"Wood ashes have about one percent phosphate and less than 10 percent potassium, but no nitrogen. They also contain about 25 percent calcium carbonate, a common liming material," said Gaylord Moore, horticulture specialist, University of Missouri Outreach and Extension.
Because wood ash has a fine particle size, it reacts rapidly and blends completely into the soil. Although small amounts of nutrients are applied with wood ash, the main effect is that of a liming agent.
It is important when applying ashes to spread them evenly and avoid dumping them in one area. It is also a good idea to know your soil pH before adding the ashes (get a soil test).
Soils that are slightly acidic (pH 6.0 to 6.5), should not be harmed if 30 pounds of ashes per 1,000 square feet of garden area are applied. Moore also recommends working ashes into the upper six inches of the soil.
"If your soil pH is 7.0 or higher, find another way of disposing of the ashes," said Moore. "It is also important to never apply wood ash to acid loving plants like potatoes, rhododendrons, azaleas or blueberries."