There is, with all of us, a lens through which we see the world. And while each of ours is different—informed by our past, by our present, where we have come from—a common lens we can all share is the recognition that our globe is changing.
While climate change receives increased amounts of attention by global leaders, there is still a lack of intention placed on one of our greatest questions—how will we feed ourselves with increasingly unpredictable weather patterns? Global predictions on staple crops are that by 2050, climate change could cause irrigated wheat yields in developing countries to drop by 13%, and irrigated rice could fall by 15%. In Africa, maize yields could drop by 10–20% over the same time frame.
Predictions are difficult, however. The author and biologist Dr. Thor Hanson has spoken to the fact that predicting climate change is really impossible, because what we are learning is that climate change will be defined by unpredictability. But what we know is that change is coming.
Agriculture has been a part of the global carbon trauma that has brought us to this point in history. Research suggests that up through the turn of our century agriculture has been responsible for one-third of global greenhouse gas emissions. The production of food makes up the lion’s share of all food related greenhouse-gas emissions—86%. How we grow food is a part of how we heal our atmosphere.
As a part of this larger movement of carbon-based practices including reduced and no-till systems, cover cropping, green manures, rotational grazing, composting, etc—biochar is illustrating the ability to decrease carbon mineralization, making carbon more stable in the soil, while also increasing biological activity. This article will cover the ways charcoal may play a role in storing carbon where it creates soil resiliency in the biological food-web.
Carbon is the word we use to describe the life that came before, all of the organics that have come and gone and now rest in the soil. And we have risen the ghosts. The question of how we as farmers interact with the carbon cycle and store it in our soils is a part of not only addressing climate change, but also preparing ourselves for it.
Carbon farming is the science of resiliency. It tries to answer the question of how to create stronger, more nutrient dense foods with the ability to withstand greater levels of stress. It’s what so many local farms are already doing, focusing on lowering tillage, cover cropping, composting, rotational grazing, and seed saving. As my farming mentor Steve Bensel says, the pinnacles and practices of the original organic food movement are built around understanding the carbon cycle. The idea is to create a soil ecosystem to feed the soil to feed the crops.
Biochar in the carbon cycle
The term biochar can sound new, but there is nothing new about charcoal. Charcoal is as old as fire, and fire has served as a central element of thinning old growth forest ecosystems. Flames would consume the smaller trees and brush that had not developed fire retardant in the bark, with the larger trees remaining alive. The small trees would often smolder in the understory, leaving behind charcoal in the soil and in the bark of the larger trees. These black scars can be seen on many forest hikes, the semblance of past flames reaching up towards the green canopy.
The Salish people of the Puget Sound also used fire to cultivate their wild foods. While the first European settlers thought the forests of the Pacific Northwest were naturally abundant with food crops, they were actually gardens. The Salish used fire to drive deer and elk into central locations where they could be more easily hunted, and burned their most common root and berry crops after harvest. Controlled burns served to replicate the role of wild fires in old growth systems, thinning and cleaning the forest floor while providing ash and charcoal to fertilize the next generation of plants. There are illustrations that fire and charcoal were also used for agriculture historically in South America and parts of Europe.
Charcoal’s role in the soil
There is a large body of research to suggest that biochar can play a role in helping other carbon-based practices go further, stabilizing the native carbon in the soil. This goes beyond what I and many people thought, which is that biochar stores roughly 50% of carbon in the biomass converted through pyrolysis (the process of making biochar), which can last as a stable carbon source in the soil for thousands of years.
A paper published in 2014 found that charcoal reduced carbon mineralization and off-gassing by between 64.9-68.8%, a phenomenon they termed the negative priming effect. This means that between 64.9-68.8% of the native soil carbon—the carbon already in the soil—was stabilized and kept in the soil. The negative priming effect was then confirmed by a meta-analysis paper that examined 395 research publications on charcoal and carbon off-gassing, finding an average of 40% decrease in carbon off-gassing, known as respiration, while also seeing an average increase of 18% microbial biomass.
It is difficult to fully predict biochar’s influence in the soil. Different biochar made from different feedstock—wood, manure, crop residues—seems to have unique physical and nutrient properties. These different biochars are going to interact uniquely in different soil types. As the previous study found, temperature and pH of the biomass converted into biochar seems to have a significant influence on its properties.
