The Use of Ash in Landscaping

Soil Nutrients

• Wood ash contains many nutrients that plants need to grow, including calcium, nitrogen, potassium, magnesium, aluminum, iron, phosphorous and sodium. Adding wood ash to soil or compost provides more nutrients for plants to absorb, making it similar to a fertilizer. In general, ash from hardwoods contains more nutrients than ash from softwoods. For example, Oregon State University recommends using the ash from a cord of oak to meet the potassium requirements of a 60-by-70-foot garden. Ash from a cord of douglas fir, however, provides enough potassium only for a 30-by-30-foot garden.

pH Adjustments

• Ash, like lime and other additives, can raise the pH of soil, making it less acidic. In areas with naturally acidic soils, lime or wood ash are necessary to make soils hospitable for many plants. Before applying wood ash to a garden area, test the soil pH with an at-home test kit or by sending a soil sample to a lab for testing. Research what's the best soil pH for the plants that will go in the garden space, and add ash only if the plants need a higher pH than the soil's current acidity level.

Ash and Urine

• Research suggests that ash mixed with human urine increases crop yields. The urine adds nitrogen, while the ash adds other nutrients and reduces soil acidity. This homemade soil additive is more sustainable and natural than commercial mineral fertilizers, and the urine doesn't pose any sort of health risk for crop consumers. Some types of animal urine, such as dog urine, can damage lawns and other plants by changing the soil pH.

Warnings

• Some soils may not have acidic pH levels, and ash can raise the pH of these alkaline soils too much for plant health. Ash can also upset crops that like acidic soils, such as azaleas. Do not use ash on soils with a pH of 7 or higher, and do not use it in soils that already have high potassium levels. Wear respiratory masks and gloves while applying ash to the garden, because it can irritate the skin and respiratory system. Keep ash out of streams, rivers and groundwater, because it can pollute them.

Ash Applications

• The Oregon State University Extension recommends applying ash by coating the surface of the soil with a layer of ash and then raking the area. The extension's website suggests using 1/2 to 1 pound of ash per year per shrub or rose bush. A 1,000-square-foot lawn can tolerate 10 to 15 pounds of ash at the most. To add ash to compost, sprinkle a layer of ash on each new layer of compost material. To prevent slugs and other insect pests, sprinkle some ash around plant bases.