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Richard Illig,

Wood ash is one of our oldest fertilizers (as in slash-and-burn).  There are 
twenty-some nutrient elements that plants extract from the soil.  Although 
two macronutrients--nitrogen and sulpher--are volatilized by burning, calcium, 
magnesium, potassium, phosporous and virtually all of the micronutirents 
remain in the ash.  Most of the nutients react as bases, but the reason for 
applying ash to soil has more to do with adding nutrients than adjusting pH. 
Fortunately, these two objectives are in harmony: ash does both.  The reason 
agricultural soil becomes acidic is due to the loss of these bases by crop 
removal and leaching.  One caution: if the wood has ben treated or painted,
you'll obviously want to be careful what gets added to agricultural soil with
the ash.

Bruce Herrick

P.S.  Because both sulphur and nitrogen are present in large quantities in 
organic matter and are volatilized by burning, you'll need to consider 
SOx as well as NOx emissions from the wood-fired boilers.