CORVALLIS, Ore. — When winter ends, many Oregon homeowners are left with piles of fireplace ash. Instead of tossing it in the trash, ash can be applied to soil — provided it is used thoughtfully.
Because wood ash comes from plant material, it contains many of the 16 essential nutrients plants need for growth, according to Linda Brewer, a soil scientist in Oregon State’s Department of Horticulture in the College of Agricultural Sciences.
Wood ash is also “a minor source of soluble phosphorus and potassium,” both of which are essential plant nutrients, Brewer said.
When wood burns, nitrogen and sulfur are lost as gas. But calcium, potassium, magnesium and other trace elements remain. Carbonates and oxides in wood ash also act as liming agents, which can raise soil pH and help neutralize acidic soils, Brewer said.
“As with all things, however, the dose makes the poison,” Brewer said.
Where wood ash can help
Wood ash is most useful on acidic soils that are also low in potassium, Brewer said.
She said ash can also be applied to lawns much as lime is used to help maintain adequate soil pH. The larger area typical of lawns helps prevent many of the problems that can occur with repeated applications in a small garden.
Before applying wood ash, Brewer recommends testing soil pH. This may be done as part of a comprehensive laboratory analysis, a well-calibrated pH meter or a home kit from a garden center or home improvement store.
If you submit a soil sample to an analytic lab, be sure to request an agronomic recommendation.
How to apply ash to soil
Brewer recommends the following steps to use wood ash safely:
- Sift first. Remove large chunks before applying. Brewer suggests using a scrap of hardware cloth with 1/4-inch to 1/2-inch openings. Window screening is too fine.
- Stick to a yearly limit. Apply no more than 10 pounds per 100 square feet per year, Brewer said. Keeping records of ash application (date, place, amount) can help you stick to that limit.
- Plan before planting. Apply ash about two weeks before planting, Brewer said.
- Skip winter applications. The forms of phosphorus and potassium found in wood ash are highly soluble in water and may wash away before actively growing plants can take them up, according to Brewer.
- If you apply ash annually, check pH, sampling the soil about the same week each year.
- Plan for excess ash. If wood is your only heat source, Brewer said, you can easily generate more ash in a heating season than a typical home garden can absorb.
When to avoid using wood ash
Brewer advises against using wood ash:
- On acid-loving plants such as blueberries, rhododendrons, ferns and azaleas.
- If soil pH is above 7.0.
- At the time of seeding, since salts in ash can burn tender seedling roots.
Safety precautions
Because wood ash is alkaline, Brewer said it should be handled with the same care as other strong alkaline materials, such as household bleach or lye.
She strongly recommends:
- Wearing eye protection, gloves, a dust mask and closed toed shoes with socks.
- Applying ash only to moist soil and raking it in lightly.
- Avoiding application on windy days.
- Never using ash generated by burning trash, cardboard, coal, barbecue charcoal or pressure-treated, painted or stained wood, as these contain harmful substances.
Avoid overapplication
Too much ash can raise soil pH to levels that interfere with plant growth, Brewer said. Repeated heavy applications in the same spot — such as using one corner of the yard as an ash dump — can effectively sterilize soil and threaten surface water quality.
Avoid leaving ash in piles. Concentrated salts can leach into the soil and damage plants.
Avoid mixing ash into the compost pile unless you keep records of ash addition. Typically, compost is a suitable mulch for any gardening situation, but if ash has been applied, it can damage sensitive seedlings and acid loving plants.
Previously titled Wood ash can benefit gardens, with precautions
