How to use biochar in plant experiments

Here are some suggestions that can help you conduct valid and meaningful biochar experiments with plants

1. Find a source of biochar. Biochar is just charcoal, but charcoal can have very different properties depending on feedstock and process conditions. Potential sources of biochar include the following:
  • Campfires often have chunks of charcoal remaining after fires. You can use a campfire to make charcoal – burn the wood until you have a pile of glowing coals. Then quench the fire with water instead of letting the coals burn to ash. You will be left with some charcoal.
  • In most regions, it is possible to purchase lump wood charcoal for grilling food. This charcoal has a high content of volatile matter and will work better if it is crushed into smaller pieces and “aged” for several months in a compost pile before use. Never use charcoal briquettes as they may contain additives that could be harmful to soil.
  • One biochar researcher uses spent charcoal from an aquarium filter in her houseplants. This is impregnated with fish waste nutrients.
  • Build our tin can biochar-making stove to produce small amounts of charcoal. Instructions for making the stove are on the Dome School wiki site at
  • Those with advanced metal fabrication skills can search the Internet for open source plans for clean pyrolysis units made from metal drums and other materials.

2. Add nutrients to biochar. Biochar is a soil conditioner, not a fertilizer. You can add plain biochar to soil along with manure, compost or other fertilizers. Since biochar is porous and holds nutrients and water, you can also pre-charge it by soaking it in a liquid fertilizer. At the Dome School, we tested fish emulsion, liquid seaweed and urine. The urine formulation did the best.

3. Pre-test your biochar. Use two simple tests, a worm avoidance test and a seed germination test (see #5 below) to screen out biochar soil mixes that have harmful compounds.

4. Decide what variations to test.
  • If you only have one type of biochar-fertilizer combination, test different concentrations of biochar in soil. A good place to start is a 2:1 by volume, soil to biochar mix. Try both a higher and a lower concentration. If you have more than one type of biochar, try several concentrations of each.
  • Pay attention to your control. Commercial potting soil generally contains all the nutrients young plants need, so you might not see a big difference between your biochar plants and your control. Try digging up some plain soil from nearby to use as your control. Research shows that biochar has the most dramatic effect on poor soils like those found in the Amazon and other tropical areas.

5. Resources for biochar experiments. The extension service of the International Biochar Initiative has a number of fact sheets, technical bulletins (including a bulletin on the “simple tests” mentioned above) and guides that can help you set up a biochar experiment. These are available at It is also worth visiting the IBI Biochar in Schools page at: to see what others have done.