Another paper found that the negative priming influence (carbon stabilization) was more pronounced with biochars produced at higher temperatures between 970 degrees F and 1,200 degrees F. A third study confirmed this finding, stating:
Consistent with previous studies, our results showed that higher pyrolytic temperatures led to increases in specific surface area and moisture retention while decreasing CEC and DOC content (Mukherjee et al., 2011; Wang et al., 2013). The characteristics of these high temperature biochars promoted greater negative priming of (Soil Organic Matter) in our study, and they have recently been advocated as the best type of char for maximizing soil C storage (Yuan et al., 2014).
The paper found that lower temperature biochars may have other benefits, showing increased ability to retain nutrients and moisture and immediately bio-available carbon. The paper concluded that both high and low temperatures of biochar may play unique roles in soil health, with high temperature biochars serving as a longer influence in stabilizing carbon levels, and lower temperature chars having a more immediate influence on nutrient retention, water retention, and biological activity within the first several years.
If you are interested in making biochar, here are two easy, low cost and low-tech ways to make charcoal from burn pile materials and/or material from your surrounding forests.
Conservation burns are the simplest and most accessible form of biochar production, as all they require is systematically stacking wood in a pile. The theory behind conservation burns is that all the feedstock reaches the same point of fully volatilizing organics, at which point the fire is put out. The method mimics many of the elements of slash burn piles, the most common method for brush and woody biomass removal, but generates a valuable product while also lessening the sterilization of soil in the location of the burn.
Conservation burns can be done on a variety of scales and with a variety of wood sizes, but requires a uniform range of diameter within each burn to work effectively. Conservation burns do require a significant amount of water to extinguish, so burns would ideally be conducted near a pressurized water source.
By lighting the conservation burn on the top, the coals in the flames are held in a less oxygenated environment as the fire moves down towards the newer material. These flames consume oxygen that would otherwise directly access the coals. Whereas the flame cap kilns (described next in this article) look to eliminate oxygen to the charcoal by raising the fire box and adding more material, conservation burns operate as one large fire zone where the operator puts the fire out at the stage they would otherwise add material to a kiln.
The success of Conservation Burns is dependent upon how the pile is constructed. Feedstock should be somewhat homogenous in diameter, with the largest pieces only several inches larger than the smallest pieces. Length can vary more significantly but keeping material to a range within 1-4 feet in differential will improve the rate of consistent biochar production. Material should be constructed in a cone, with the largest material placed in the center of the burn, with material graduating to smaller diameters as the pile moves towards the bottom and the top. This is because the center of the fire will produce the highest heat, and thus can char the larger material.
Once the pile is constructed start a fire at the top. If there is wind, the pile should be lit on the downwind side 2/3 of the way towards the bottom. Drier material is more important for conservation burns than other forms of production, as higher moisture content in the feedstock will result in a slower development of the fire, resulting in the top material turning to ash before the lower sections of biomass convert to charcoal.
The goal of the conservation burn is to have the pile torch quickly, with the fire moving from the top of the pile to the bottom in between 2-20 minutes depending on the size of the pile. The ideal time to extinguish the burn is after the fire has reached the bottom of the pile and a slight coat of ash can be seen on all the material from top to bottom.
Conservation burns, conducted with dry material, consume most volatile gases in the fire box, which feeds the fire and decreases emissions into the atmosphere, especially in comparison with traditional slash pile burns.
Extinguishing the fire
The ideal way to extinguish a conservation burn is by spraying the fire with pressurized water. It is important to make sure the pile is fully saturated before leaving, as often the pile can appear out but have a small amount of live coals that can dry out the surrounding charcoal and start the pile burning again.
Flame cap kilns
Flame cap kilns use a metal cylinder or box, with either an open or closed bottom. One of the significant advantages of flame cap kilns for farm and forestry production is that material can be loaded throughout the burn process, with the conclusion of the burn occurring when the kiln is filled with biochar. This allows for increasing the amount of biochar produced as well as eliminating larger quantities of slash material, making it ideal for forestry or on-farm production.
Flame cap kilns eliminate oxygen access on the sides of the fire. The area of active flame rises with the addition of new material, consuming the oxygen that would otherwise access the charcoal below. Kilns can be made from a variety of diameters and heights, with larger diameter and shorter dimensions resulting in hotter, quicker burns and lower efficiency of converting biomass to biochar. Taller and more narrow kilns result in cooler, more efficient burns. A general guideline for an ideal kiln size is 1.5:1 ratio of height to diameter, however many different ratios can be effective.
