What is Biochar?

And other Frequently asked questions


“Biochar is a solid material obtained from the carbonization or thermochemical conversion of biomass in an oxygen-limited environment. 

In more technical terms, biochar is produced by thermal decomposition of organic material (biomass such as wood, manure or leaves) under limited supply of oxygen (O2), and at relatively low temperatures (<700°C). This process mirrors the production of charcoal, which is perhaps the most ancient industrial technology developed by humankind. Biochar can be distinguished from charcoal — used mainly as a fuel — in that a primary application is use as a soil amendment with the intention to improve soil functions and to reduce emissions from biomass that would otherwise naturally degrade to greenhouse gases.”

The International Biochar Initiative  

How many different kinds of biochar are there?


There are many different kinds of biochar or carbonized biomass. The most common feedstocks for biochar include wood, grasses, bamboo, crop waste (nut shells, corn cobs, fruit pits, rice hulls, etc.), manure and bone. These materials will have very different properties. For instance, manure biochar has a higher content of ash as well as nutrients. Most grasses are high in ash and minerals. Dense nut shells make a dense biochar. Of all the feedstocks, wood biochar is probably the easiest to use. It can make a very pure carbon biochar that is easy to handle and inoculate with desired nutrients. 

How does the carbonization temperature affect biochar properties?

Biochar production temperature has a big impact on biochar properties. At lower temperatures, only some of the carbon will be converted to highly porous charcoal. Tars and oils will remain that plug up the pores, limiting their ability to absorb nutrients and hold microbes. Charcoal briquettes and lumps used in the BBQ are made from low temperature char, because the tars and oils provide additional fuel for cooking. At high temperatures, like those found in a biomass boiler or furnace, most of the tars and oils are burned out, leaving a pure carbon char that is mostly carbon and minerals from ash. This high temperature biochar rivals activated carbon in the amount of reactive surface area that it contains.

How is biochar different from activated charcoal?

Activated charcoal is made from charcoal (often from dense coconut shell or bamboo charcoal) using either steam or acid chemicals to open up more pores in the material, creating additional surface area. This energy intensive process has a conversion efficiency of about 20%, converting one ton of charcoal to 200 pounds of activated charcoalwith a surface area of about 1200 square meters per gram. ROGUE BIOCHAR™ is highly porous, with about 450 square meters per gram. In some applications, it could be preferable to use three times the amount (600 pounds) of ROGUE BIOCHAR™ instead of 200 pounds of activated charcoal. Six hundred pounds of ROGUE BIOCHAR™ will provide more surface area, and prevent the loss of 800 pounds of charcoal carbon being oxidized into CO2. 

How is biochar different from BBQ charcoal?

Charcoal that is meant for fuel is produced at low temperature in a smoldering process that is highly polluting. In the US, charcoal kilns have been fitted with afterburners to clean the emissions. Cooking charcoal is not very porous because its pores are clogged with condensed soot, tar and oil. This is fine for fuel, but not a material you would want to put in soil. It will not perform well in other biochar applications such as water filtration that require high surface area.


How would you go about matching a type of biochar to a particular application?

It’s best to work with an agronomist who is already familiar with biochar. Also useful is the International Biochar Initiative’s (IBI) biochar classification tool. This tool focuses on these variable properties of biochar:

  • Carbon storage value
  • Fertilizer value (P, K, S, and Mg only)
  • Liming value
  • Particle size distribution

Depending on your application, you may want to choose a biochar that is long-lasting for carbon storage, or you may be more interested in a biochar that has significant fertilizer value, such as manure biochar. ROGUE BIOCHAR™ has superior carbon storage value, based on the Hydrogen to Carbon ratio (H:C). It has limited fertilizer value, but it does have liming value of about 10% of the pH neutralizing capacity of lime, on a dry weight basis. ROGUE BIOCHAR™ has an ideal particle size for many applications. Half of the material is between 1-2 mm in size, and the rest is mostly less than 4mm, with very little fine dust. This granular size is easy to blend with soil or other substrates, and it works well in water filtration applications.

Do I need to charge my biochar with nutrients and microbes before putting it in soil?

Sometimes charging biochar is absolutely necessary, and sometimes it is not. It depends on the soil (especially soil pH) and the amount of biochar you use. If you want to add a lot of biochar all at once, it is best to add nutrients at the same time. The easiest way to charge biochar with both nutrients and microbes is to compost it with other organics. Here is a good article from David Yarrow on how to prepare biochar for soil: Biochar: Prepping it for Soil Application, by David Yarrow.

How much biochar should I use?

In soil, it is best to add biochar in small amounts every year and allow it to slowly build up in soil where it gets charged with nutrients and soil life. Depending on the soil type and pH, the crops you are growing, and the other additives, nutrients and inoculants, you may add as little as 1% and as much as 20%. A small amount of biochar banded under seeds where it will work in the root zone can be very effective. Up to 20% biochar added to container media along with appropriate nutrients can also work well. Be aware that large amounts of biochar can significantly change the hydraulic conductivity of the soil. If not covered with mulch, soil with large amounts of biochar can wick water to the surface where it evaporates and dries the surface of the soil. 

How hard is it to change the pH of biochar?

Because ROGUE BIOCHAR™ is made in a clean, high temperature, open combustion process, it includes a small amount of wood ash. This ash contains valuable minerals, but it also raises the pH of the biochar to around pH 8.5. This alkalinity can be a concern if applied unchanged to alkaline or calcareous soils. If applied to acid soils, such as those found in the Pacific Northwest region, the liming capacity of ROGUE BIOCHAR™ can be a great benefit. There are several remedies that can lower the pH of ROGUE BIOCHAR™ if it is a concern. These include composting it with other organics, fermenting it with a lactic acid bacteria culture, rinsing it with water, or treating it with acidifiers such as vinegar, citric acid or liquid smoke. All of these treatments will also enhance the biochar with additional nutrients and/or beneficial microbes.


How long will biochar last in my soil?

Different kinds of biochar have different residence times in soil. Generally, biochar made at lower temperatures will have more degradable compounds that can become food for microbes over time. Biochar made at higher temperatures contains more stable carbon that is harder for microbes to break apart and use as food. Biochar stability is estimated by determining the ratio of hydrogen to carbon. Hydrogen is mostly burned off during the carbonization process, but some of it always remains, usually as part of a compound with carbon (hydrocarbons). The more hydrogen is detected in the material, the more hydrocarbons are present. The threshold value for distinguishing between biochar and other materials like uncharred wood, is an H:C ratio of 0.70. The H:C of ROGUE BIOCHAR™ is 0.25.

What safety precautions should I take when using or applying biochar?

Dust can be a hazard when mixing or applying biochar. Workers should wear a dust mask such as the 3M™ 8511 Particulate Respirator - N95 to protect their lungs when working with dry biochar. Moistening biochar can help a lot with dust control. ROGUE BIOCHAR™ is shipped with about 30% moisture content to help with dust control. Protective eye-wear will reduce the chance of getting dust in the eyes. Gloves are also a good idea. 

Where can I find more information on biochar?