Biochar is a special kind of charcoal that seems to have many benefits as a soil amendment. It holds water, acts like a fertilizer and grows bigger plants. While doing all this it is also eco-friendly and sequesters carbon in the soil for many thousands of years. Sounds like a win-win-win.
What is biochar? Are the claimed benefits real? Should gardeners be using this product to amend their soil? Let’s check it out.
What is Biochar?
Everyone agrees it is charcoal made from pyrolyzed plant-based organic matter including such things as manure. The term biochar is quite new and was coined by Peter Read in 2005.
Several sources including the UC Davis biochar database and Wikipedia say it is charcoal that is primarily used for soil amendment and not for heating. That is interesting, but does not really describe what it is.
It is created in a low, or no, oxygen environment so that it maintains its fine grained structure.
The International Biochar Initiative says it’s produced at low temperatures and then goes on to define this as <700°C. The UK Biochar Research Center says it should be made above 250°C in a zero oxygen environment. “In general higher pyrolysis temperatures mean a smaller amount of char, but containing a greater proportion of highly stable carbon”. Others say it needs high temperatures, over 500°C.
Biochar vs Charcoal
We have all seen charcoal and maybe even used it in a barbecue (real charcoal, not the briquettes). How does this differ from the new biochar?
After much searching all I can determine is that the end use determines the name of the product. Some sources hint at the fact that the manufacturing processes are slightly different to produce products with different characteristics, but nobody gives details.
A report by the Illinois Prairie Research Institute says Biochar is not a new product, just a new name for material that is used to amend soil. In fact it says biochar covers a wide range of products including char, charcoal and soot.
This certainly would explain why nobody seems to be able to describe the difference between charcoal and biochar. It is charcoal when used for burning, and biochar when used to amend the soil.
Biochar Varies a Lot
Biochar is a variable product and its chemical and physical characteristics depend very much on the input ingredients, the process used to pyrolize it ( temperature and length of heating) and any chemical treatment after production.
This makes it difficult to make any definitive statements about its use.
Claims for Biochar
The claims made for biochar include:
- increase yields
- increase fertilizer efficiency
- remove pollutants and pesticides
- mitigate climate change
- increase soil moisture
- increase soil pH
- increase soil microbe populations
- increase cation exchange of soil
Most of the studies that have led to these claims have been done in the lab and very few field tests exist.
Both charcoal and biochar absorb chemicals. They do have negatively charged sites which increases the CEC (cation exchange capacity) but if they absorb pesticides or nutrients then this can be detrimental to plants since it makes these products less effective. We also know that activated charcoal, which is used to scrub chemicals, gets saturated and needs to be replaced. I would expect that Biochar in soil will also become less effective over time, but nobody mentions this. There are no long term studies.
Biochar does increase the pH of soil and holds more water, neither of which are benefits in alkaline clay soil. It does increase microbe populations.
One review of current studies reported, ” In biochar studies reviewed, half reported an increase in plant yield after adding black carbon or biochar, while 20% noted decreases in plant yield, and 30% reported no difference in plant yield from the addition of biochar.”
A review of a large number of studies concluded that the application of biochar can be both positive or negative depending on the parameter being measured, and that it is site specific. “This review has indicated many benefits, complexities and effects of biochar; however, more research is needed”
Field studies are mixed on the yield issue with some showing an increase and others showing a decrease. Some studies show positive results when used with fertilize, or on acidic, low fertility, soils.
A four year study looking at growing vegetables in a simulated back yard garden, at three different sites, found mixed results; some increases in yield and some decreases in yield.
It is important to keep in mind that the characteristics of Biochar vary a lot and soil plays a key role in determining how it might affect yield. It is therefore not surprising to see mixed yield results.
It is clearly not the cure-all bullet some claim it to be.
How Does Biochar Affect Microbes?
The effect depends on the type of soil and crops. Wood-derived biochar can have short term positive effects but amendments can also reduce microbial biomass. Biochar can increase pH resulting in a modified community, which may not be beneficial.
