Using ashes as a lawn amendment is a practice passed down through generations of gardeners. In lawn care, “ashes” almost always refers to wood ash, the powdery residue remaining after burning untreated wood in a fireplace or woodstove. Many believe this byproduct acts as a fertilizer, helping grass grow greener and thicker. Whether wood ash benefits a lawn depends entirely on its chemical properties and the specific condition of the soil it is applied to.
The Chemical Composition of Wood Ash
Wood ash represents the mineral content of the original wood, concentrated into a fine powder. It includes several plant nutrients, primarily Potassium (K) and Calcium (Ca). Potassium, often called potash, is important for grass strength, aiding in water regulation, disease resistance, and root development.
Ash also contains significant amounts of Calcium (10 to 25 percent of its mass), along with smaller quantities of Magnesium (Mg) and Phosphorus (P). These nutrients give wood ash a fertilizer value, sometimes approximated as 0-1-3 to 0-2-10 in N-P-K ratios. Since the burning process causes Nitrogen (N) and Sulfur to escape as gasses, wood ash contains virtually no Nitrogen, making it an incomplete fertilizer.
How Ash Interacts with Lawn Soil
The most significant effect of wood ash comes from its high concentration of alkaline Calcium compounds, primarily calcium carbonate and calcium oxide. Wood ash has a high pH, typically ranging between 10.0 and 12.0. When applied to soil, the ash acts as a liming agent, neutralizing acidity and raising the soil’s pH level.
This liming effect is beneficial because most turfgrasses thrive in a slightly acidic to neutral pH range (6.0 to 6.8). If soil is too acidic (below pH 6.0), nutrient availability is reduced, and grass health suffers. Applying wood ash quickly corrects this acidity, improving the environment for nutrient uptake and microbial activity.
However, the rapid change in pH can be problematic. If the soil is already neutral or alkaline (pH 7.0 or higher), adding wood ash will raise the pH further. When soil becomes overly alkaline, nutrients like Phosphorus, Iron, and Manganese become chemically bound, making them unavailable to the grass roots. Therefore, wood ash is only helpful if a soil test confirms the lawn needs liming; otherwise, it can be detrimental.
Practical Application Guidelines for Lawns
Before application, conduct a soil test to determine the current pH level and nutrient deficiencies. Soil testing confirms if the soil is acidic and requires the potassium and calcium present in wood ash. Without this step, applying ash is a gamble that could harm the grass by making the soil too alkaline.
Once a need is confirmed, the ash should be prepared by sifting it to remove large, unburned chunks that could cause clumping and scorching. The recommended application rate is conservative: typically no more than 10 to 15 pounds of wood ash per 1,000 square feet. Applying more than this can quickly over-alkalinize the soil and create toxicity issues.
The best time to apply wood ash is during the dormant season, such as late fall or early spring. This allows the material time to break down before the grass is actively growing. Spread the ash as evenly as possible using a fertilizer spreader or by hand on a calm day to prevent wind dispersal. Immediately watering the lawn helps dissolve the ash and move it into the soil profile, preventing the caustic, salt-containing material from burning the grass blades.
Ashes to Avoid and Potential Risks
Not all ashes are safe to use on a lawn, and certain types must be avoided due to the risk of toxic contamination.
Ashes to Avoid
Ash from wood that has been painted, stained, or pressure-treated contains chemicals and heavy metals like arsenic, which can poison the soil. Similarly, ash from charcoal briquettes should not be used because they often contain petroleum products, fillers, or chemical additives. Coal ash must also be strictly avoided, as it contains significantly higher concentrations of heavy metals compared to wood ash, including lead, cadmium, and mercury.
Potential Risks of Wood Ash
Even safe wood ash poses risks if misused, primarily due to its high salt content and alkalinity. Applying too much ash, or leaving it in concentrated clumps, can cause chemical burn on the grass, known as “scorch.” This condition causes the grass to turn brown or yellow. This burning effect results from the high salt content drawing moisture out of the plant tissue, similar to excessive fertilizer application.