This is a follow-up post to the one I wrote about organic matter soil amendments in October 2017. The question proposed is one frequently asked in my Master Gardener classes all around the state. I did not include biochar in my post on organic matter soil amendments, because it is not currently broadly commercially available to individuals. Additionally, it has some unique properties compared to the organic matter sources I covered; hence, I am giving it its own space.
Biochar is the charcoal-like byproduct of the process of pyrolysis, or the anaerobic (meaning without oxygen) thermal decomposition of organic materials (generally plant-based residues) to produce fuels in the form of bio-oils and/or syngas used to produce heat or electricity, or both. The interest in biochar stems from the fact that this pyrolysis residue may have myriad uses as a soil amendment to, among other things, sequester stable carbon in soil, provide water and nutrient adsorption capacity to soils, and even suppress soil-borne plant pathogens. Many research projects around the world have been, and are underway to explore its potential benefits and limitations.
Just to be clear, biochar is NOT ash. Ash is the result of typical open burning or oxidative (meaning oxygen-rich) thermal decomposition of biomass. In an oxidative process, almost all the carbon volatilizes as carbon dioxide, leaving behind primarily mineral components of the biomass rich in potassium and other salts. Therefore, ash is not typically a desirable soil amendment—it is just too salty.
Pyrolysis of biomass leaves much of the carbon in the eventual biochar depending on both the biomass source and the temperature at which the process is undertaken. At lower temperatures (400-500 C or approx. 750-950 F) as much as half the carbon from woody biomass is preserved. At higher temperatures (above 700 C or approx. 1,300 F) most of the biomass converts to syngas for energy production with only about 10-20% of the resulting residue as biochar.
In either case, the biochar left over has desirable properties as a soil amendment. In fact, the lower-temperature products are very similar to materials proposed as responsible for the development of highly fertile “terra preta” soils in South America (see additional reading section for info on these soils).
As potentially desirable as it seems to be, there are varied results with biochar use in the research literature. In very broad-brush summary, the higher the biochar production temperature, the higher its adsorptive properties. High-temperature biochars have been observed in short term studies to cause a bind-up of nutrients, and a negative impact on soil productivity. Measures to nutrient-enrich the material before application, or simply let nature take its long-term course may be the solution to those issues, but may not be understood by the general public looking for short-term positive results.
Moreover, biochar, unlike other organic matter soil amendments we have discussed previously (see October 2017 post at https://extension.usu.edu/dirtdiggersdigest/) does not readily feed soil biota, does not break down and cycle plant nutrients back to the soil, or form the organic glues important for building soil structure. It is mostly to be seen as additional nutrient and water storage enhancement in soils that may be stable for more than a thousand years.
The best I can currently recommend is to become educated about this potentially beneficial product so that when it becomes more commercially available, one will be prepared to make an informed decision on how to use it effectively. It may, in the end, most simply present itself as a desirable, socially conscious way of participating in carbon stabilization for the average person concerned about the environment.
Cornell University’s website on Terra Preta soils:
John Hopkins University publication on Terra Preta soils:
Colorado State University Extension bulletin on Biochar:
POST-POST NOTE: I have provided academic sources of information on Biochar and Terra Preta soils. Much of the popular press in “.com “ sources is promotional, or emotional in nature, thereby clouding its value as educational. Study responsibly!