What follows is my experience with biochar in my garden in southwest Virginia (USDA Zone 6b). Since 2009, I have been watching what happens when I add ďbiochar compostĒ to potting soil and to my permanent vegetable garden beds.
I have used biochar made in a pit by snuffing a burning heap of woody, prickly yard waste, and I do think there may be something to excavating a pit every few years and using it as an incinerator for organic matter (read here of my pepper-smoking pit). However, the most pleasing results so far have come from mixing wood charcoal gathered at a local campground with almost-done compost. Used in spring as the compost component in potting soils and planting holes, the cured biochar compost has a fresh fragrance and crumbly texture. I like using it.
So far I have found no vegetable crops that respond negatively to the incorporation of cured biochar compost into their root space. I have observed no miracles in terms of plant growth or productivity, though preliminary side-by-side plantings have made me curious about possible benefits to tomatoes in terms of early growth and disease resistance. Read below about my experiences with Black Krim and Constoluto Genovese in 2010.
Soil time is slow time, but now that Iíve been adding biochar compost to planting holes for three years, Iím seeing charcoal taking a permanent place in the soil profile. Two beds now contain noticeable amounts of biochar. One is planted with spring carrots and the other has garlic, so Iíll relate what I learn about those crops later this summer.
Cabbage grown with and without biochar grew to comparable size.
Broccoli produced similar-size heads with and without biochar in 2010.
This Year's Garden Biochar Projects
By Barbara, May 29, 2010
What kills me about biochar is this: we understand very little about how it works and its true benefit to soil and plants, but it sounds so interesting that some people are using it as an excuse to burn stuff that doesnít really need to be burned.
Glad I got that off my chest.
As I reported last fall, I mixed wood charcoal gathered from campground fire pits with finished compost and bagged it up to cure along with bags of the same compost without charcoal. This spring, I used the two batches of compost in several very small pot experiments. The rest went to in-garden observations.
This year, I want to observe the effects of cured biochar compost on several garden plants. Last year I saw productivity gains in winter squash. This year I wanted to look for effects in other species, because in my experience winter squash are very adaptive to weird soil conditions (they grow happily IN compost). What would happen if biochar became part of the rhizosphere (the soil biota in the root zone) of more sensitive plants like broccoli or tomatoes?
At this point I can report these observations, with more to come as the plants mature:
Cured biochar compost does not bother broccoli, cabbage, tomatoes, or peppers when mixed with potting soil at a ratio of 50:50 and introduced when seedlings have 3 true leaves.
Cured biochar compost mixed into the planting holes of broccoli and cabbage does not appear to affect growth one way or another. All of the plants are doing great, which is encouraging. The question now becomes one of residual nitrogen availability. In other words, does biochar compost in soil reduce N depletion from heavy feeders like broccoli? If I provide the soil samples, Iím hoping to get some help from Virginia Tech on that question.
Biochar compost may slightly enhance growth of tomatoes, but itís too early to say. Again my observation is very small in scale, and to be honest I resisted using tomatoes as a test crop because I didnít want to have to weigh every last fruit. But the truth is that I work for you, and you want to know about biocharís effect on tomatoes. So, if the blight doesnít get them, Iíll be weighing every fruit from side-by-side pairs of ĎBlack Krimí and ĎConstoluto Genoveseí grown with and without biochar in their seedling containers and planting holes.
There is a cucumber observation in progress, too, but it involves pickling cucumbers, and I'm not ready to commit to counting or weighing every one of those guys. Getting them into a pickle jar is much more important.
Like a lot of other people, I want to know how to best use biochar to enhance my gardenís health and productivity. If youíve been following the biochar story, you know the challenge: adapting a nearly miraculous soil-building method from an ancient Amazon culture to our own back yards.
Reviewing the research, it is clear that biochar (or horticultural carbon, as itís called in some parts of the world) benefits plants as long as (1) the soil is well-enriched with organic matter and plant-available nutrients and (2) the biochar comprises no more than 30 percent of the soil mass (relevant in containers). And, although all biochar is not alike, it appears that a one-inch deep layer of coarse crumbles is about the right amount to try as a soil amendment in vegetable gardens.
As for the making of biochar, Iíll let resourceful farmers and talented engineers figure out how to do it while keeping the air as clean as possible. For my garden research needs, Iíve found plenty for free in the fire pits in a nearby campground. The tent camping area, where responsible campers routinely douse their campfires with water, offered up especially fine pickings.
Now I must design small experiments that will help answer your (and my) questions on gardening with biochar, and its link to compost. The International Biochar Initiative has published an excellent guide for doing just this. With my limited time and resources, hereís my thinking:
Pot experiments, in which biochar is mixed with potting soil to compare plant growth with and without it, are good baby steps in this type of research, so Iíll set aside some of my supply to use during seedling-starting season in spring. I grow a lot of onion seedlings and havenít heard of much biochar work with onions, so right now they are topping my list along with eggplant, which I grow in containers all season anyway.
Iíll also do some micro-field experiments, in which Iíll compare plant growth with and without cured biochar compost. Last year I worked with squash, but this year Iím considering peas and beans.
As shown in the photo above, Iím ďactivatingĒ biochar by mixing it with moist, mature compost made from materials from my garden and kitchen. This mixture will be set aside in a cool place through the winter Ė its culture period, biologically speaking. By the time the biochar compost goes into the garden, the zillions of open nooks and crannies in the char should be well colonized by fungi and bacteria.
If plants do perform better with cured biochar compost around their roots (and there still are many ifs), it will give us a great old/new way to make the benefits of compost last longer in soil. All this and saving the planet, too? No wonder biochar is such a hot topic.
Combining biochar and compost
Campground treasures for the garden
Biochar vs. Layered Crater
By Barbara August 22, 2009
The fruits have been harvested and weighed, so my first experiment using biochar in the garden (begun in May and described below) is ready for final report. The plants grown without biochar in a layered crater grew faster early on and bloomed earlier, but in the end the winter squash grown with biochar won the contest. With about 15 pounds of butternut and delicata squash from the layered crater, and 20 from the biochar bed, I win all round.
But I still have questions. Was it simply later blooming that made the difference, or did the plants in the biochar bed produce better pollen or other reproductive organs? There are still many questions to ask and answer about biochar, so next year I'll run another experiment and keep you posted. Meanwhile, let us know about biochar studies you've been conducting in your back yard.
A few months ago I wrote about biochar for Mother Earth News, which scientists all over the world believe may be a boon to agriculture and environmental restoration. I wondered: would biochar made in a trench in the garden benefit vegetables?
Last fall, I began my experiment by establishing two new beds:
>The Biochar bed was dug 14 inches deep, then piled 4 feet high with hard-to-compost materials including brambles, invasive weeds and shrub prunings. I burned the stuff about halfway, then snuffed the fire with moist soil. A second burn was done 6 weeks later. In spring, as I filled in the last few inches of the trench with reasonably good soil, I mixed in a standard application of a balanced organic fertilizer.
>The Layered Crater bed was dug 14 inches deep, then filled in with alternate layers of garden compost, soil, old mulch, more soil, and so forth, along with the same amount of organic fertilizer used in the biochar bed.
Two seedlings each of ĎDelicataí and ĎEarly Butternutí squash were set out in each bed on May 20. Since then, we have had unusually abundant rain.
As of June 11, the plants in the Layered Crater are noticeably larger than the ones in the Biochar bed. Stay tuned! Updates to come, along with details on the next Biochar experiment Ė studding buckets of curing compost with chunks of charcoal from the wood stove.
Above, two species of winter squash appear to be growing well in a Layered Crater.
Below, the same squash are falling behind in the biochar bed.