Answer: Wood ashes are a traditional source of potash and some phosphorus, but the nutrient value varies based on the type of wood burned. They also tend to leach very quickly which means they can burn tender plants. Another concern is that if you add too much, the soil can become too alkaline for some plants. You want your soil pH to be around 6-6.5 for most crops. Finally, if other waste was burned along with the wood, there maybe toxic contaminants in the ashes. Charcoal is nothing more than processed hardwood, and can be thought of in the same terms as wood ashes, but if it is the type that was impregnated with starter fluid, I would be concerned about putting it in my soil.
If you opt to use them, you could use them as a side dressing (keeping them several inches away from seedlings or newly planted plants) at a rate of about 5 pounds per 100 square feet; as a rough guide, hardwood ashes would be about 10 percent potash. Sprinkle them lightly or push them through a sieve to achieve even coverage. Reduce the use of other sources of potash accordingly.
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