Recycling Wood or Briquette Ash

by Catherine Haug, July 10 , 2011

I have a small wood stove and an old fashioned briquette barbecue, both of which generate ash. In the past I added this to my heavy-duty yard waste (twigs, branches and fir cones) to become part of the compost. But is this the best thing to do? Can ash be added to regular compost? Are there undesirable contaminants in briquette ash? What about ash from burning paper?

As a chemist, I understand that ash is high in mineral content, and as such, is quite alkaline. In fact, wood ash mixed with water produces lye (sodium hydroxide, NaOH) or potash (potassium hydroxide, KOH) which are used for soap making (and also to preserve cod fish as lutefisk, but I digress). If this is made a part of garden compost, can it be bad for the flowers and veggies?

Topics discussed in this article:

  • wood ash in compost and garden;
  • biochar;
  • other uses for wood ash;
  • briquette ash.

Wood ash in the garden or compost

I asked several local compost experts, and also did some research on the web. The answer, it seems, is yes and no to all questions.

What’s in wood ash?

From Clemson Univ.: Wood Ash as an Agricultural Soil Amendment (4), wood ash is about 0-1-3 (N-P-K) and contains:

  • Calcium (Ca) is the most abundant mineral, giving wood ash properties similar to lime
  • Potassium (K)
  • Phosphorus (P)
  • Magnesium (Mg)
  • Aluminum (Al)
  • Micronutrients needed in trace amounts for adequate plant growth.

Wood ash may also contain a few heavy metals that pose environmental problems, but these are typically low, and not readily extractable or available.(4)

[NOTE: Clemson Univ. is in Georgia, where the soil tends to be acidic, a condition that may not be the case in your Flathead garden, as Jeffrey points out, below.][Further note: a reader pointed out (see comment) that Clemson is in South Carolina, not Georgia, and SC soil tends to be more alkaline, not acidic, so is more like our alkaline soil, making the original note invalid.]

See also UC Davis: Wood Ashes as a Garden Fertilizer (5).

On compost

The Humanure Handbook, by Joseph Jenkins (1) is my primary resource on all things to do with compost. [see my earlier post: What Makes Your Compost Tick? (15)]. Mr. Jenkins does not recommend adding wood ash to compost, because the compost could be used where the alkalinity of ash is not needed. Instead, this book recommends adding ash directly to soil, only where needed.

In the garden

“Using wood ash is equivalent to using a 0-1-3 (NPK) fertilizer for your plants” according to (23).

Before deciding to use ash in your garden, test the soil’s pH; don’t just guess. If your soil is too acidic, wood ash can be added to raise the pH (make it more alkaline). This is especially useful for those veggies and flowers that prefer a sweeter soil, provided your soil is not already alkaline. Garden Wise: Using Wood Ash in the Garden (3) has a list of herbs, ornamentals and veggies (such as beets and other root veggies (except potatoes), spinach, cabbage) that benefit from sweet soil.

Also from Garden Wise: Using Wood Ash in the Garden (3):

  • Because it raises the pH, apply ash only to those areas of the garden, or particular plants in the garden, that benefit from sweet soil, such as some vegetables, lavender and lilacs.
  • Do not apply it to plants that need acid soil, such as rhododendrons and blueberries.
  • If you compost ashes, be sure it doesn’t make up more than 5% of your total compost pile.

See also Hume Seeds: How to use wood ash in the garden (22) for lots of good info.

Fall/early winter is the best time to distribute ash in the garden; that way it can settle in and release its nutrients the following spring.

  • Generally ash is applied every two years at an application rate of 2 pounds per hundred square feet (.9 kg per 38 square metre), and is lightly raked into the top inch of the soil prior to planting.
  • For established plants, sprinkle 1/4 cup (50 mL) within the drip line every two years.”

In agreement with Garden Wise article (see above), Jean H and Don B sprinkle fireplace/woodstove ash over a snow-covered garden in late fall; the point being a very light sprinkling of the ash, so as not to overwhelm the soil.

