Why are the alkali and alkaline earth metals present in the ashes of burnt wood? The alkali earth metals form the two left-most column of the periodic table of the elements, other than hydrogen. See wikipedia articles:
http://en.wikipedia.org/wiki/Alkali_metal
http://en.wikipedia.org/wiki/Alkaline_earth_metal
Now "alkali" means that these are in a sense the opposite of "acid". One of the ancient uses of alkali materials is in the conversion of corn into hominy (or, for those in the southern US, grits similar to posole, polenta or farina ).
The conversion of corn to hominy is of ancient origin. From a nutrition point of view it is useful because it increases the availability of the lysine and tryptophan proteins in the corn. See http://www.practicallyedible.com/edible.nsf/pages/hominy This helps prevent the nutritional disease pellagra http://en.wikipedia.org/wiki/Pellagra
In ancient times, hominy was manufactured by soaking corn in a mixture of water and ashes. This is equivalent to the modern method, which is to soak the corn in lye.
So my question is this: why is it that ashes are alkali?
 A: Where I live we have a problem with a bushy plant called Tamarix.  This is a nasty invasive plant that takes over everything.  It is also practically indestructible.  It is called salt cedar because it salinates soils.  These voracious bushes have long tap roots which reach the water table and pull up water with alkaline salts.  The plant then secretes most of this into the soil, further damaging ecosystems.  
This is an extreme example, but trees usually tap root into the water table where there are dissolved salts and alkaline metal compounds.  These then accumulate in the xylem.
A: Partial answer: A fire increases the temperature. This separates things which are volatile from things which are non volatile. Examining the periodic table of the elements we see that the base forming oxides are on the far left (i.e. the alkali and alkali earths), while the acid forming oxides are on the far right (i.e. the halogens). So what is happening is that the acid forming atoms are more volatile.
Consequently, the problem reduces to the question, "why is it that the oxides of the alkali and alkali earths metals are non volatile, while the halogens and their oxides are volatile?" This is a question that can be answered completely in physics; it's due to their electron configurations.
