I just found this....

Source:
http://hearth.com/what/ashremoval.html

Differences in Ashes

If the ashes are from wood, they make an excellent addition for
gardens because they are high in potassium, calcium, sodium,
magnesium, and phosphorus. In areas where there is high rainfall, the
soils there are typically high in acid. Wood ash can actually help
decrease that acid level. Dry areas, however, tend to be more alkaline
in nature. Wood ash in dry areas can exacerbate that problem. Since
regions will vary, it's best to have your soil tested through a local
extension agent or garden center to see if wood ash can be helpful to
your soil prior to applying.

If the ashes are from burning coal, then you must be more aware of
proper removal for many reasons. First, you will have much more ash to
remove versus wood. Second, coal ash is not beneficial to a garden
because its potassium and phosphorus content are very low compared to
wood. Third, coal ash also contains a great deal of other elements,
such as cobalt, boron, and arsenic among others, which are toxic to
plants, animals, and people. You must be careful when burning coal
because ash can trap unburned carbon which means that more unburned
fuel is tossed away with coal ash versus wood. What else can you do
with the ashes once removed from the appliance? In more rural areas,
people will place them on their driveway to help overcome a slick
surface during winter. With coal ashes, you may do the same, but coal
ashes can affect springtime vegetation as it washes away during the
late winter. After a period of time when you feel that there are no
longer any potentially "live" coals, you should bag and dispose of
coal ashes at a transfer station or the local landfill.

---pete---