Jeffrey F responded to my query on using ash in compost or garden as follows:

“I am in agreement with the author of the Humanure Handbook (1) that ash ought not be added to compost directly. The rule of thumb regarding adding ash to the garden has, I believe, its origin in the Eastern US, where soils are typically acidic, and the ash is typically from hardwood trees. Here the soil is typically basic [alkaline] and so adding alkaline wood ash, regardless of source, would have a negative effect. In fact, in the the Flathead valley, it is much more likely that sulfur [acidic] would be an appropriate amendment to balance pH, not wood ash [alkaline].”

Supporting this argument, Is wood ash good for garden soil? (2) advises:

  • If your soil is very acidic (5.5 of lower), wood ash can improve your soil pH.
  • If your soil is neutral or alkaline to begin with, adding wood ash could raise the pH high enough to interfere with plants ability to take in nutrients.
  • Wood ash should also be avoided around acid loving plants like rhododendrons and blueberries.

“Bottom line is that a small amount of wood ash will add some nutrients and be beneficial to most soils. Large amounts should be avoided”. (2)

David Brown, who doesn’t have a fireplace so no wood ash, but recycles human waste, sent this interesting article, from The Best Years in Life: Grow Hearty Tomatoes Using your Bladder and Fireplace (15).

What about the charcoal (biochar) in the ash?

Wood ash usually contains bits of charcoal in the mix. Should that be screened out before spreading the ashes? Don B comments:

“This is unnecessary and probably unwise. Recent research on ‘biochar’ indicates that charcoal can play an important role in capturing minerals and improving soil quality.”

Remember, as discussed above: if your soil is already alkaline, adding ash may not be a good idea, and may be harmful to your plants.

Additionally, there appears to be a difference in the benefits of ash vs biochar, so that separating them could be quite useful. While ash provides valuable minerals and raises the pH, biochar provides a wide range of benefits (11, 12):

  • It has a unique ability for attracting and holding moisture, nutrients, and agrochemicals, even retaining difficult to hold nutrients like nitrogen and phosphorous;
  • Its immense surface area and complex pore structure …provides a secure habitat for micro-organisms and fungi. Certain fungi form a symbiotic relationship with plant root fibers and this allows for greater nutrient uptake by plants.
  • Biochar’s long-term persistence in soil (more than 2,500 years and counting) makes its effects long-lasting.

See also Walden Effect: Wood Stove Biochar Experiment (12) and Wikipedia: Biochar (13) for good info on biochar.

Making your own potash

Potash is potassium hydroxide (KOH), a component of wood ash. See Landscape Juice: Making your own Potash (7) for more.

Other uses for wood ash

In the home

As mentioned above, mixing ash with water produces lye, which can be used to free up clogged drains, though I don’t recommend this use for those unfamiliar with the extremely caustic nature of lye. Another, more common use for lye, is for making soap (see The EssentiaList: Making Soap at Home, with Kathy Mansfield (21))

Ash is also used in the brine to cure and preserve several types of foods:

  • Making the fruit of the olive tree edible. See All Things Sicilian: Pickling olives using wood ash (14);
  • Curing cod fish to preserve it (making lutefisk), per Cat;
  • Making hominy using hardwood ashes from birch, per Jeffrey.

Minimize odor in the privy

When I was caretaker of an old homestead cabin on the Oregon coast, we used wood ash from the wood range to minimize odor in our indoor privy. Similarly, Don B has used wood ash in an outhouse to minimize the odor. This makes sense because human solid waste tends to be acidic (from the presence of bile and sulfur).

From WikiBooks: Self-Reliance Handbook/Disposal of Human Waste (16):

“The best method of reducing the normal unpleasantness [in a standard outhouse] is to have a bucket of wood ash and to put one scoop in after relieving yourself. This method assists in eliminating moisture and smell.

After the outhouse fills to within a foot of the ground level, fill in the top of the pit and clearly mark the area so that no one accidentally falls in. The ground should be very firm due to the addition of the wood ash over time.”

