Re: When a log is burned, why does it turn into ash?

Ash is the residue left after a fire has consumed all the organic (carbon- based) materials. Wood is composed mostly of cellulose and lignin. When completely burned, these compounds are oxidized to carbon dioxide and water, both of which leave the fire as gases. Small amounts of nitrogen and sulfur are also present in wood, but these, too, have gaseous oxidation products.

However, the wood also contains small amounts of minerals, which do not combine with oxygen to make gaseous compounds. These inorganic compounds compose the ash. Wood ash is a source of potassium, calcium, magnesium, and other trace minerals. They are found as carbonates or oxides. The composition varies with species and location.

Here’s an Internet article on using wood ash as fertilizer from the extension office at Oregon State University.

This article from Clemson University includes a table showing typical composition of wood ash.

And finally, here’s a "pdf" (requires Adobe Acrobat Reader) file with more information than you probably want about wood ash! While it’s highly technical, page 9 of the file has a table giving wood ash composition by species.