Charging Biochar
Many sources recommend that you charge biochar before using it. This can be done by mixing the biochar with compost, or just adding it right into a compost pile and composting the whole thing. You can also soak it in manure tea or just add some synthetic fertilizer to the biochar.
I am not surprised charged biochar works better. All of the active negative sites on the biochar will now be saturated with nutrients. When it is added to soil, it can add nutrients to the soil and make them available to plants. On the flip side, un-charged biochar is like a magnet. When added to soil it sucks nutrients away from plants, at least for a short period of time. This may explain why some studies show a decrease in plant productivity when the soil is amended with biochar.
Are the reported increased yields due to the added fertilizer?
Does Biochar Sequester Carbon
Even if biochar does not increase plant growth, it is believed to be a really good solution to our CO2 problem. If plant material is left to compost, it produces CO2. If it can be turned into biochar, and then added to soil, it will essentially bury our CO2 problem.
The efficiency of this all depends on how long biochar remains in its charcoal form. People have suggested it is very stable and will last thousands of years but there is almost no science to prove this. A report from the Earth Island Institute says, “Field trails proved rare; only five such studies were found, which between them tested biochar on 11 different combinations of soil and vegetation. In only three cases did biochar result in any additional carbon sequestration. In most cases, there was either no measurable difference in soil carbon, or even a reduction in soil carbon. These results from short-term studies —none spanned more than four years — fly in the face of repeated claims that biochar will sequester carbon in soils for tens, hundreds or even thousands of years.”
A recent peer-reviewed study revealed that 72% of the carbon was lost in the first 20-30 years. “There is little basis for confidence that charcoal will retain carbon in soils“.
There are also environmental costs for collecting the raw material, creating the biochar and redistributing it back to soil. Maybe the whole process is not ecofriendly?
Organic Matter or Biochar
One of the big benefits of biochar is its ability to hold onto positively charged nutrient cations (calcium, potassium, magnesium, etc). Clay and organic matter do the same thing. The organic matter turns into humus which sequesters carbon long term, but maybe not as long as charcoal.
One has to wonder if it might be better to just add the organic matter to soil and let nature take care of things?
Biochar as a Byproduct
Most producers of commercial biochar are more interested in producing biofuels and biochar is a waste product for this industry. Using it for soil amendment in this case may be eco-friendly.
Can You make Biochar at Home?
Some sources say you can’t make it at home because you can’t get things hot enough and control oxygen well enough. It is clear that simply burning things in a fire or in a pit won’t make good biochar, and if it is not made properly it can release harmful gases such as methane and carbon monoxide.
Others have designed special DIY stoves that seem to make reasonable biochar, although the process is not tightly controlled. Campfires can reach 1,100 °C, so a properly built system could reach the temperatures needed. Stoves built with two chambers, one for burning to make heat and one for making the charcoal in low oxygen conditions might work. With the right system you can make biochar at home.
Since there is no clear definition of what biochar is, I guess it is hard to say you can’t make it at home. But keep two things in mind:
- Just because you make some black charcoal does not mean it will have any of the properties reported for biochar. Its properties depend on how it is made.
- If it is not made properly you will be creating more pollution than you save.
It really is better not to make your own.
Biochar in a Potting Mix
Biochar has been shown to inhibit seed germination so it should not be used in a seedling mix.
It can be used to replace some of the peat moss in soilless mixes where the increase in pH counterbalances the acidic nature of peat moss.
Should Gardeners Use Biochar?
Biochar has been shown to have some benefits in the garden and it might be a good amendment for soilless potting mixes, but here is the problem.
Biochar is not one product. It can be made from many different input ingredients, in many different ways. The result of each process is a different product with its own special characteristics. There are no standards so you can’t pick up a bag and say, “this is the one that produces good results in my soil type”.
You are left with buying a bag of something and hoping it works.
You have the same issue with buying compost – who knows what is really in the bag?
But there is one big difference between compost and biochar. Unlike biochar, compost decomposes in a few years. A bad batch of compost is a short term problem. Biochar may not last a thousand years, but it certainly lasts a lifetime. What do you do if you add a bad batch to your soil?
Many studies have shown that too much biochar is harmful to plants and soil. You can’t correct this problem without removing the soil.
