Should we use biochars in our gardens?

In the last few years, I’ve had a number of people ask me about biochar:  what  is it and what does it do?  Should they add it to their garden?  Should they make their own biochar?  So while the subject deserves a longer review, I thought it would be useful to discuss it briefly on the blog.

In the strictest sense, “biochar” refers to charcoal that’s made as a byproduct of biofuel production.  Various crop residues, livestock manures, and just about any other organic material you can imagine has been studied for this purpose.  Biofuel production not only helps diversify our energy resources, but the biochar itself also boasts several benefits, not the least of which is that it serves as a long-term, stable repository of carbon.  Since the carbon in biochar decomposes so much slower than the parent organic material, it is often considered to be a “carbon negative” material.

Even more exciting is that biochar offers some distinct, tested benefits to agriculture.  It is a porous, charged material that has been used to remediate soils by binding contaminants such as pesticides and heavy metals.  It offers a physical environment to mycorrhizae, which often benefit from biochar amendment.  It binds nutrients such as nitrogen, preventing runoff or leaching, and releases these nutrients to crops, most of which are shown to benefit from biochar additions.  The scientific literature is robust in examples, worldwide, of how various biochars benefit agricultural soils and crops.

But before you rush away to buy (or make) your own biochar, there are some significant caveats.  First, there is a sophisticated process used to make biochars, whose characteristics will vary tremendously depending on how they are produced.  Differences in temperature, for instance, will produce very different biochars from the same parent material.  (And you would be hard pressed to do this at home:  temperatures can range from 100-700C.)

Second, there is little, if any, research on the use of biochars in nonagricultural situations other than soil remediation.  This means no information on how it affects trees, shrubs, home gardens and landscapes, and other urban greenspaces.  As readers of this blog should know by now, there are many agricultural production practices that do not translate well to the home garden or landscape.

Third, biochars are generally very alkaline, often with a pH close to that of lime.  While this might be fine for some soils and plants, naturally acidic soils and their respective acid-loving plants are not going to react kindly to a more alkaline soil environment.

Finally, I hate to see people (and they are out there) who are now taking their pruning debris, arborist wood chips, and other organic material, burning it, and burying it.  Ideally, both bio-oils and biochar are made from excess crop residues and other debris generated in agriculture.  Arborist wood chips and other plant debris generated in a home landscape need to go right back onto the soil as part of a compost/mulch layer.  To burn this valuable resource strikes me as the classic “penny wise, pound foolish” mentality.

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Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - "The Garden Professors" Facebook group - Books:

22 thoughts on “Should we use biochars in our gardens?”

  1. Linda, are biofuel-makers selling,
    or giving away, biochar for use in gardens or commercial farming operations? I have not heard of this term before. I have been spreading my woodstove ash in the garden and orchard for years, and have always noticed that it creates a nice structure to the soil. We have very acidic soils that could use some “sweetening” around here.

  2. Anne, you can purchase biochar (even on eBay!). Wood ash is similar in the way that it raises soil pH, but biochar is buried and doesn’t degrade as quickly (so you benefit from storing carbon). It really depends on what kinds of plants you are growing on whether using wood ash or biochar is a good practice: acid loving plants will not like this, but most turf grasses will, for example. In any case, it would always be worth doing a soil test to see what the overall nutrient balance was before, and after, adding biochar or wood ash.

  3. Linda, have you heard of the machines that supposedly deal with restaurant organic waste by shredding, heating, and dehydrating? They don’t make biochar, but a “composty” type of material.

  4. Very interesting topic, thank you for posting this. Like Anne I hadn’t heard of biochar before. However, I’d not be tempted to use it as our soil is slightly alkaline.

  5. My grandfather and father always used the fallow section of the fields as the burn pile area. They piled all the brush and leaves and other larger organic matter there each fall and burned it, then spread the ash/charcoal over the field. Their gardens were always great. I thought that was the natural way of dealing with the large stuff.

  6. Don’t wood ashes basically just provide soil with potassium/potash, while charcoal effects structure, microorganisms, nutrients, etc? Aren’t they two different things as far as the soil/soil structure is concerned?

  7. Biomass should never be just burnt, instead it should be fractionated to it’s high value uses.
    Biochar systems achieve this, to fill in gaps and hopefully expand your story & research , particularly concerning Christoph Steiner’s new work with Biochar and NH3 conservation in composting systems.

