1. What is the difference between charcoal and biochar?

Some of the materials that we would call biochar could also be called charcoal. The term biochar is used if the charcoal was produced intentionally for carbon management or agricultural/environmental applications. Thus, I wouldn’t call your BBQ charcoal “biochar”.

2. Is biochar sustainable?

Producing biochar can be a good idea, but it can also be a bad idea. The big questions I always ask are (1) what is it being produced from, and what would have happened to that biomass if it weren’t used for biochar? (2) where is it going to be applied, and what will its effect be on the soil? (3) why is it being produced (e.g., for its energy, carbon, biomass management, agricultural effects, economic reasons or a combination of all of these)? (4) who is producing it, and did all parties involved give prior, informed consent to its production, application, and management? As you can see, biochar systems are complex, which makes them exciting to study, but also means it is important to consider all aspects of their introduction.

3. Is biochar carbon neutral or carbon negative?

See the question above. The answer to this question depends on the baseline scenario (what would have happened to the biomass if it weren’t turned into biochar), whether energy is captured and how that is used, and what happens to the biochar after it is produced, among many other things. It is possible to produce biochar in a way that results in less carbon being in the atmosphere than otherwise would have been there, but this is highly system-specific.

4. Should I buy biochar for my garden?

This depends heavily on the specific biochar and your soil, and what you want to get out of it. Many biochars have a basic pH, which could help acidic soils, but if your soil is not acidic to begin with, then these biochars might harm your soils. Lower-temperature biochars produced from wood, however, tend to be more acidic. Knowing your own soil and the biochar is important. Even if you are more interested in the carbon management side of biochar, you still want to be sure to do no harm.

Soils are complex and fascinating systems, and there are diverse effects biochar could have on your garden. This is what my lab and research groups around the world are studying to try to better understand and predict how a given biochar will interact with a specific soil type.

As another example, if the number one goal you have is to improve nutrients in your soil, especially nitrogen, you might be better off composting the biomass and returning that to the soil. Although some nutrients remain after biochar production, much of the nitrogen is lost as a gas, and the remaining nitrogen may be less available to plants and microbes than it was originally.

The International Biochar Initiative has taken some important steps toward the standardization biochar labelling with its characterization standard, developed using a transparent and inclusive process. There will likely be a certification program in the future, which will help individuals understand just what is in the biochar they are buying.

5. One study says biochar does X, but another study found it does the opposite – what’s going on?

An essential element of understanding biochar’s effects comes back to the definition we started with – a diversity, or continuum, of materials gets called “biochar”, and these diverse materials can have different properties and effects, which interact further with a specific system, environment, or soil type. A biochar produced at 300°C is likely very different from a biochar produced at 700°C. A biochar produced from pine shavings is likely very different from a biochar produced from dairy cow manure. If someone told you, “I put biomass on my plants, and it did wonders,” and you don’t know whether they added manure, maple leaves, or butterfly wings, how could you expect to do the same thing in your own garden?

Part of my own and others’ research is characterizing biochars, so we can understand and predict its effects on soils, and, ideally, develop quick tests to help us do so. A good starting point is always to ask what temperature the biochar was produced at and from what type of biomass.