Many people burn wood for heat or ambiance in the winter. The use of this heating method has had its peaks and valleys as fuel prices change and the popularity wanes or grows. For whatever reason we use the fireplace, stove, chimney or fire pit, disposal of the ashes is necessary.
Wood ashes are very alkaline and have little nutritional value. As a rule, it is not a good idea to put ashes on your gardens. They can be recycled into the lawn and garden, but only with care and understanding of their effects.
The content of purchased fertilizer is expressed in terms of percent of nitrogen (N), phosphorus (P) and potassium (K). A balanced fertilizer would carry the percentage NPK numbers as 10-10-10 or 15-15-15. Fall applications would be similar to 20-5-1 or 30-10-5, both high in nitrogen. Wood ashes contain a very high amount of potash, which is a source of potassium, and only small amounts of phosphorous and no nitrogen. The NPK analysis of wood ashes would be something like 0-1-8. Potassium is an important nutrient for plant root development; however, Kansas already has an abundance of this nutrient native to the soil. Likewise, phosphorus is generally not deficient in Kansas. Nitrogen is our biggest need, none of which will be provided by the ashes.
Wood ashes are very alkaline, and, with the fine particle size, act as a fast-acting liming material. Lime and ashes raise the pH of the soil. Microorganisms are unable to break down organic material, and some plants find it hard to take up nutrients in high pH soils. Many plants do better in soils with low pH, such as potatoes, blueberries and azaleas. Most plants like a slightly acidic to neutral soil. As little as 10 pounds of wood ashes on a 100-square-foot area could easily move your soil out of a well-tolerated pH range. Soil tests are a great way to determine your soil's pH.
Mulch piles can tend to be acidic - low pH - so addition in moderation to mulch piles would be of benefit in raising the pH. Small mulch piles, in the home garden range, would require only cupfuls and not pounds as might be available. The city even prefers not to have ashes recycled in its mulch pile.
Mixed with water, wood ashes produce lye, which burns plants. This would definitely harm germinating seeds and seedlings.
Spread around the garden edge, thrown up into trees, folklore has it that wood ashes control insects and diseases. This is an exaggeration at best, however; the insects probably don't like to crawl in the gritty texture of the lye. Your best bet is to bag the ashes tightly and discard them with the trash. Be sure the ashes are cold before bagging.