I think burn temp matters and what you add to the fire as to the quality of the ash. I burn plain paper, card board (without tape), and untreated wood. Mostly just wood. It's in a high efficiency wood burner, which means it is fine dust when I am through with it. No plastic would survive. Ash has been used a garden fertilizer for thousands of years. Potash aka potassium garden fertilizer. I use it as a side dressing or around the base of trees. No problems yet. I might watch quantity on blueberries, but we have enough acidifiers here I don't worry.