Since Roman times, wood ash has been recognized as a useful amendment to the soil. In fact, North America exported wood ash to Britain in the 18th century as a fertilizer, and today, 80 per-cent of the ash produced commercially in the Northeastern United States is applied to the land.
Wood stoves and fireplaces are great for warming gardeners' chilly hands and feet. So, what can we do with the ashes? Since wood ash is derived from plant material, it contains most of the 13 essential nutrients the soil must have for good plant growth and health.
When wood burns, nitrogen and sulfur are lost as gases, and calcium, potassium, magnesium and trace element compounds remain. The remaining carbonates and oxides are valuable liming agents, raising pH, thus neutralizing acid soils. Soils that are acid and low in potassium benefit from wood ash. However, acid-loving plants such as blueberries, cranberries, rhododendrons and azaleas would not do well at all with an application of wood ash.
Wood ash has a very fine particle size, so it reacts rapidly and completely in the soil. Although small amounts of nutrients are applied with wood ash, the main effect is that it is a liming agent. The average ash is equivalent to a 0-1-3 (N-P-K)
Do not mix ash with nitrogen fertilizer as ammonium sulfate or ammonium nitrates or urea. These fertilizers lose their nitrogen as ammonia gas when mixed with high pH materials such as wood ash.