It's my understanding that charcoal, ie burnt wood, is basically inert and unusable by plant life.

I was at a soil management talk once, out in a paddock, and we dug a few holes looking at biomes and general soil quality. When we dug up bits of charcoal, the man said nothing has been burnt here for at least 50 years. It's my experience that wood is better at storing water: if interested have a look at hugelculture/ hugelkultur. It's a method that deliberately buries timber for nutrient and water retainment and as it composts it continues to feed the soil nearby.