And from Sootypaws: Living off the grid (16):

“Human waste is dealt with simply. We have a privy where urine and fecal matter are kept separate. There is almost no odor near the privy except in the hottest time of the year. Urine is dumped periodically onto large leaf piles where it helps accelerate decomposition.

Fecal matter is buried under smaller leaf piles. It decomposes more slowly, taking two years or so. We then use the compost in the garden for non-food crops. In the privy, wood ashes from our wood stove are dumped in the bucket after use. The ashes help diminish odor and when mixed with our heavy clay soil help give it better texture.”

Working with metal

Jeffrey,  a blacksmith and metals artist, uses ash “to insulate hot iron and slow the cooling for metallurgical purposes, and for neutralizing acid solutions used in my work.”

What about briquette ash?

If your briquettes are simply charcoal, with no additives (such as starting fuel, heavy metals for flame color, etc.), the ash can be combined with other wood ash as described above. If, however they include other ingredients, recycling is not recommended.

Lawn and garden

eHow offers good advice and suggestions for recycling briquette ash: What to Do With Leftover Briquette Ash (18), including for lawncare (grass likes a sweeter soil):

“Rather than use lime and potash as a fertilizing material, collect ash in a metal bucket over the course of a grilling season and then spread the ash on your yard in an amount of no more than 10 to 15 pounds per 1,000 square feet. Charcoal also can help bind important soil nutrients such as potassium and prevent them from leaching away before they can be beneficial to a garden.”

and in the garden as a barrier to slugs:

“Charcoal ash can serve as a barrier to slugs. Spinkle a small amount around the outer edge of a plant to keep slugs from destroying its leaves and roots.”

Making your own briquettes

While we’re on the topic of briquettes, did you know you can make your own for use as fuel? This process has been used for centuries by traditional peoples, and is described by the Pace Project: Making Fuel Briquettes from Everyday Waste (pdf) (19).

References, and for More Information

  1. Humanure Handbook, by Joseph Jenkins (see Amazon for a peak inside)
  2. Is wood ash good for garden soil?
  3. Garden Wise: Using Wood Ash in the Garden has list of plants that like sweet soil (with ash added)
  4. Clemson Univ.: Wood Ash as an Agricultural Soil Amendment
  5. UC Davis: Wood Ashes as a Garden Fertilizer
  6. Gardening Tips ‘n Ideas: What is Potash
  7. Landscape Juice: Making your own Potash
  8. Improve your Garden Soil: Potassium And Soil (Potash Fertilizer)
  9. Oregon State Univ. Extension: Wood ash can be useful in yard if used with caution
  10. Colorado State Extension: Charcoal is Not a Good Soil Amendment in Colorado
  11. US Biochar Initiative: Soil & Water Benefits of Biochar
  12. Walden Effect: Wood Stove Biochar Experiment
  13. Wikipedia:Biochar
  14. See All Things Sicilian: Pickling olives using wood ash
  15. The Best Years in Life: Grow Hearty Tomatoes Using your Bladder and Fireplace
  16. WikiBooks: Self-Reliance Handbook/Disposal of Human Waste
  17. Sootypaws: Living off the grid:
  18. e How: What to Do With Leftover Briquette Ash (does not discuss contaminants in the briquettes)
  19. Pace Project Making Fuel Briquettes from Everyday Waste (pdf)
  20. The EssentiaList: What Makes Your Compost Tick?
  21. The EssentiaList: Making Soap at Home, with Kathy Mansfield
  22. Hume Seeds: How to use wood ash in the garden
  23. on wood ash in the garden


2 Responses to “Recycling Wood or Briquette Ash”

  1. Vivienne Long-Speer says:

    Clemson is in SC, not Georgia.

    Georgia has more clay (alkaline soil) vs. acidic.

  2. Catherine says:

    Thankyou Vivienne, for pointing out my error. I have crossed out the incorrect text and add a note right after that text explaining my error.