Burning wood over the winter can produce a significant amount of ashes. An average cord yields about 20 pounds of ash, equivalent to one five-gallon bucket. If you burn wood frequently, a lot of ashes will accumulate.
Spreading the ashes on your lawn or garden can add essential nutrients but may not always be the best disposal method, as excessive ash can raise soil pH too high and harm plants.
Benefits | Disadvantages | How to Apply | Safety Concerns | Using in Compost Pile | More Information
Benefits of Wood Ashes on the Garden
Wood ash can benefit the soil because it contains essential plant nutrients. Depending on the type of wood, the ash may contain 5 to 8% potash (potassium), up to 20% calcium, about 1% phosphate (phosphorous), and trace amounts of micro-nutrients such as iron, manganese, boron, copper, and zinc. Its approximate fertilizer analysis is 0-1-3 (N-P-K), varying with the wood type.
Ashes are effective at raising soil pH and can substitute lime, but their calcium carbonate equivalent (CCE) varies from 25 to 59%. The CCE measures how effective a product is at raising soil pH. Lime has a CCE of 90-95%. To match lime's effect, you may need 2-4 times more ash. When the CCE of wood ash is not known (a frequent occurrence for home gardeners), it is best to estimate it at about 50%.
Disadvantages of Wood Ashes on the Garden
Iowa soils tend to have high pH, so applying wood ash can raise it too high. Wood ash also has a higher water solubility and changes the soil pH more quickly than lime.
Farmers add lime to raise the pH in fields all the time. Why is using wood ash (which can be a lime substitute) a problem? Farmers raise soil pH in corn fields due to acidic (low pH) soils caused by fertilizers and other amendments. Garden soils are not managed this way and rarely need their pH raised.
Wood ashes should not be applied around acid-loving plants like azaleas, rhododendrons, and blueberries. Additionally, avoid applying too much around vegetables as most prefer a slightly acidic soil and the amount of potato scab on potatoes is significantly increased in higher pH soils.
Wood ash can contain trace levels of heavy metals like lead, but levels are low and usually not of concern, especially when spread out over a large area. If concerned, conduct a soil test.
How to Apply Wood Ashes to the Garden
A soil test is the best way to determine whether you should apply wood ashes to garden soil. Avoid applying wood ashes if a soil test reveals a pH above 7.0.
An annual application of no more than 20 pounds (~ 1 five-gallon bucket) sprinkled over 1,000 square feet of the lawn or garden in the winter or early spring is safe for most plants in the landscape and garden.
Spread ashes evenly on a calm day. If possible, apply to moist soil and incorporate into the top 2 to 4 inches. Avoid heavy watering to prevent nutrient leaching.
Avoid spreading ashes around germinating seedlings. The salts present in wood ashes negatively affect germination.
Safety Considerations
Wood ashes can be caustic, so avoid contact with the skin. Protect yourself by wearing long sleeves and pants, gloves, eye protection, and a mask to prevent any issues.
Never mix wood ashes with other fertilizers. The ammonium in some nitrogen fertilizers (urea, ammonium nitrate, ammonium sulfate) can produce ammonia gas when mixed with high-pH materials like wood ashes.
Never use wood ashes from the burning of treated or painted wood, and never use ashes from the burning of other materials (such as garbage) as they are more likely to contain toxic materials and/or higher levels of heavy metals.
Avoid stockpiling ashes on the ground, as high concentrations can effectively sterilize the underlying soil and adversely affect water quality. Store wood ashes in a container with a lid. Excess moisture can cause the ashes to cake-up and become difficult to spread.
Using Ashes in the Compost Pile
Add wood ashes sparingly to compost piles. No more than 5% of the total volume should be ashes, as high pH can harm compost microorganisms. Less is better to avoid disrupting the balance of the compost.
More Information
- Fertilizing in the Home Garden
- Using Manure in the Home Garden
- Natural (Organic) Fertilization for Turf (PDF)
- Garden Soil Management (PDF)
- Composting Yard Waste (PDF)
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