Your Biochar Questions, Answered

In previous blog posts we discussed the history of biochar, we offered our thoughts on its possible application, and examined potential controversies. Most recently, we interviewed Kelby Fite about his work with biochar and held a webinar to discuss what designers need to know about the product. The following post is a collection of questions and answers from the webinar with Peter MacDonaugh, FASLA, Kestrel Design Group and Kelby Fite, PhD, Bartlett Tree Laboratory.

Since the quality of biochar can be so variable, how should people go about buying it and assessing its quality?

KF: The International Biochar Initiative has set standards for methods of testing biochar. Beyond the expensive chemical tests, you can do simple hand-drawn assessments. For example, you can rub it in your hands to feel how greasy it is, which will give you an idea of how many volatiles, tars, and resins are still left in that material. You can also put biochar in a tin can and place in the toaster oven to see how much smoke boils out.

We also highly advise you to ask your supplier to provide any type of analyses of the product they carry before purchasing it from them. If they don’t understand what you’re talking about, that’s probably not the supplier you should be purchasing from.

Are there any open source specifications that are available to professionals who want to specify biochar on a project?

PM: There is not a lot out there in terms of CSI specifications relating to biochar. The International Biochar Initiative is the primary source of standards, which have not been translated into specifications or guidelines for use by contractors.

As we know, biochar can help remove pollutants. Once that process has taken place, does the material need to be disposed of? Does the biochar itself become hazardous?

KF: It depends on what we’re trying to filter or cleanse. Every situation is different but the idea is not to have another toxic waste product that we have to dispose of. For instance, fresh biochar is used to clean the effluent coming from dairies. This results in a relatively nutrient-free effluent leaving the facility and is a good way to sorb phosphorus and charge biochar with nutrients. This makes biochar more valuable and prepares it for use in an agricultural or horticultural setting – it’s now more valuable than it was before.

Another example where biochar is being used to remove pollutants is in mine reclamation work. Heavy metals are bound to the biochar particles, instead of available for leaching or at  toxic levels to plants. After sorbing the heavy metals, biochar is left on site as part of the soil continuum.

Is one application sufficient for the life of an urban tree, or does the application procedure vary based on the intended outcome (such as improved stormwater management)?

KF: To the best of my knowledge, we consider it a ‘once and done treatment’ if we can achieve 25% by volume. These recommendations may change over time with additional research. A few years from now we may find out that subsequent applications do give us more of a response, or as a tree grows we may make an application on what is now the critical red zone. But we have to keep in mind that as that tree grows and the root system expands, we may need to improve larger volumes of soil over time. This process would not be considered as reapplication, but as an expanded application.

Can you speak to the effect of biochar as a soil amendment on pH? Is that something to worry about?

KF: Yes! The pH tends to depend on the type of feedstock that’s used to produce the biochar and the type of pyrolysis unit that produces it. Although primarily alkaline, its pH varies and can be as high as nine. There are also pH’s that are in the nines but there’s only residual ash left on the particle and once a couple of gallons of water is leached through it, it has a relatively neutral pH.

The high pH is something we need to keep in mind when it comes to specifications and parameters of biochar quality. It could either be an advantage or a disadvantage depending on the location. In the Midwest, for instance, the last thing you want to do is an alkaline amendment to soil because of calciferous soils that are there. But in Charlotte, North Carolina with acidic leached clays, a biochar amendment would work really well.

With biochar, as with any other amendment, it’s always good to know the pH of the soil you are applying it to.

Watch the full webinar here.

Has there been any work on biochar regarding handling saline runoff as a result of de-icing?

KF: Amending the soil with a particle that has a high cation exchange capacity is beneficial. With that said, we don’t know that for sure, because there hasn’t been a specific study looking at sodium concentrations and the ability of biochar to bind sodium ions. That’s certainly an area of interest not only for us, but the industry as a whole.

What are the considerations regarding the use of biochar in rain gardens or bioswales in cold climates?

PM: Biochar is a very persistent material and I don’t see that it’s going to be a lot different in terms of its persistence in colder climates. We have used biochar in zone 5a, where according to the USDA zone parlance the temperatures in the winter are at least -25° Fahrenheit. We’ve also used biochar in zone 4b in Minneapolis, St. Paul and have not seen any issues with it in terms of breaking up during a frost.

Are there safety concerns for workers doing the amendments to application? Why do they need to wear respiratory masks in applying the material?

