What is Biochar?

An orange blaze flickered through the air as smoke swept skyward from a six-paneled metal box — a flame cap kiln — holding a slow, steady burn of wood and brush. On this sunny Mother’s Day at Mermaid Farm, Maggie Craig, clad in a yellow fire jacket and face shield, stood close by, steadily feeding the flames. What would eventually emerge from the embers wasn’t ash, but something far more valuable: biochar. 

“Biochar is a legacy of the Indigenous burning practices,” Maggie said after she lit the kiln — also known as the Umptopia 3.0, her third flame-cap kiln on the Island, built from sheet metal, roofing, and barn rails.

Maggie has been championing biochar on Martha’s Vineyard for several years. As a second-year Martha’s Vineyard Vision Fellow, she’s leading a biochar project in partnership with Island farms, conservation groups, and waste management facilities. Her sponsoring organization is the Martha’s Vineyard Commission. She collects wood waste, builds kilns, and gathers data on biochar production. She also researches its uses in soil and water systems, and spreads the word through hands-on demonstrations at places like Mermaid Farm, Whippoorwill Farm, Allen Farm, Native Earth Teaching Farm, and John Keene Excavation. 

So … what exactly is biochar?

Picture a block of charcoal — that’s essentially biochar. It’s created by burning wood waste or other organic material (biomass) in a low-oxygen environment through a process called pyrolysis. Maggie uses a flame cap kiln, which creates high heat while limiting oxygen below the flame cap — a key condition for pyrolysis to occur. The result is a jet-black, carbon-rich, porous material that can be used in gardens or on farms to improve soil health, store nutrients and water, increase plant growth, and sequester carbon. Biochar can also treat water. It’s a way to turn local organic waste into a powerful climate solution. The carbon-based molecules in wood are converted into stable forms that resist microbial decay, which means biochar can remain in the soil for thousands of years. Smaller pieces mean more surface area for absorption, so it is recommended to crush biochar to less than half an inch, though in soil and compost, some larger pieces are okay “because it will help keep things aerated,” Maggie says. 

Maggie sources wood waste used to make biochar from a wide network of partners: South Mountain Company saves construction cutoffs, BiodiversityWorks dries out invasives like bittersweet collected during their community blitzes, and local landscapers and arborists contribute waste from their jobs. “When you start to do these processes, you cease to think of it as waste,” Maggie says 

That shift in thinking is part of what drew her to this work. A longtime summer resident, Maggie returned to the Island in October 2020 after finishing a summer internship at Capital Press, an agricultural newspaper in Oregon. She had just earned a biology degree and hoped to pursue science writing.

“It was a very interesting time out west. Not just the pandemic — there was a social movement underway; there were massive wildfires,” Maggie says. While working on a story about turning conifers into biochar to restore oak habitat, she was struck by the material’s potential. And back on the Island, she kept seeing landscape crews with “truckloads of fodder perfect for a flame cap kiln,” Maggie says. “It seemed like it had so many solutions to environmental and potentially social issues … I moved here [in 2021] with the hopes of exploring it more.” She worked as a mobile mechanic while she looked for grant opportunities, and in 2023, she found one. “The Vision Fellowship has been an excellent match,” Maggie says. 

The kiln Maggie uses was designed by her mentor, Ken Carloni, who’s based in southern Oregon. She first met him while reporting on that original story. The double-walled kiln at Mermaid Farm is the third she’s built on the Island. The first two — the Umptopia 2.0 and the Daisy Chain (a series of 55-gallon barrels riveted together in the shape of a daisy) were also based on Carloni’s designs. Maggie’s kilns are designed to be portable and modular.  

“Ideally, the kiln is set up where the wood waste is generated,” Maggie says.

Her burns typically last about four hours, during which she steadily feeds the flames to maintain an even temperature. The wood at the bottom is pyrolized and falls into the oxygen-deprived zone beneath it, turning to char. 

“Biochar is happening in a regular open burn pile, but within a flame cap kiln it’s controlling the airflow in a way that makes more biochar,” Maggie says. “It becomes really artful how you operate a flame cap kiln to make as much char as possible,” she adds, noting that her process must be manual — machine loading can smother the flame. 

At Mermaid Farm, Maggie and a few volunteers layered the smaller bits of yard waste on the bottom of the kiln, then slowly added larger and thicker pieces of wood on top. A top layer of dry kindling was added for ignition, and the kiln was lit from the top, creating a flame cap that helped burn up smoke efficiently. As the initial pile burned down, Maggie added more material, and she included larger pieces as the kiln heated up. The larger boards were crisscrossed in order to allow a small amount of air to flow through the entire pile. Volunteers chucked hay and small kindling on top to keep it going.

After the burns, Maggie often hands out samples for people to experiment with at home. But before it can go into the garden, biochar must be “inoculated.”

“When you make biochar, or biologically activated char, it’s not biologically active yet if all those pores are still empty,” Maggie says of the material’s porous surface area. “They need to get charged up with nutrients to be able to offer that up to the plants in lieu of them sucking the nutrients away from the plants or locking them up in the soil.” 

