On balance it should return minerals and trace elements to the soil. The
potash is alkaline but should be converted to carbonate (pKA 8) then
bicarbonate (pKA5.8 to-6 ) quite quickly by atmospheric water and carbon
dioxide.
Potassium salts are then a good plant food.
I think the answer is to spread it thinly on the surface but don't put it on
acid loving plants.
I usually put it on the compost heap which tends to acidify naturally.
Copper is a catalyst for dioxin formation frrom organic material and
chloride in flames and fires. I would not put wood ash from treated timber
on to edible crops.