KF: Yes, absolutely. In a dry state, biochar is extremely dusty, so a P95 dust mist respirator is absolutely required. One way to minimize the exposure is by pre-wetting the biochar, which makes it easier to work with and eliminates or reduces dust exposure. Although pre-wetting the material is a standard practice, respirators are definitely required when working with biochar.

There is a concern in the industry about the exposure to polycyclic aromatic hydrocarbons (PAHs), which are known carcinogens. However, according studies (here are two examples) the PAHs are volatilized in properly pyrolyzed biochar in higher degrees temperature of 400-600° Celsius. The concern is with pyrolization in lower temperature, because poorly prepared biochar is akin to cooking charcoals. The carcinogen exposure is much higher there and becomes a human health concern.

But to put this information in perspective, please know that the exposure in an urban environment from a single exhaust from a city bus has potentially more carcinogens than in a lifetime of exposure to biochar.

Is activated carbon another name for biochar?

KF: No, activated carbon is a similar product, but functionally it’s very different from biochar.

Activated carbon is a charcoal product that has either been chemically or steam activated to liberate all the volatile compounds that are in the char particle.  The absorption capacity of activated carbon is really high. The concept is that once it has been activated and applied, the contaminant, whether it is a heavy metal or hydrocarbon, is then permanently locked away – it’s absorbed, not adsorbed. So the goal of an activated charcoal is usually in filtration such as water or air filtration. The idea there is to lock away any contaminant on a permanent basis. By contrast, biochar locks away nutrients and water on a temporary basis, giving them back up for uptake by the plant.

As we know, it’s necessary to activate biochar. Can you provide any more details about what type of fertilizers are used in research, such as the NPK ratio or the type of compost?

KF: In blending biochar and fertilizers, we use one of two formulations. One is a 24/7/7 in areas where phosphorus is limiting, and the other formulation is 30/0/12. In both these cases it’s greater than 50% in soluble slow-release nitrogen. We’ve also worked with organic fertilizers with an 8/0/3 formulation. So again, organic source greater than 50% slow-release nitrogen or a water-insoluble nitrogen. Those are the two or three standard formulations we’ve worked with over the years, but certainly our recommendation is to base the nutrient need on a sample from that site.

PM: In regards to the compost, you need a stable, mature compost that’s not hot, with a carbon and nitrogen ratio somewhere between 15:1 to 21:1.

What is the saturation, or water retention capacity, of soil that has been treated with biochar versus conventional soil or one that has not been treated? How significant is the difference?

KF: That’s going to vary on soil type, tremendously. Biochar buffers the extremes – so in saturated soil conditions the biochar is going to drain more quickly, open that pore space up for oxygen and air infiltration. And in drought scenarios, the micropore space of biochar particles are going to hang onto water longer from the plant availability standpoint.

PM: If water is available, it would take an awful lot of drying out for biochar to reach below the wilt point. It would also take a long time to get to the saturation zone. It’s going to be more in that zone where it’s available to plants. I don’t know of any studies where they’ve done a weight comparison with biochar in terms of how much moisture it’s holding over a period of time.

Does biochar move vertically through the soil over time or does it tend to stay where it’s applied?

KF: That’s a great question and one we don’t really have the answer to. That’s one of the plots that Dr. Bryant Scharenbroch put out at the Morton Arboretum, it’s just purely from a top dressing standpoint. We certainly know that as mulch degrades, or top dressing of compost through microbial activity and earthworm activity, will migrate down through the soil profile. The questions remains unknown if the same thing will happen with biochar.

PM: Anecdotally, from the Terra Preta it would seem that biochar is fairly stable within the profile.

Thanks, Kelby and Peter!


Engineering for ChangeCC BY-SA 2.0


  1. There actually has been some research on biochar’s effects on saline soils. Not a lot but worth noting


    Biochar mitigates negative effects of salt additions on two herbaceous plant species.

    Effect of biochar on reclaimed tidal land soil properties and maize (Zea mays L.) response.

    Leaching and reclamation of a biochar and compost amended saline–sodic soil with moderate SAR reclaimed water

    Furfural and its biochar improve the general properties of a saline soil

    Using Willow and Wheat Straw Biochar as a Soil Amendment in the Reclamation of Saline Soils

    Improving salt leaching in a simulated saline soil column by three biochars derived from rice straw (Oryza sativa L.), sunflower straw (Helianthus annuus), and cow manure

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