For instance, an acidic soil might benefit, but mixed with an alkaline calcium/lime rich soil it would be detrimental. I remember as a child up north we had, I assume, acidic soils. My mother would save our egg shells for the compost heap, among other things. Don't know if she used wood ash or not. My current soil is alkaline, so I don't think using my wood ash would be helpful.
Other potential uses:
It might be usable in some building materials. Wood chips and fly ash are used in some types of 'green' building materials like Faswall blocks. Here's one site about this:
After World War II amidst the rubble and destruction a way was discovered to take the huge volumes of wood waste, grind it into chips, mineralize the chips to neutralize the natural sugars that cause rot, and bond them to cement to form a building block. This is the genesis of the Faswall™ ICF Wall Form.
Since then, tens of thousands of homes and commercial buildings have been built in Europe and Asia and North America with this remarkable material. This remains one of the preferred methods of building in Europe. It has been available in North America for close to 30 years. In 1987 Hans and Leni Walter of K-X Faswall International Corp. advanced the state of the art with a patented mineralization process that allows virtually any wood or cellulose fiber to be bonded to cement.
The wall form blocks are manufactured in 24" long modular units to aid the designer. They are designed to be under 30 lbs to aid the installer. Simply stack the wall forms on top of one another without mortar. The interlocking end design keeps the wall forms in place. Rebar is set both horizontally and vertically within the stacked forms prior to filling the cores with concrete. This creates a "post and beam" grid effect which makes the wall exceptionally strong. Essentially, Faswall™ is a efficient method of building a reinforced concrete wall with built-in thermal, acoustical and fire protection. Our wall-forms can be used above-grade, below-grade, for commercial or residential applications.http://oikos.com/products/thermal/shelterworks/