What is Biochar?
Florida soil is really bad. Soon after a heavy rain it is dry as a bone. You add fertilizer but it is soon washed down out of the reach of plant roots, or worse it runs off into the waterways. Or you add mulch to preserve moisture and nutrients, keep the roots cooler, and fend off weeds, but before you turn around the mulch has broken down and needs to be replaced.
We don’t have it as bad as the rain forests of the Amazon basin, where conditions can quickly wash away any soil life. Farmers who clear a field using slash and burn may have one good crop (because of the ash–potash from the fire) but over the next year or two, the soil becomes dead and they must move on to the next part of the forest. But there’s a better way. It was known to the ancient people of the area, but they were wiped out by disease brought by European explorers centuries ago. Only recently have their secrets been re-discovered. There are areas of the rain forest that contain deep rich black soil that is tremendously fertile even after hundreds of years. It turns out these fertile areas were man made. [BBC documentary “The Secret of Eldorado” 1 hour]
The secret? Terra preta, “black earth” in Portuguese. What’s the difference between Terra preta and typical dead soil? Charcoal. Slash and burn creates polluting smoke and soot but leaves nothing behind but a cleared field and some ash. What the ancient Amazon people did might be called slash and char. They still cleared the field, and still burned the plant material but did so in a closed pit that kept out oxygen. They were left with charcoal. Today we call it biochar, or charcoal created and used as a soil amendment.
Biochar retains all the networks of fluid carrying vessels found in the original plant material. That gives it an incredible amount of surface area compared to the volume. Plus it has a slight electrical charge which attracts to it both water and nutrients, allowing it to store those elements for later use.
Most of the organisms that give soil life are really tiny. A horse needs 1-2 acres of good pasture. That’s as much as half a ton of livestock per acre above ground. But that same acre could contain many horse’s weight of other life forms that live below the surface: a teaspoon of pasture soil could have a billion bacteria, a million fungi, thousands of amoebae. Organic material is broken down over time and consumed by these soil organisms. The microorganisms that break down plant material don’t break down charcoal. Instead they live in it. And they share. Mycelium, the vegetative part of fungus, colonizes plant roots and extends out in the soil to bring in water and nutrients for the plant in exchange for sugars made by photosynthesis. Adding biochar to the soil provides a storehouse for water and nutrients that can be used by plants, and more importantly by the tiny life forms hidden from view.
Too much carbon dioxide in the atmosphere is causing climate change. Simply reducing the amount of greenhouse gases produced will at best only slow the damage. There is a tremendous amount of biomass produced and wasted, much of it taken to the landfill. It breaks down relatively quickly and the carbon returns to the atmosphere as carbon dioxide and methane (which is a far worse greenhouse gas). Even if the biomass is treated as a resource, composted and returned to the soil, the carbon would still soon return to the atmosphere. Making biochar involves burning off some of the material. Not more than the natural breakdown, just faster. But a significant amount of the carbon turns into a stable form that remains in the soil over time. The half-life of compost is a few years up north, maybe one season in the sub-tropics. The half-life of biochar is hundreds of years. The result is that biochar is carbon negative–it removes carbon from the cycle and puts it to good use. Adding biochar along with compost and mulch creates a winning combination.
Videos:
- Wae Nelson TED talk–Biochar: The oldest new thing you’ve never heard of (10 min) Wae used to publish Florida Gardening Magazine
- Rob Lerner TED talk–Biochar, Putting the Carbon Genie back in the bottle (12 min) Biologist, environmentalist, and entrepreneur
- Dr. Johannes Lehmann–Biochar: up to 300% greater productivity
(9 min) Cornell U. Professor & Chair of International Biochar Initiative
Making Biochar: Biochar is made using a process called pyrolysis. Here, that means charing at high heat in the absence of oxygen. Oxygen is then re-introduced in an after-burner to burn the volatile gases, smoke and soot–at the same time increasing air flow via the chimney effect. Since the oxygen is consumed in the initial burn, there is not enough oxygen left above the burn line to turn the charred material into ash. Once the burn is complete, the container is either sealed or drenched with water to stop the material from smoldering into ash.
- Tiny scale in a single tin can: so simple, really no excuses not to try it.
- Small scale in a gallon paint can, handout from Wae Nelson
- Small scale using a wood stove (13 min) – EdibleAcres: make biochar while heating and cooking.
- Large personal-scale operation (15 min) – John Rogers in Melbourne using 4 sets of 55 gallon drums. You could also use 30 gallon or 5 gallon metal containers.
- Biochar workshop in 5 parts (about 40 minutes each) scaling up, using the energy generated from the process and minimizing the emissions. Worth the time to get an in-depth class on biochar.
- New biochar production facility Living Web Farms creating biochar for their farm in Florida, refining the above workshop design, capturing excess energy for use in the greenhouse and drying, separating volatiles for use on the farm; experimental continuous-feed machine.
How much do you get? About 50% of the biomass turns into biochar, about 10% is required to create the charcoal, and about 40% can be re-captured as energy if your setup allows. That translates to about 1/4 of the original biomass in volume. The rest is water and volatile gases that are burned off.
Charcoal vs Briquettes Biochar is NOT the same as charcoal used in a backyard cooking grill. Briquettes are a chemical mix with many unknown additives; I wouldn’t even save the ash for the garden.
Natural charcoal is available for use in the grill, but I have found some brands are extremely hard with large chunks more like coal, that do not easily break up into suitably small pieces even with a hammer.
Charging Biochar: If you add raw biochar directly to a garden you could lose a season’s productivity, because it will absorb water and nutrients from the soil, robbing them from your plants. So before applying it to the soil, it is best to “charge” biochar for several days or weeks with water and nutrients (compost or worm tea, fish emulsion, urine, manure). Alternatively, mix it in while making compost, or add it in the off-season up north or if you leave a garden bed fallow. Once charged, biochar will continue to capture water and nutrients before they wash away.
Quantity to use in soil: 1-2 cups per square foot of soil minimum, 5-15% by volume of the top 6″ of soil is ideal. That would be from 1–2 cubic feet of biochar for a 4′ x 8′ bed mixed into at least the top 6″ of soil. Once applied, it is very stable; unlike compost or mulch, it does not break down over time.
Biochar can be applied under a layer of mulch for existing plantings; mixed with the soil more deeply for newly planted trees or planting beds; placed in a trench outside the drip line of a larger tree; or broadcast over a field. It can also be mixed in while making compost and later used as side dressing. Over time, worms will mix it deeper into the soil.
Summary: Biochar has many benefits. It provides a boost to soil microorganisms which improves the life, vitality and texture of the soil. It also improves the ability of the soil to absorb and hold water and nutrients, keeping them available to the plants over time while reducing runoff and down stream pollution. And it can help reduce greenhouse gases by creating a stable form of carbon that will remain in the soil for centuries.
Also be sure to check the “Building Soil” page under the Resources Menu.
By the way, char can be quite useful when you go camping. Create Char-cloth.