mikeandgerry wrote:No benefit to coal ashes in the garden. Could hurt over time.
To clarify: the downside of ashes over time is that the continued application of a non-beneficial mineral simply crowds out the access of the plant to beneficial minerals. The downside is minimal and the ashes are not detrimental in and of themselves except that they are alkaline which may not be good for some vegetation. Toxics are minimal but present in the form of heavy metals and sulphur.
The one good point of the ash is that it may sustain hydration in soil (hold water longer). The fine powdery ash does this, not the chunky stuff. But again, too much ash is not a good thing. Use it sparingly. Best bet is to have your soil analyzed and use only what the soil needs.
If you are mixing soil and ash for making an embankment or something similar, there are engineering standards for percentages used. I cannot recall exactly but once you approach certain levels the embankment becomes dangerously unstable, that is, it can slide or will fail to support a load. The research was done by some Japanese engineering school.