute to acid rain, not something I want on my garden.
I know that growing up we always used to dump them on the garden with no bad effects, but now that I understand what is in charcoal I just throw them away. Just my opinion though.
I just did a Google search on charcoal ashes and gardening and found that opinions are split about 50/50. Some say never put them on the garden while others say they can be treated just like wood ash and do provide nutrients for the soil. I guess it all comes down to what you are comfortable with.
Tim