It may be more beneficial in acidic soil where the increase in pH is desirable. It has also been shown useful in reducing compaction in lawns and poorly drained soil.
If you want to try it, start small and use some test areas. Keep the amounts small. Document the product you use so you can buy the same product again.
I think I will sit on the sidelines and stay away from it, for now. Compost seems to offer most of the same benefits without the downside.
Most everything that I do, attempt, or consider then my first step is looking into what happens in nature.
I can’t help but wonder about the fires that take place naturally or even by the use of controlled burns with the seemingly beneficial results of the new life and growth.
Maybe if one chooses to use or apply any char then it might be that it’s most beneficial if just done so with a topical application instead of mixing or working it into the soil itself? After all I don’t think anyone is working it into the forest floors. Just a thought.
I have been making my homemade char for ten years now. I merely douse the embers of my garden fire with water (No it does not spit in my face, it just turns a gorgeous black)
I have been writing about it extensively on my blog (Just go there and press the search button)
I use it to add to my soil and make my potting mixes when I mix it with soil.
I heartily endorse it needs to be prepared by exposing to the elements for a year or so and heartily fortified with compound fertiliser.
I do not know whether it is slightly inferior to commercial char or vastly superior!
Must be on my hundredth large barrow load by now
Depending on definition, what comes out of a fire pit, is not biochar, it is just charcoal. But if nobody can properly define biochar, then I guess stuff created in a fire pit qualifies.
As I see it biochar or charcoal are one in the same. Adding it to soil increases the carbon content. As anyone who know anything about basic soil science, if you want the carbon to have a benifit then you also need nitrogen.its the basis of the C:N ratio. The variance in the results experience is all down to lack of feed stock i.e. nitrogen. inoculating the biochar before adding to soil elevates this problem. This issues can also be seen when uncomposted brown material such as bark mulch is added to the soil. There is frequently a negative effect on plant growth the following year but in subsequent years there is increased plant growth as the materials in broken down by soil microbes and microbial action return to normal levels and critically makes nitrogen more readily available again. Using biochar is simply away of speeding up the natural processes. If you have bi-product, say from forestry, and you can capture the heat from the process it probably worthwhile doing. Otherwise I’d say stick with conposting.
You only need nitrogen when microbes are decomposing the organic matter. They require a ratio of about 30:1. One of the main reasons for using biochar is that it is very stable – it does not decompose. Therefore no nitrogen is required to keep a C:N ratio.
I have a lot of experience with biochar. The biochar I have is the water heating byproduct of my enkistove wild+ which is created at around 700°C and has great physical characteristics. Hot burned has a high pH (9) especially when using non-wood feedstock and MUST be pH adjusted before use. When adding phosphoric and nitric acid in quantities to meet plant nutrient needs you can use biochar pure as long as it’s toxin free (no PAH etc.) and the pH is around 6.
I contend that if you have a given amount of wood, the soil gets more benefit if that wood decomposes on or in the soil than if it is converted to biochar and added to the soil. For more elaboration, see my blog post: http://www.leereich.com/2013/05/rational-in-spring-no.html. The only concrete benefit about which I’ve heard is in the tree “pits” along sidewalks in cityscapes, where the biochar provides a long-lasting physical (and some other benefit) in those soils that can get little management once planted.
Charcoal becomes BioChar once it is in the ground for a while and absorbs nutrients. Once functioning in the soil and inoculated with ingredients that plants like, it is BioChar. You can speed this process up by housing worms in it or urinateing on it.
Nobody I found uses that definition. Do you have a reference?
I followed the germination suppression link (thank you for including it) and it states that “High pH was likely a cause of poor lettuce germination in 50% biochar mix”. Which is an effect I would expect, and which I assume you are in a position to appreciate and anticipate these kinds of not so subtle nuances in natural materials. Some biochar is black ash, which may be volumetrically >50% C, but is >75% ash by weight. Freshly made biochar can be very caustic (pH12+) and if this is substantially ash by mass, this type of biochar can have a huge impact on soil pH. However, if that high pH biochar is allowed to season, meaning allowed to equilibrate with atmospheric CO2, it drops the pH to 8.3-. And if the ash% on a m/m weight basis is low, the liming potential on a CaCO3 equivalent basis is miniscule. Some liming effect is desirous for germinating seeds. My use of 10% v/v seasoned biochar in 2″ soil blocks (yep, IMA soil blocker) shows improved germination rates, earlier germination, and better sproutling survival than prior to my use of biochar. To read your blanket advice to people to avoid biochar due to germination concerns, it kind of upsets me, to be honest.
