John Fech, a horticulturist at the University of Nebraska, replies: There are numerous pros and cons to the use of wood or charcoal ash. First, it’s worth noting a few facts about ash. This finely powdered material contains roughly 1.5 percent phosphorus, 7 percent potassium, and no nitrogen. The calcium carbonate content, also known as lime, is usually about 21 percent. Adding lime to soil raises the pH, making the soil more alkaline.
Whether it’s worthwhile to use ash depends largely on the characteristics of your soil. A basic soil test by a reputable independent lab would indicate soil pH, organic matter content, and levels of nitrogen, phosphorus, and potassium. If the soil-test report indicates a need to raise the soil pH, or that the soil is deficient in potassium, then adding ash may be an inexpensive way to amend it. In most cases, ash is free, either generated in your own home or by a neighbor or friend who wants to discard it. When amending soil with ash, make sure it is thoroughly mixed in.
If the soil test indicates a pH of 6.5 or above, then there is no need for lime. In general, soils in the eastern third of the United States tend to require the addition of lime, Midwestern soils already have plenty, and Western soils can be variable.
Because of its finely powdered nature, ash tends to slow down the rate at which water moves downward. If the soil drains excessively well, such as at a riverside property, then adding ash could be beneficial. However, the potential for leaching of the potassium into the ground water is relatively high, since such soil tends to have poor nutrient-holding capacity. If your soil does not drain adequately, adding ash will further inhibit the process. In that case, coarse organic materials, such as leaf mold and compost, make better soil amendments than fireplace or grill ash.
As with most fertilizers, be careful not to use ash near germinating seeds. The intense concentration of nutrients, along with the higher pH level, tends to discourage the germination of most seeds or to stunt the growth of seedlings. Also avoid using ash near acid-loving plants such as blueberries, hollies, rhododendrons, yews, and mountain laurels.
As a final note, coal ash is generally not recommended for gardens because it does not offer any significant value.