I am like Scott, I really don't want to be in a soil war, start one, or to be considered a troll. But, I am also curious like Oso - what purpose would it have and what could one expect it to do? I can see where it would not hurt, just acting as part of the "dirt" the tree grows in. Other than that, I don't see what purpose it would serve.
I see a lot mentioned of it being a filter or that it absorbs certain chemicals. How much charcoal are you adding to your soil to use it as a filter? I would think that it would have to be a significant amount (at least 60% of your soil mass, or more) to have any effect, seeing how it would have to come in direct contact with what you are wanting to filter or absorb. What does it absorb/filter , and, to what extent? Does it absorb good chemicals as well as bad ones, thus taking away from the tree? If it absorbs the chemicals, does it bind them up making them unusable by the tree, or do you have little concentrated nuggets of noxious stuff all throughout your soil, leeching back into it? I expect that if it absorbed anything at all, the effect would be short term, not more that a week or two. There is a finite amount that it can absorb. The cartridges used in respirators (gas-masks) are short lived, and the higher the concentration the shorter the saturation time, until it becomes ineffective.
I assume that the absorption/ filtering characteristics are wished to be applied to impurities in the water. I would be better, more economical, and more efficient (from a purification standpoint) to run your water supply through a charcoal filter before it hits the pot than throwing a handful of charcoal into your soil mixture.
Just Curious,
John