Wood ash can be a great addition to garden soil in many cases. When used correctly, wood ash provides nutrients that benefit plants and helps improve the soil structure. However, there are also some potential downsides to using wood ash that need to be considered. In this article, we’ll explore the pros and cons of using wood ash in the garden so you can make an informed decision about whether it’s right for your soil.
What is Wood Ash?
Wood ash is the powdery residue left over after wood burns. It contains minerals like calcium, potassium, and magnesium that were inside the original wood. Small amounts of phosphorus, iron, manganese, copper, zinc and boron are also present.
The exact composition of wood ash depends on the type of wood burned. Hardwoods like oak and maple contain more calcium and magnesium, while softwoods like pine and fir are higher in potassium. Bark ash tends to be higher in manganese and zinc.
In general, wood ash increases soil alkalinity and provides potassium, phosphorus, and calcium. It’s often compared to agricultural lime as an amendment that raises pH, but wood ash also has fertility benefits lime lacks.
Benefits of Wood Ash for Soil
Here are some of the top benefits of using wood ash in your garden soil:
1. Raises pH of Acidic Soils
Wood ash is highly alkaline, with a pH commonly between 9 and 13. When mixed into acidic soils, it can help raise the pH closer to the ideal range of 6.0-7.0 for most plants.
This neutralizing ability makes wood ash helpful for gardens with chronically low pH. It provides similar pH benefits as agricultural lime, though it acts more quickly since the minerals are already broken down into a readily available form.
2. Provides Potassium and Other Nutrients
Most soils can benefit from more potassium, an essential nutrient for plant growth and health. Wood ash contains a soluble form of potassium that plants can readily absorb.
In addition to potassium, wood ash supplies calcium, magnesium and trace nutrients. These nutrients become available over a longer period, as rain helps leach them from the ash.
Nutrients in wood ash are fast acting and long lasting, making just one or two light applications a year beneficial.
3. Improves Soil Structure and Texture
Wood ash contains calcium and magnesium, two cations that help “glue” clay soils together into stable aggregates. This improves soil structure, creating a better environment for plant roots and beneficial soil microbes.
In sandy soils, the minerals in wood ash can help improve water retention and nutrient availability. The carbonates enhance the soil’s ability to hold nutrients like nitrogen and potassium.
4. May Help Deter Pests Like Slugs and Snails
Sprinkling a thin band of wood ash around vulnerable plants can help deter soft-bodied pests like slugs, snails and insects. The abrasive texture of ash deters slugs from crossing over it. Ash also extracts moisture, making the area less hospitable for these moisture-loving creatures.
5. Provides an Alternative to Lime for Raising pH
While lime and wood ash both raise pH and provide calcium, wood ash offers additional nutrients. It’s a helpful alternative for gardeners who want to avoid introducing more lime to their soil.
Wood ash also acts more quickly than lime. Its pH-raising effects will show up in soil test results within weeks, while lime takes several months.
6. A “Free” Soil Amendment
For those burning wood for home heating or cooking, ash is often readily available at no cost. Wood ash from a fireplace or wood stove provides a free, locally-sourced amendment for acid soils.
Ash generated onsite avoids the carbon emissions of trucking in lime or other amendments. It’s a way to recycle waste back into soil health.
7. May Increase Soil Organic Matter
While wood ash itself has no organic matter, research shows it can help boost soil organic carbon levels. The minerals in ash promote soil microbial activity and growth of mycorrhizal fungi.
These beneficial effects on soil biology can increase the overall amount of organic matter held in the soil over time. More stable soil aggregates also protect organic matter from decomposition.
8. Has a Long History of Traditional Use
Gardeners have used wood ash as a soil improver since at least Roman times. Its traditional use speaks to the observation over centuries that modest amounts of ash enhance plant growth.
The right amount – not overdone – provides a slow-release source of potassium, calcium, magnesium, phosphorus and trace nutrients. Ash was traditionally spread in the fall so nutrients would be available for spring root growth.
When is Wood Ash Not Recommended for Soil Use?
While wood ash has many benefits, there are some cases where it’s not advised:
In alkaline soils
Soils with a pH above 7.0 do not need more alkalinity from wood ash. It may push the pH too high and create micronutrient deficiencies in plants.
Near acid-loving plants
For rhododendrons, azaleas, blueberries and ferns, keep wood ash away. These acid-loving plants prefer a lower soil pH. Even small amounts of ash near their roots can stunt growth.
Around seedlings and tender plants
Ash is abrasive and quick acting. Direct contact with concentrated ash can damage tender roots and seedlings. It’s better to apply ash around mature, established plants.
For compost piles
Wood ash makes compost too alkaline, slowing decomposition. Small amounts of ash may be okay in layered compost piles, but avoid throwing large quantities of ash straight into your compost.
As the sole fertilizer
Ash supplies potassium, but little nitrogen and phosphorus. It should not replace a balanced fertilizing program, but instead complement it.
In large, concentrated amounts
Wood ash is often recommended at rates of 1 to 15 pounds per 100 square feet. Too much at once can overwhelm plants with alkalinity and salt content. Moderation is key.
Where sodium content is a concern
Ash from burned treated lumber or salt-water driftwood contains excess sodium. This sodium can damage soil structure, so avoid using these types of ash. Most wood ash has minimal sodium.
