I've been switching to raised bed gardening and I've found biochar to be an interesting way to "manufacture" my own soils. I love the texture and fluffiness biochar adds, similar to adding perlite, but it's almost free if I am willing to make it myself. I've noticed biochar can hold a lot of moisture, which is great for getting you through those dry periods.
WHAT IS BIOCHAR?
Biochar is a name for charcoal when it is used for particular purposes, especially as a soil amendment. In a sense, when we experiment with biochar or agrichar we are "rediscovering" an ancient farming technique that could revolutionize food production in modern times. Although the verdict is still out on how useful it may be in temperate climates, biochar has been used very successfully back to the time of Christ in the Amazon jungle, and some of those "Anthropomorphic" soils, also called Terra Preta de Indio or Black Soils, are still amazingly fertile to this day. In fact these soils seemingly grow of their own accord, and folks down their "harvest" it for potting soil, and it seems to be able to replenish itself if harvested wisely.
BENEFITS OF BIOCHAR
Biochar has a tremendous capacity for holding and storing nutrients until they are needed for your crops. This prevents important nutrients such as nitrogen from leaching away and creating an environmental hazard. In fact when you first apply biochar, you are supposed to supply extra nitrogen with it or it will tend to suck it all out of your soil. Some folks charge the biochar before hand by soaking it in a liquid fertilizer, compost tea, or even urine.
Biochar has a tremendous amount of surface area, and basically provides "condominiums", places for many beneficial soil microbes and fungi to live. This dynamic ecosystem seems to be the key to the productivity of "growing soil" in the tropics, in soil that under usual conditions is very low in organic material, due to high oxidation. This is the big question: can we develop these same kinds of systems in a temperate climate, like where I live in Iowa where the soil is frozen almost half of the year.
Carbon in the form of biochar is more or less permanent, it breaks down extremely slowly. It may turn out to be a useful way to sequester carbon, instead of letting it get into the atmosphere and cause climate change. Also, with traditional farming and gardening, you are always having to replace the carbon in the form of organic matter, in terms of constantly putting back organic matter in the form of mulches, composts, manures, green manures, etc. In fact the more you till the faster you tend to oxidize your organic matter. Biochar is much more resistant to this. Finally, biochar also reduces soil acidity and need for liming.
Pictured at left is how I make biochar in my backyard for my garden - I call it my "Three Barrel Retort". I used to punch holes in the outside barrels but found this is easier, and the barrels last longer if you don't do that. To start things out, I fill a 15 gallon barrel with chunks of wood, the smaller the better, from tree trimmings, or from 2x4 chunks I get for free from the lumberyard. These split quiet easily with a hatchet if you want. Make sure this wood is good and dry for a clean, efficient burn. The more you can stuff in your barrel the better. If you can find a 15 gallon barrel with a lid that is great because you have to flip your barrel upside down. Just hold the lid on with one hand while you flip it, and let it set loosely on top of its lid.
For the next step you slip a 55 gallon barrel over the top of your 15 gallon barrel, so that it's resting on 4 bricks or other similar spacers, so you have an air intake on the bottom. Then you pack the area between the two barrels with scrap-wood from the lumber yard or other sticks or small firewood. This firewood you light on fire, and roast the wood in the inner barrel to produce charcoal. This inner barrel then starts to ooze wood gas, a combination of smoke (particulate carbon), carbon monoxide, hydrogen, and a little methane. This wood gas doesn't get wasted, but bursts into flame, and that heat travels upwards and continues to roast that inner barrel and produce more wood gas to keep the process going. The first place I saw this system explained was by a guy named Folke Gunther, just Google his name and you will find his site. There are quite a few variations of this method on the web nowadays.
Traditional charcoal making is a pretty dirty and time consuming business, and involves a lot of smoky air pollution due to all the careful smoldering and tending involved. This method eliminates most of that pollution. I also go the extra step of creating a chimney/afterburner, to make sure I burn up any smoke or gasses that get to the top of the barrel. What I have been doing is putting a 55 gallon lid on top with about a 1 foot hole cut in the center, then I lay 4 railroad spikes as spacers, and put a 15 gallon barrel on top with both ends cut out. This the third barrel in my "Three Barrel Retort", and will also help increase the draft in your system.