Answer: Two authoritative texts in our library claim that coal ash can contain potassium, but it also has sulfur and iron in levels that can harm plants. Though both sulfur and iron are plant nutrients, too much of a good thing can be toxic. Coal from different sources has different amounts of these materials, so the rule isn't hard and fast, but there are other, less risky methods for adding potassium and improving clay soil. If you aren't sure how to dispose of your coal ashes otherwise, contact the state or district solid waste disposal office.
If you're looking for ways to add nutrients to your soil, compost, animal manures, yard wastes and green manures (cover crops that are tilled into the soil) are the best bet. It would surprise me if potassium is lacking in your soil, since one of recommended fertilizers is granite dust! Maybe that gravel does have a purpose! I hope this information helps settle the question.
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