If you are cutting a metal cylinder or box to create a kiln, it is important to discover if the container has previously stored any flammable products. If so, fill the container with water before cutting, as a spark can catch residual gases left in the container, providing the potential for an explosion.
Flame cap kiln method
Starting the fire: Make a hot fire using kindling and small sticks. If the kiln has an open bottom, allow air to enter the kiln from the bottom to facilitate starting the fire, which can be done by simply shoveling out several access points where the cylinder bottom meets the soil. When the fire is burning well, usually 10-20 minutes after starting the burn, seal the bottom of the kiln with dirt so that no air enters from the bottom perimeter.
Adding material: Add material when you see the top of the existing material develop grey ash on the exterior. It is helpful to add material of similar size, moisture content, and wood type for each loading, as the material will transition to a similar stage of charcoal at the same time, maximizing the efficiency of the burn. Place the largest diameter material one third of the way through the burn process, as that is the point of maximum heat.
Continue adding fuel until the kiln is full of char or you run out of fuel or time. As the level of charred wood rises in the cylinder, the lower level of the outside of the cylinder should be cooler than the area of active burning, indicating that no oxygen is entering the kiln from below.
Analyzing the fire: The color of the smoke coming from the kiln will give some indication of the temperature and how clean of a burn you are conducting:
A clean burn with heat waves is the ideal.
White smoke is a result of steam off gassing and is not a significant issue as it is mostly water entering the air.
Blue smoke illustrates that some volatile gases are not burning and that the fire is cooler than ideal. Stop adding fuel or begin adding drier fuel to correct.
Yellow and greenish smoke is the production of methane (harmful greenhouse gas) and other volatile gases, which means that the fire has gotten too cool and gases are not being consumed by the fire. This is an important time to stop adding fuel and let the burn clean itself up.
Extinguishing the fire: There are two different methods for extinguishing Flame Cap Kilns: excluding the fire from oxygen with a lid or using water. The lid method is ideal for forestry application where water access may be limited.
To use the lid method, create a lid 1-2 inches smaller than the diameter of the kiln, so that the lid can sit inside. The ideal material for lids is used metal roofing that can be screwed together with roofing screws and cut with an angle grinder or Sawzall. Put 10-15 gallons of water evenly around the top of fire, and then add the lid inside the kiln. Seal the gap between the lid and the edge of the cylinder with soil. Steam leaks will indicate the need for a better seal.
Once the cylinder has cooled off entirely (this takes at least two days), tip it over to retrieve the biochar. Be very careful with fresh char, as it can hold heat for a surprisingly long time. If a still-live coal is exposed to air, it will burn. Make sure there is no heat in the char before transporting.
You can use the water method if there is plenty of water available close to the fire source (80-100 gallons). This method is more ideal for kilns that have closed bottoms, as the water will rise in the container and submerge the coals. If the kiln does not have a closed bottom, spray down the top of the fire and then tip the kiln over either by hand or by mechanical means, such as a tractor or come-a-long.
Spray the charcoal with water while raking it around. Continue stirring and wetting the char while looking for hot spots. Any steam issuing from the pile after 30 minutes of applying water indicates that the fire is not out. After an hour or two, return to the pile and spray and rake it again. It is highly recommended that the char be inspected several times more before assuming that the fire is out, and the char is safe to handle and transport.
We still have a lot to learn about the effects of putting charcoal in the soil. Biochar is not the answer to our agricultural carbon crisis, but it might be a part of it. Whether you think biochar is worth putting in your soil or not, hopefully we can agree that finding ways to store carbon in the earth is a part of our role as farmers to do our part in addressing climate change.
It is in our interest to find ways of communicating our use of regenerative practices to customers, and helping consumers understand how storing carbon in our soils is part of a climate solution and increased soil health. Regenerative agriculture is becoming a brand name, with the Rodale Institute launching Regenerative Organic Certification for farms using carbon-based practices.
While that program is still in its pilot phase, the concept is important enough that regenerative organic is terminology that can be used with customers. Let’s invite our customers to know how they are impacting the climate through their food purchases. Tell them the carbon story, and how it can be one of storing carbon in the soil and not the air.
Kai is the executive director of Forage, an ecology journalism nonprofit, focusing on studies and projects around plant resiliency in a changing climate. He runs a research market garden on his homestead in the San Juan Islands, where he examines the efficacy and economics of carbon-based cultivation practices.