Wood ash is the inorganic and organic residue that remains after burning untreated wood, bark, sawdust, leaves, and woody debris. The combustion of wood results in 5 to 10% ash on average.
The properties of ash can vary depending on temperature of combustion, and the cleanliness and type of wood fuel. The effect of applying wood ash to forest soils for nutrient enhancement depends on the specific soil characteristics and ash quality.
If good management practices (soil and ash testing) are followed, there are no adverse environmental effects from the land application of wood ash.
Soil amendment: Managed forests can become nutrient deficient over time. Wood ash can be used as a lime substitute and nutrient source for plant growth. Ash is composed of many major and minor plant nutrients. As a good source of calcium, it gives properties similar to agricultural lime. It also provides potassium, phosphorus, and magnesium.
In addition to these macro-nutrients, wood ash contains many micronutrients needed for plant growth such as boron. Heavy metal concentrations are generally low.
Disposal: Ash disposal can incur landfill costs. Application to forests can off-set these disposal costs.
The application rate of wood ash to land is based on the desired soil pH and nutrient requirements of the crop. To determine this rate, soils must be sent for testing by a certified soil testing laboratory.
Ash quality must also be tested prior to application. Testing includes analysis for nutrients, such as nitrogen, phosphorus, potassium, calcium, and magnesium, as well as lime equivalency. Testing should also include an analysis for heavy metals: arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc.
Once the nutrient contribution of the wood ash is known then fertiliser use can be reduced where appropriate.
Useful links for using wood ash as a soil amendment and fertiliser: