Does Wood Ash Kill Grass?

Soil Changes

• Wood ash makes specific changes to the soil in your lawn. The calcium in the wood ash raises soil pH, making it more alkaline. Wood ash also adds potassium to the soil, a key ingredient in helping your lawn stay healthy. It changes soil composition, making it hold water better -- this can benefit lawns grown in sandy soils, but it can harm lawns where the soil is mostly clay.

Soil Test

• Before spreading wood ash over your grass, test the soil to determine the pH and potassium levels. Garden supply stores often sell home test kits, and many states have cooperative extensions that can test your soil, often for a small fee. Wood ash will harm, or even kill, your grass if your soil already has a high pH -- optimum pH is between 6 and 7 -- or high potassium levels. Apply wood ash only if the soil test confirms that you need to raise both pH and potassium.

Safe Amounts

• Applying too much wood ash can burn or kill your grass, which means that proper application amounts are key. Adding wood ash once a year typically is sufficient. Keep the amounts fairly low, about 10 to 15 pounds per 1,000 square feet of lawn, to ensure that you don't overdo it. Test the soil each year before adding more wood ash; you might need to lower the amount or skip a year of wood ash application based on the results of the tests.

Products to Avoid

• Other additives provide similar benefits as wood ash; so, don't use wood ash and the other components at around the same time. If you spread limestone two months ago to raise the soil's pH, don't use wood ash, even if a soil test is still showing a low pH -- limestone can take up to six months to work, while wood ash works more quickly. Wood ash is more water soluble than limestone, which means that it makes changes to the soil faster. Using both products can raise your pH levels too high to sustain grass. Or, if you recently spread wood ash, be careful about adding other fertilizer. Wood ash isn't a good source of nitrogen or phosphorus; so, you might need to apply a fertilizer containing those ingredients. Stay away from those with high potassium levels -- it's best to find fertilizers with no or low potassium concentrations.