Biochar is a form of fine-grained charcoal that can hold onto tremendous amounts of microorganisms, nutrients and water when added to the soil. It is created by slowly burning organic matter in a restricted flow of oxygen, a process called pyrolysis. Once the material is reduced to charcoal, the burning is stopped and the charcoal is crushed and added to the soil.
In the burning process, the plant materials are heated to several hundred degrees, which drives off the moisture and releases the plant oils and gasses. The charcoal is left with many tiny holes and spaces that increase the surface area of each particle – spaces that are filled with microorganisms, nutrients and water.
Biochar is not a new idea. The ancient civilizations of the Amazon rainforests made Biochar from the abundant biomass as early as A.D. 500 and created the Amazonian dark earth, or “terra preta.” The tribes grew cassava, corn and fruit, made rich with mulches, compost and Biochar. They continued farming this way until the arrival of the Spanish in the 1500s brought European diseases that destroyed the civilizations. Even 500 years later, these soils remain fertile.
So how do you use Biochar in your garden?
If you buy it, it usually comes as a coarse to fine black powder. If you make your own, it will be chunky, but easily crushed with a shovel or hammer. A recipe I found on a bag of Biochar I purchased at Northwest Seed suggested mixing one part Biochar with four parts potting soil or compost and using the mix in containers. Another way is to mix Biochar into a compost pile. It will soak up the nutrients, water and microorganisms already living in the pile and be fully alive when it is added to the soil. Biochar doesn’t break down over time, so once in the soil, it will be available for a very long time.
Making your own Biochar is not difficult, but it does take some planning. On a small garden scale, gather dried plant debris, small sticks from pruning work or dried animal manure. Dig a trench deep and wide enough to hold the material and pack it into the hole. Set it on fire and burn it until the smoke turns from white to gray and thins. Now cover the trench with an inch of soil to block the air from reaching the fire. When all the material has been reduced to charcoal, apply water to put the fire out. Dig out the charcoal and break it into small pieces.
Now, here’s the big caveat. You have to be in an area where it is legal to burn, and you must follow the clean air rules dealing with smoke. You also absolutely can’t do it during fire season. We all know what might happen if you do.
Pat Munts is the co-author, with Susan Mulvihill, of “Northwest Gardener’s Handbook.” Munts can be reached at pat@inland nwgardening.com.