    Recent NATURE STUDY;
    Sustainable bio char to mitigate global climate change

    Not talked about in this otherwise comprehensive study are the climate and whole ecological implications of new , higher value, applications of chars.

    the in situ remediation of a vast variety of toxic agents in soils and sediments.
    Biochar Sorption of Contaminants;

    Dr. Lima’s work; Specialized Characterization Methods for Biochar
    And at USDA;
    The Ultimate Trash To Treasure: *ARS Research Turns Poultry Waste into Toxin-grabbing Char

    The uses as a feed ration for livestock to reduce GHG emissions and increase disease resistance.

    Recent work by C. Steiner, at U of GA, showing a 52% reduction of NH3 loss when char is used as a composting accelerator. This will have profound value added consequences for the commercial composting industry by reduction of their GHG emissions and the sale of compost as a nitrogen fertilizer.

    Since we have filled the air , filling the seas to full, Soil is the Only Beneficial place left.
    Carbon to the Soil, the only ubiquitous and economic place to put it.

    Thanks for your efforts.

    Erich J. Knight
    Chairman; Markets and Business Committee
    2010 US BiocharConference, at Iowa State University

  8. @Daniel, no I hadn’t heard of that…but anything that creates compost from otherwise discarded material is thumbs up in my book! @SandyG and @Moni, yes, ash would be quite different from charcoal, as the latter adds a physical structure to soil that ash would not do. I would think the benefits of charcoal would be more diverse than those of ash. And @Erich, yes, the field of research on biochar is much bigger and broader than I could do justice to in a short posting. Thanks for the additional links.

  9. This morning there was an article in the SF Chronicle ( about biochar. It is being suggested as an additive to potting mix in their roof garden, specifically six year-old, collapsed potting mix (which they call “potting soil”). I also write for the Chronicle (Golden Gate Gardener column, also in today’s paper.) I was curious about biochar, so I looked it up on the web with your name, and found this blog post. Beyond trying to learn more about biochar, I was curious to know if there is any evidence that it is a good material for use in potting mix, or for refreshing old, collapsed, potting mix. (I would have suggested replacing the potting mix or adding a lot of compost.)

    1. Pam, Nicobar will help soil but it needs to be composted with a mix of rabbit and chicken manure. (That’s what I use). It will absorb the nutrients from this mix and slow release it back into soil. Estimates vary up to 51% higher yield on certain crops if biochar is used as a soil amenity, in composted form.

  10. Hello Linda,

    I thought of another question. I have always heard that it is not a good idea to add incompletely burned charcoal briquettes to compost or soil. I have assumed they are not simply charred wood, but contain some toxin. Do you know about this? I am thinking that if the media start writing about biochar, people will start thinking that the stuff left in their barbeque after the party is the same thing.

  11. first part of the article is OK but the 2nd 3rd and 4th points have not been researched very well and throws an unnecessary negative slant on biochar. Biochar has need recommended for use in many areas such as for cleaning up brownfield sites, fighting fungal infection on hedging plants and much much more. The author obviously does not understand the process of charring or she would not refer to it as burning, that just leaves ash not charcoal/biochar. Even though charcoal may have a pH of 9 only 0.5 is transferable.

    1. Hi David – The points you mention summarize well over 100 peer-reviewed articles that I read. There is no “slant” – I just report what’s there. Since this is a publication for home gardeners, there is no inclusion of brownfield cleanup. I found no scientific articles on fighting fungal infection in hedging plants or anything else related to home garden use. If you have titles of such publications, please include them in a comment here so I can use them when I update the fact sheet.

  12. No one ever answered my question about the difference between biochar, as sold, and the burned charcoal residue from a barbecue. I have always heard that burned briquettes are not to be added to compost. Does anyone know why?

  13. Linda, thank you for the primer. In the table at the end it indicates that biochar could waterlog heavy soils, but since biochar is granular and improves drainage I don’t understand this concern. Yes, the granules hold water, but their structure opens up the soil structure. When I build side by side beds with and without biochar (at 10% by volume) I find that I can’t get water to pool on the biochar beds due to the improved drainage while it readily pools on the biochar free controls.

    1. It’s the same result you would get when you add sand to clay soil – it makes drainage worse, not better, unless you add so much that you are able to change the textural nature of the soil.

  14. Thank you. Makes sense. So go with organic matter addition over time and let the worms distribute it down in, as well as perhaps some lime for clay?

    1. Topdressing with OM is the ideal way to ensure a sustainable incorporation into the soil. I would certainly test my soil before adding lime, however, to make sure you don’t already have sufficient calcium. You’ll also find that a decent arborist chip mulch works wonders on clay soil tilth.

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