When to Use Ashes as Fertilizer

Chemical Composition

• Wood ash is derived from burning plant materials such as wood, leaves and bark. The chemical composition of wood ash varies, depending on factors such as the type of wood burned and the temperature and cleanliness of the combustion method. According to the University of California, most wood ash consists of approximately 25 percent to 50 percent calcium, 5 percent to 7 percent potassium and 1 percent to 2 percent phosphorous. Wood ash also contains trace amounts of many other nutrients, including iron, manganese, boron, copper and zinc. With an average pH around 10 to 12, wood ash is similar to lime in alkalinity.

Uses

• Wood ash is used to fertilize and increase the pH level of acidic soils. Its fine, powdery consistency is easily assimilated into the soil. Wood ash is best used on soils with a pH level of approximately 5 to 6.5. Before deciding whether to use wood ash as a soil amendment, perform a soil test to determine the pH and nutritional content of your soil. Wood ash can be spread around the base of plants to repel slugs and snails or mixed into compost piles to control acidity and assist in the decomposition process. It is sometimes spread on lawns in place of lime.

Contraindications

• When pH levels are high, nutrients such as phosphorus, iron and potassium become chemically bonded to the soil, making them more difficult for most plants to absorb. Therefore, highly alkaline soil can stunt growth or kill many plants. Wood ash should never be used on soils with a pH level higher than 7. It should not be used on acid-loving plants, particularly potatoes, as alkaline soil can increase the possibility of potato scab.

Application

• Wood ash is generally applied to soil at the rate of about 5 to 10 pounds of ash for every 100 square feet. Rake it into soil evenly. You must be careful not to overapply wood ash as this can lead to a buildup of salt in the soil. Wood ash should be applied to recently tilled soil at least one month before planting. Because heavy rain causes ashes to lose nutritional value, application in the fall when soil is drier is sometimes recommended. Wood ash can burn tender seedlings, but it may be used around mature plants during the growing season.

Precautions

• Because wood ash is alkaline, care should be taken to prevent contact with the eyes, nose, mouth or skin. It should never be applied on windy days, and protective glasses, long sleeves, gloves and a breathing mask should be worn. Never use ash obtained from burning paper, cardboard, charcoal or painted or pressure-treated wood, as these materials may contain harmful chemicals.