Biochar is highly absorbent and negatively charged, meaning it’ll attract positively charged nutrients. That’s good — once it’s charged — but if added raw, it can deprive plants of nutrients.

“It acts like a mirror to whatever is already there,” she says. 

To avoid this, Maggie recommends mixing biochar with something nutrient-rich, like compost. “The suggested portion is 10% by volume into compost,” she notes. (See box below for more details on recommended biochar-to-compost ratios.)

There are other options, too. “Urine is a really, really good one,” she says. “It has a lot of nitrogen. Honestly, this is a big win. If people are peeing into a bucket full of char, letting it sit for a couple of weeks, you’ve got ready-to-go char for the garden — and that’s urine that doesn’t end up in the pond. It’s very easy DIY urine diversion.”

After it’s charged, “biochar in the soil is holding water, holding nutrients, and providing habitat for microbes that are very beneficial to the soil, and it kind of charges the soil,” Maggie says.

Master gardener Roxanne Kapitan started using biochar after meeting Maggie two years ago. Maggie gifted her a sizable bag, and Roxanne has since incorporated it into three areas of her gardens. She mixed it with rainwater and urine, let it sit for about a month, and diluted the mixture before applying it. (Rainwater contains a lot of ambient nitrogen, Roxanne notes.)

She’s seen real results. A beech tree that had shown signs of viral decline is now leafing out. Lilacs that never bloomed produced profuse flowers. A native dogwood that hadn’t bloomed in years came alive again. 

 “I feel like everything I applied biochar liquid to — after the dilution process — and then scattered the remaining char at the base, it made a difference,” Roxanne says. 

She’s now layering it into compost to see how it performs over time. “I think there’s merit in it,” she says. “People just need to be patient. Most gardeners want instant results from fertilizer, but that has such a large carbon footprint — from production to packaging. I just think biochar has huge potential.”

“It’s really for people on the cutting edge of gardening — beyond Miracle-Gro and granulated fertilizer,” she adds. “It requires dedication and some attention.” 

Maggie also conducted a study with Island Grown Initiative to compare how well char charged by different resources could grow calendula. She tested urine, fish fertilizer, compost extract, and chicken manure. For the chicken manure, the char was used as bedding and incorporated into the feed of chicks at North Tabor Farm, with help from longtime farmer Matthew Dix. The results were promising. “The urine was very consistent — the calendula was robust through the whole season,” Maggie says. “And at the very end of the season the chicken manure kicked in.”

For additional insight on the poultry side, Maggie consulted Jefferson Monroe, a pig and chicken farmer formerly of the GOOD Farm. He uses biochar regularly and swears by its benefits: It cuts down on smell and helps prevent disease. “He told me one time he ran out of biochar, the smell was so bad that he wouldn’t be a pig farmer if he didn’t have biochar,” Maggie says. 

Biochar can also clean water. Maggie recently launched an experiment in Tashmoo Springs Pond, near the pumping station, to see how well biochar can remove excess nitrogen and phosphorus from runoff. “There are lobster pots full of char in laundry bags,” she says. “And I’m going to be sampling the water above and below the char pots over the next six months.” The pond has long battled algae issues, and the char could offer a natural filtration tool. “The char that’s in the pond is raw char,” Maggie says, “so it removes the [unwanted] nutrients from the water.”

For now, biochar isn’t commercially available on Martha’s Vineyard, though Maggie shares her own. Nearby, Bob Wells of New England Biochar in Eastham produces and sells biochar compost blends. He also fabricates retort kilns — midsize, stationary kilns that burn biomass. 

Sheriff’s Meadow Foundation has produced some biochar using an air curtain burner — a box-like incinerator they obtained special permission to use during their southern pine beetle mitigation efforts. Much of the infested material was processed through this system, and the resulting biochar is slated for distribution to the Town of Tisbury and Morning Glory Farm. Tisbury plans to use it to filter stormwater runoff from a storm drain at Owen Park before it enters the harbor. Morning Glory will mix it with compost and apply it on the farm. 

“We also had a nice offer from SBS to bag it up and sell it at the store,” says Adam Moore, president of Sheriff’s Meadow. The organization won’t be able to produce more, however — the incinerator must be returned to its owner in New York. Sheriff’s Meadow had the finished product tested at a lab, Moore adds, and it came back 90 percent carbon. “We’re all learning from each other, and I think there’s a lot of potential for this,” he says.

Biochar’s origins trace back thousands of years. In the Amazon basin, Indigenous people created “Terra Preta” — dark, fertile soil enriched with charred biomass. Maggie also learned about the Umpqua tribe in Oregon, who used fire to maintain oak habitats. That sparked her curiosity about indigenous fire stewardship on the Island.

“I believe there’s a lot of evidence that the Wampanoag used fire to steward the land as well,” she says. “Biochar is very clearly a legacy of indigenous burning practices.” 

