Minerals in ashes

I was wondering if anybody knows what are the main chemical components left
after you burn wood, or charcoal?  I know it's fairly caustic.  What I was
wondering was, if we removed all the soluble components from the ashes, if what
remained would be a reasonable source of some trace minerals for plants in
particular magnesium (oxides of magnesium).  The ashes from fires contain many
valuable minerals for plants but in tropical regions, I think they are then
subject to erosion and loss through run off water.  The tropical jungle
ecosystem is very rich but is easily destroyed when men attempt to domesticate
the land.  I struck me that the composition of minerals left in plant ash should
be just about the correct proportion needed by those plants.  Comments?

 - Steve