Wood ash contains 1-2% phosphorus, 7-10% potassium, plus micronutrients such as iron, manganese, boron, copper, and zinc. The largest component of ash is calcium carbonate, making it useful as a liming agent to neutralize acidic soil. The book recommendation is to apply 5-10 pounds of ash per 100 square feet of garden. For lawns the recommendation is 10-15 pounds per thousand square feet. Care should be taken not to apply too much, as this could result in soil that is too alkaline for proper plant growth. Soil testing is a good idea if you’re unsure. Be sure that freshly spread ashes don’t come in contact with germinating seeds or sensitive new plant roots. Avoid using wood ash around blueberries or other acid-loving plants.
If you store some ash until spring, it can be used for insect control. Spreading a circle of wood ash around the base of plants has been used as a deterrent for maggots, cutworms, cucumber beetles, squash borers, spider mites, potato bugs, and slugs. If you store ash for later use, protect it from rain prevent nutrients from leaching away. Important note: ALWAYS let wood ash cool before storing or disposal. Hot wood ashes have caused many a wildfire and house fire. They can hold heat and embers much longer than is realized. To play it safe you should wear gloves and protective eye wear when handling ash, as it is an alkaline and could irritate the skin and eyes.
Historically ash was used by early pioneers to make lye by leaching it with water. Lye is a key ingredient to make soap. If you want to try your hand at making soap from scratch, there’s plenty of information on the internet.