This is an interesting discussion!
I've worked in the electric generation industry for a few years and spent most of that time in a coal plant in central PA. From my own personal experience studying coal ash to increase plant performance, I would not
say that it makes a good companion to two slices of bread and some grape jelly. And as far as it being similar to soil, I've never worked in my garden wearing a resperator and full body suit plus obey the signs that say "absolutely no eating, drinking, or smoking". In the plant the areas exposed to ash are called "regulated areas".
The stuff is actually pretty nasty! The levels of arsenic are highly carcinogenic (that means the stuff causes cancer). But I would actually be much more concerned about the mercury and other heavy metals. Check out OSHA's website to find out how much this stuff is regulated to keep workers safe.
As far as the discussions about beneficial use of ashes from power plants, the standards are pretty stringent. In the plant where I worked the stuff was way too acidic (that's right, lower than pH5.5) which is the problem of most coal ash. Where coal ash is used for mine reclamation it is mixed with very high concentrations of lime (like 25+%) to raise the pH to acceptable levels. We could not do any of this so we did landfill the stuff. We spent millions of dollars installing liners and catching the rain water and run-off from the big pile of ash (we didn't just dump it on the ground in a pile). In the old days ash was used for all the reasons previously listed in this thread. It was thrown around like it was
dirt. This is part of the reason PA has so many dead rivers and streams. Ahhhh, but those were the "good old days" of pcb's and asbestos too. We've learned a lot since then.
Beneficial use such as concrete and flowable fill are solids, I don't think there's much risk if the stuff isn't airborne or can't be absorbed into the skin.
Speaking of clinkers, I've fired thousands of rounds of 12ga shotgun to shoot the clinkers out of our boilers. Some clinkers I've seen were the size of two (2) school busses and had to be blasted out with dynamite. This is what happens when the fire in the boiler is too hot and the ashes "melt" and fuse together. We tested the coal to find its ash fusion temperature (suppliers had limits they had to meet).
Even still... I'd dump it on my driveway... I just wouldn't breath it in.