One of my garden experiments this year was to make biochar to increase my soil's capacity to retain nutrients. Soils have varying abilities to hold elements such as calcium, magnesium, phosphorous, potassium, copper, boron, zinc, and manganese that are necessary for healthy plants. Imagine that soil is the (-) end of a big Energizer Battery. Most of the chemical nutrients are the (+) end of the battery. Soil with a lot of stable organic material (humus), clay, or biochar (a fancy term for finely-ground charcoal) has more (-) battery ends and attracts more (+) nutrients and prevents the nutrients from leaching away during heavy rains. Plants, a.k.a. nature's biochemical wizards, obtain their nutrition from this soil battery. This whole battery concept is known as the soil's Total Cation Exchange Capacity (TCEC - a topic for another day...and for hardcore science/garden nerds). It's all about nutrient dense gardening. For more reading on TCEC and soil nutrients, check out Steve Solomon's book The Intelligent Gardener: Growing Nutrient Dense Food. I sent a soil sample to an agricultural laboratory and found that my garden is a pretty weak battery.
So, in addition to the regular applications of compost, I will add biochar to the garden next season as part of my ongoing effort to build powerful soil. My source of charcoal is the leftover briquettes from my last barbecue. I grind the briquettes into a fine powder using a rock and store the resulting biochar in a 5-gallon bucket.
I also have quite a bit of tree bark left over from woodsplitting. The bark makes a nice campfire and produces ample charcoal that is easy to grind (when fully extinguished, of course!). I maximize the amount of charcoal by dousing the fire with water rather than letting it burn down to ash. I think it's so cool that a byproduct from one operation becomes the input for another. There's no waste...just resource cycling.
I layer biochar into the compost heap rather than adding it directly to the garden. Based on my research, this step is critical. Raw biochar has to be "charged" with nutrients and biological life in order to make it effective in the garden, otherwise it can inhibit plant growth for a year or two. The compost heap is loaded with nutrients, bacteria, and fungi and is a great place to age the char for a year or two.
Biochar is also a simple way to sequester carbon, since the char molecules will persist in the soil for hundreds or thousands of years without releasing carbon dioxide to the atmosphere. Pre-Colombian Amazonian farmers used biochar to fortify notoriously poor rain forest soils. These ancient farmlands are still fertile as a result of biochar. Let's see if it works in my garden!