I've been reading lately about the incredibly fascinating Terra Preta ("Black Earth") in the Amazonian rainforest. Most tropical rainforest soils are pale, nurtient poor, prone to leaching, and virtually impossible to sustainably farm. The Terra Preta, on the other hand, are dark, nutrient rich, resist leaching, and produce high yields year after year. Here is a picture of the two soil types from wikipedia:
I should emphasize that no one has yet conclusively shown that biochar (charcoal) is THE factor in creating these soils, but a number of studies (nicely reviewed here by Warnock et al) support the hypothesis that adding charcoal to the soil does dramatically increase populations of beneficial soil microorganisms.
So, I'm thinking of trying adding charcoal to some of my beds this spring, and seeing if I can tell a difference. There are lots of sites with information on making your own charcoal, but it seems regular charcoal (chunks, not briquettes, which have added chemicals to glue them together) should work just fine. So we'll see! I'm not expecting my soil to instantly morph into super-fertile terra preta, but it should be fun to play with.