Biochar, simply put, is charcoal that is used for agriculture. There is a lot of technical information available on the production of biochar and the benefits of its use as a soil amendment. No attempt will be made in this write-up to cover all of that information. What follows consists of some observations and opinions of the author, along with some basics which should enable anyone who wants to give biochar a try to do so.
First, let's get something out of the way. When you get charcoal to put in your grill, it often comes in the form of briquettes which have other things in them besides charcoal. Most of those "other things" are things that you don't want in your soil. Natural lump charcoal should be good, just make sure it has nothing added. The closer we can get to pure carbon, the better.
Much is made of the fact that biochar is charcoal which is made by excluding oxygen. This is marketing hype, pure and simple. There are some qualitative differences in biochar made at different temperatures and from different materials. ALL charcoal is made by excluding O2. This may be done by sealing the burning coals in a container, burying them or by dousing or submerging them in water. This needs to be done after the wood gases are driven off but before the remaining carbon turns to ash. One may also seal the raw material to be converted into biochar inside a container and apply heat. This process is known as pyrolysis. When pyrolysis takes place, volatile gases are released. These volatile gases must be vented and may be used to generate heat to assist the pyrolysis. If the wood gas is cooled, condensation occurs and the resulting liquid contains pyroligneous acid (wood vinegar).
Turning wood or other carbonaceous material into biochar is a good start but there are still a couple of steps before this soil amendment is ready to be applied to your farm or garden soil. But, before we cover that, let's cover the question I know you are just dying to ask. Why would I want to put this in my soil? Good question, I'm glad you asked!
Biochar in the soil has been compared to coral reefs in the sea. Much is being learned about the role of microbial life and fungi in the soil. Biochar provides a habitat for this microbial life. A single gram of this material, by some estimates, can have a surface area of more than a thousand square yards. This is due to its complex pore structure. With a useful life of more than 2500 years, it may outlast me and you. Add biochar once and you're done. Being almost pure carbon, biochar adsorbs and holds nitrogen, which is otherwise often leached out by heavy rain. There are other advantages, but hopefully these are enough.
Back to preparing biochar to be put in the ground. The next step is to crush it, grind it, pulverize it. The big boys often use a commercial roller mill. On a small backyard scale, I have used a sturdy metal tub and a two by four. I've heard of people putting biochar on a slab, placing a sheet of plywood on top and then driving a truck back and forth on the plywood. Some have used a chipper shredder with good effect. As one might guess, it is a messy proposition. Judicious use of water (with or without nutrients added) is highly recommended to keep black dust from blowing everywhere.
One last step, and one that is too often overlooked. This amazing material may temporarily tie up nitrogen in your soil. To avoid that, we will give it plenty ahead of time. This is often called "charging". In some parts of the world, biochar is just given a long soaking bath in human urine. Your mileage may vary. Compost tea, manure tea...you get the idea. Place the crushed charcoal in a water tight container, cover with burlap or similar porous material, weight it down and cover the whole thing with nitrogen rich liquid. Biochar may also be added to compost. If you do the compost method, soak the biochar in water first and add it after the pile has heated.
Someone discovered biochar a very long time ago. Terra preta soil is found in the midst of some of the poorest soil in the tropics. It is so rich and resilient that it is sometimes mined for potting soil. The local businesses mining it say that it regenerates itself. That is a good thing, isn't it?