The average gardener is not going to measure pH and ash in any product they buy – so to be safe it is better not to use it for seedlings.
If the ash content were written on the biochar packaging material along with simple guidelines on amount of biochar to add to most common soil types we may not have to measure pH and ash.
I attended a soils symposium at UC Davis last year, and when asked about bio-char, the professors on the panel indicated that they have seen no benefits to using bio-char in terms of production.
you don’t mention the use of biochar thousands of years ago in the Amazon – https://biochar.co.nz/about-biochar – which is doubtless the basis for the assertions about thousands of years
I almost included info on it, but the article was getting long.
The idea for biochar originated from Terra Preta soils of the amazon. But then you have to ask the question, how does the production of biochar today compare to how it was made then? How do the differences affect longevity in the soil? I did not get into any of that.
I make and use biochar. I have always been taught that biochar is different to charcoal in 2 main ways, 1st is temperature used to produce it and 2nd is that charcoal is normally made from wood cut for the sole purpose of making it (questionable enviromental impact) where as Biochar is made from waste or byproduct of a product. Some good examples of this are sweetcorn husks, nut shells, olive seeds etc. It shouldnt be made from wood cut just for the purpose.
Interesting you say there is no evidence from a thousand years ago as to how effective biochar is! I was under the beleif that Terra Preta soils or black gold are evidence of its effectiveness.
In my own experiance the soils I have used biochar in produce better results than the others. The only difference between the beds is the use of properly activated biochar. I use the same organic Korean Natural Farming fertilisers on all the beds as I use to charge the Char.
Concur with your comment with regard to seed raising. In my experiance it almost halves the success rate, how ever using it as part of my potting up mix has not shown any detrimental effects. I add about 5% raw char to my mixes and leave it for at least 6 months before use.
If it is temperature dependent, then why is the definition of biochar not something like, charcoal made between X and Y degrees? Found no definition that says that.
Re: “charcoal is normally made from wood cut for the sole purpose of making it” – that may be true, but cut wood can also be used for making biochar so it is not really a differentiator.
I was also very surprised about the longevity. There are certainly old sources of charcoal in the ground found by archaeological digs. But did these start out as charcoal or biochar? Clearly in the Amazon, they did not make modern day biochar. If we accept that fact that biochar is somehow different than just charcoal, then Terra Preta soils are also different than soils with biochar in them.
I think most people would agree biochar probably last a long time, but scientifically there seem to be no studies to show this. So it is now an educated guess.
I am guessing that many people believe in the long-term persistence of biochar based upon the long-term persistence of buried coal deposits (hundreds of millions of years). Is coal a relevant material to project characteristics onto biochar? Maybe, maybe not.
I think coal and biochar are quite different.
Hi G Negus,
I often wonder if terra preta enthusiasts aren’t overthinking the biochar manufacturing process. I’m sure Incas didn’t have advanced chemistry degrees so it seems more likely that the ash and charcoal left over from the cooking fire (which would include dry corn husks, straw, dung, and anything else readily available… as well as bones, feathers, etc which are better off burnt than rotting away in the hut) was simply tipped into the toilet pot (an excellent source of ‘activating’ the charcoal) to ‘sweeten’ the pot and stop it stinking out the hut and then, when the pot was full, the mix was buried in the garden… that process seems to tick all the boxes. Pyrolysis doesn’t produce a superior charcoal but simply more charcoal from the amount of organic matter being burnt… inessential but more efficient.
Proper pyrolysis could create a different structure in the charcoal. When I started this post I assumed I would find some clear properties of the “real stuff” so it could be distinguished from regular charcoal. The fact that this is not readily available does not mean there is no difference.