Even routine landscape burning deposits pulses of char into the soil. Maggie recalls a conversation with Wampanoag elder Kristina Hook. “She remembers gathering with her whole family every year to burn the garden,” Maggie says. “Her grandfather would go out to specific berry patches to burn them. We have this impression that it’s ancient history, but it’s still very present.” 

Maggie plans all the burns herself, but gets plenty of volunteer help. A grant from the Ag Society’s Healthy Soils Fund has helped pay for extra hands at demonstrations. You may have seen her outside last year’s Climate Action Fair at the Ag Hall. 

As awareness grows, so does interest. Maggie now does private burns at homes, like one in Edgartown where neighbors contributed yard brush. She turned it into biochar on-site. The homeowners plan to convert their front yard into a native plant meadow — and hope biochar will give it a healthy start. 

Due to open burning regulations, these projects can only take place on private property between January and May, with permits and coordination from local fire departments. But with each new burn, Maggie’s network grows — along with curiosity about turning waste into soil-building, water-cleaning carbon gold. She plans to continue her work and expand on it by merging the kiln into forest management.

As she sees it, biochar isn’t just a soil amendment. It’s a tool for reconnection — with land, with waste streams, with ancestral knowledge, and with each other. 

“I think we could all slow down, and the world benefits,” Maggie says. 

Want to host your own neighborhood burn? Email [email protected]. Maggie’s biochar project also accepts donations via check, made out to the Martha’s Vineyard Commission attn. Biochar Fellowship. This will help compensate flame-cap kiln workers. 

Lucas Thors contributed to this story.

Recommended Ratios When Mixing Biochar With Compost 

The general recommendation is to add 10% biochar by volume to your compost pile. “But it’s more complicated,” Maggie says. Depending on the biochar, the materials available, and the end goal, the recommended range is anywhere from 3% to 25%.

Biochar will alter the C:N (carbon-to-nitrogen) ratio of compost. This is the balance of browns and greens that make up an active compost. Without biochar, the C:N ratio is 30 to1. Biochar can alter it up to 100 to 1.

Carbon: Biochar is mostly carbon, but only 5 to 30% of that carbon will break down in the compost. The range is due to the pyrolysis temperature. Biochar made in high-temp pyrolysis is a more stable form of carbon — microbes live in it, but can’t eat it up.

Nitrogen: Biochar absorbs nitrogen, so you need more nitrogen to compensate for the nitrogen that gets locked up in the char. Adding biochar without compensating the nitrogen it locks up means that it will just take longer for the same ratio of ingredients to make soil.

Another factor to account for is moisture. Biochar holds water, so an enclosed system will need more watering to keep the pile active. 

If you already have a composting system figured out, the easiest way to incorporate biochar is to add biochar to a high-nitrogen substance first (i.e. manure, fish fertilizer, urine), and then into your regular compost C:N regimen. This is recommended by The Biochar Handbook, by Kelpie Wilson, a good guide for all things biochar.

Preserve Nutrients With Effective Microorganisms

The following is an excerpt from Kelpie Wilson's The Biochar Handbook (Chelsea Green Publishing June 2024) and is printed with permission from the publisher.

When you have very high-nitrogen ingredients like food waste, urine, grass clippings, and fresh manure, add biochar as the waste is generated to preserve nitrogen that wants to volatilize. In my vegetable production system, I use bokashi [fermented food scraps] to pickle and preserve nutrients from my food waste in a precomposting process before I add it to the compost pile. Here is my recipe for handling food waste. 

Pickled Kitchen Waste

Kitchen waste can go in your outdoor compost pile, but it may attract pests and cause other problems, especially if you have dairy or meat scraps. Processing it first in a bokashi bucket is the way to go. You can keep the bucket under the sink in the kitchen, in the garage, or outside on a porch or balcony. Pickling kitchen scraps reduces odors and preserves the nutrients in the food scraps. 

Ingredients

• 1 gallon (4L) dry biochar

• ½ gallon (2L) dry bokashi 

Procedure

1. Mix together half of the biochar with the bokashi. Put a couple of inches (5 cm) of the remaining biochar in the bottom of a 5-gallon bucket. Make sure it is as dry as possible because it needs to soak up all the liquid from the food waste. Some advocates of using bokashi with food waste recommend using a bucket with a drain to remove the liquid periodically, but biochar can just absorb it. If the food is really wet, add more biochar. 
   
2. Start adding food scraps as you generate them. Whenever you add a couple of inches of food scraps, sprinkle about a cup of bokashi/biochar over the top. Use a board to tamp it down and remove air from the waste. Soon you will see a white fuzz growing on top. This is a beneficial, filamentous bacteria called actinomycetes. Keep the lid on tight, and when the bucket is full, you can either dump it onto an ongoing compost pile, or add it to a larger barrel … The pickled nutrients will be preserved. This is a much more potent fertilizer than you get if you just toss food scraps in an open compost pile where they mostly dehydrate and lose nutrients.