Without testing current soil pH
It’s a good idea to test soil pH before deciding to apply wood ash. Unnecessary alkaline amendments can do more harm than good by disrupting soil biology.
How to Use Wood Ash in the Garden
Wood ash is simple to apply, but there are some best practices to follow:
Only use ash from untreated, unpainted wood.
Ash from construction debris, painted or pressure treated wood may contain toxins. The safest choice is ash from your own fireplace or wood stove burning clean firewood.
Apply lightly and infrequently.
Follow recommended rates of 1-15 lbs per 100 square feet, once every 1-3 years. Heavy applications can overload soil with salts and alkalinity. Moderation is key.
Spread ashes evenly over dry soil.
Then water gently to soak it in. Avoid dumping in piles, or applying over wet soil. Either can “burn” plants with concentrated alkalinity and salts.
Focus on pH deficient areas.
Test soil first and only apply where the pH needs raising. Avoid over-liming already alkaline soils. Targeting specific areas gets the most benefit.
Till or dig in after applying.
Mixing ash into the top few inches of soil reduces pH shock to plants. It also prevents ash from blowing away in windy conditions.
Alternate wood ash with acidic amendments.
Rotate wood ash with acidic options like pine needles, peat moss or sulfur to maintain ideal pH balance. The goal is moderation, not wild pH swings.
Use less when the ash is coarser.
Finely sifted ash has more reactive surface area, so lower rates are advised. Coarser ashes with small pieces of charcoal require more by volume.
Store ash dry if saving some.
Ash absorbs moisture from the air. Damp ash loses its alkalinity and becomes acidic. Keep stored ashes dry in a container with an airtight lid.
Wood Ash Precautions
When using wood ash around plants, it’s smart to take some basic precautions:
- Wear a dust mask and gloves when handling and spreading ash. This prevents irritation to your lungs and skin.
- Avoid inhaling plumes of ash. Dampen ash slightly before spreading if very dry and windy.
- Wash produce grown in ash-amended soil before eating to remove any ash residue. Certain elements if over consumed could be a health concern.
- Prevent ash from entering streams or waterways. The runoff can be alkaline and contain heavy metals that impact aquatic life.
- Use moderate rates as recommended, and retest soil pH every 1-2 years. It’s easy to raise pH too much with excessive wood ash.
- Monitor for leaf burn, especially on new transplants and seedlings. Adjust application rates if too strong.
- Recognize wood ash provides plenty of potassium, but little nitrogen. Supplement with a balanced fertilizer containing nitrogen.
- Be cautious using ash from mixed sources or unknown origins. Utility company fly ash, for example, contains toxins.
Frequently Asked Questions
Below are answers to some common questions about using wood ash in the garden:
Does wood ash increase nitrogen in soil?
No, wood ash does not provide a significant amount of nitrogen. It is highest in potassium, calcium and magnesium. To balance out the potassium from ash, make sure to also fertilize with a nitrogen source like compost or manure.
What’s the difference between wood ash and charcoal ash?
Charcoal ash tends to be higher in potassium content, while wood ash provides a broader range of macronutrients. Charcoal ash also has a more neutral pH, while wood ash is highly alkaline. Otherwise, their soil benefits are fairly similar.
Is ash from burning trash, cardboard, or paper safe for gardens?
No, only use ash from clean, unpainted and untreated wood. Ash from burning plastics, metals, chemicals, or other waste often contains toxins that can contaminate garden soils. Burning cardboard and paper is not recommended either.
Should wood ash touch plant leaves and stems?
Avoid getting wood ash directly on plant tissue. The salts, alkalinity and abrasive texture can damage leaves and tender plant tissues. Light dustings of ash are okay, but prevent heavy coatings on plants.
Is it okay to spread wood ash in the winter?
Yes, spreading wood ash in the late fall or winter allows time for rainfall and snowmelt to soak it in gradually. Winter application prevents burning plants when growing most actively. Just avoid spreading on frozen or snow-covered ground.
How often can I apply wood ash to my vegetable garden?
For garden beds, apply a 1⁄4-1” layer once every 1-3 years. More frequent, lighter applications work well too. For new gardens, start with less – a dusting to 1⁄4” layer – and give plants time to adjust to the mineral content before adding more.
What’s the fastest way to raise soil pH with wood ash?
To increase pH quickly, apply a light dusting over soil and water thoroughly. Repeat weekly for a rapid pH change. For longer-lasting effects at slower pace, till or dig heavier amounts into soil before planting.
Should I wet wood ash before spreading?
You can dampen excessively dry ash slightly to prevent drifting in windy conditions. But avoid spreading wood ash onto wet or even damp soil as it can rapidly raise pH in concentrated areas. It’s best applied to dry soil and watered in afterward.
Conclusion
For most gardens with acidic soil, wood ash is a valuable soil amendment that provides potassium, raises pH and improves texture. It can be a money saver by recycling ash created onsite as a heating byproduct.
Follow recommendations on application rates, and partner wood ash with nitrogen-rich organic matter. Test soil pH periodically, and rotate in acidic amendments to maintain ideal balance. By thoughtfully incorporating wood ash, most garden soils can benefit from this traditional soil improver.