Level up your garden with this ancient DIY soil booster.
We knew there must be a secret. For decades we’d lived and gardened in zone seven, where winter temperatures dropped below 14 F and the only tropical fruit could be found in the grocery store. Yet here we stood in a greenhouse not far from our own where avocados, bananas, passion fruit, olives, and thirty-two varieties of citrus thrived. How was this possible?
The owner of the greenhouse told us that she used a soil conditioner, something she made herself from organic waste. This ingredient, she said, made all the difference.
She called it biochar.
What is biochar?
We soon learned that biochar is a form of charcoal created by heating organic matter in a low-oxygen environment.This process converts biomass into a stable form of carbon that can be added to soil to improve its quality. In this case, that environment was a metal bowl that looked something like a commercial fire pit.
A small-scale commerical grower used this metal bowl to make biochar for her tropical crops.
Best known for the terra preta (dark earth) soils created by indigenous peoples in the Amazon basin, biochar has been used for centuries to augment soil fertility, condition soil, and support microbes.
Unlike other additives, biochar does not decompose quickly. This means it can help mitigate climate change by locking carbon in the soil and reducing greenhouse gas emissions. Thanks to its ability to improve soil structure, support plant growth, and sequester carbon, biochar is an important tool in regenerative agriculture.
How does biochar help garden soil?
Biochar enhances soil fertility by providing a medium for everything a plant needs to thrive. Its highly porous structure helps retain water and nutrients, making it particularly valuable for degraded or nutrient-poor soils. By reducing the loss of important nutrients, biochar reduces the need for chemical fertilizers and other inputs.
Even better, biochar is good for all soil types: it improves sandy soil’s ability to hold water while simultaneously making drainage easier in clay soils by preventing compaction and waterlogging.
Lemon trees nourished by biochar-infused soil in a zone 7 greenhouse.
The porous nature of biochar also offers a great home for beneficial microbes and the carbon feeds them, which helps suppress diseases and keep nutrients cycling. The extra carbon also feeds beneficial fungi. Since biochar is generally alkaline, it can help neutralize acidic soils and improve pH balance.
What is biochar made from?
Biochar can be made from any material rich in carbon, such as wood, crop residue, food waste or manure, collectively known as ‘feedstock.’ And what you use matters. Biochar made from wood has a higher carbon content, whereas biochar made from animal waste has a higher potassium content. Knowing your soil can help you choose which materials are best for your garden.
If you live near the ocean or desert, remember that it’s best not to use salty driftwood, salty desert plants (like mesquite) or toxic plants like oleander, as some of the salts and toxins may remain in the finished product.
It’s also important to avoid using materials containing chemicals that might end up in your biochar. This includes wood treated with creosote or paint, and pressure-treated wood, due to contamination concerns. Also pay attention to the container you use to make your biochar: using materials with galvanized or painted metals is not recommended. A stainless steel option is a safe bet where available.
Biochar feedstock is commonly made from small, woody cuttings from shrubs and trees.
The process used to make the biochar can also affect its properties. Drying out the wood or other waste before making biochar is an important part of preparing your feedstock. You can do this passively using exposure to sun and time or with heat from previous batches.
How to make biochar
Biochar is commonly made through a controlled burning process called pyrolysis, where organic materials are heated under low oxygen. This process converts the biomass into a stable material while releasing less carbon than regular burning (making it less impactful on the environment).
The key with whatever method you choose is ensuring the material burns slowly, without turning to ash.The most common methods of creating biochar through pyrolysis include pit burning, drum kilns, and retort systems.
Pit burning
For home gardeners, a simple conical fire pit dug into the ground makes a good base for making biochar. Dig your hole about three feet deep with sides that slope inward. Layer with heaviest organic materials on the bottom, topped with thin, light kindling. Burn until the feedstock is red hot, adding kindling periodically to draw the fire up through the pit. At the crucial moment, douse with water to prevent material from turning to ash. Cool and grind finished charcoal before spreading.
In-ground biochar systems are simple, though often less efficient than other designs.
Drum kilns
A metal drum (often outfitted with small air holes top and bottom) makes a perfect kiln for brewing up biochar. As above, add your heaviest materials first, before filling the remaining space with lighter materials and kindling. Once the material has burned down six inches or so below the barrel top, some like to seal the barrel with soil to further restrict oxygen.
Finished biochar in a simple drum kiln.
Retort biochar systems
A retort kiln consists of two metal drums, one inserted inside the other. The feedstock for the biochar goes into the smaller drum, which is then sealed and placed inside the larger drum and surrounded with more (larger) feedstock. Light the feedstock in the larger drum before sealing the entire unit to retain heat, reduce oxygen, and reburn circulating gasses. Retort biochar systems often have a chimney and ventilation holes to regulate oxygen flow. The Eartheasy biochar kiln is one example of this model.
How to "finish" biochar for the garden
Once you’ve created your biochar, cool and crush it into smaller particles before applying it to the garden. There is no perfect size, but a mix of crumbles, small stems, and dust is common.
Raw biochar can be highly absorbent, so it’s beneficial to “charge” it by mixing in compost, manure or compost tea before adding it to the soil. This allows biochar to provide immediate benefits to plants and fosters beneficial microbial activity. It also avoids sterile biochar, which can lock-up nutrients and make them less available in the short term.
Do this by mixing the two ingredients together and letting them sit for three to four weeks. You can add small amounts of compost or manure to biochar or small amounts of biochar to compost, depending on your goal (see more below).
If you prefer to apply biochar directly to your soil, we recommend doing so along with other soil amendments so the biochar doesn’t monopolize soil nutrients when first introduced. Soaking biochar in worm tea, with an aerator to increase microbes, is one of the most efficient ways to charge biochar.
How to use biochar
Be sure not to over apply biochar to your soil. While this amendment can greatly improve most soils, too much can degrade soil over time by altering pH and nutrient balances. This repels some beneficial soil dwellers, leading to less organic matter over time. Testing your soil regularly will help guide you.
Related: Know Your Garden Soil
The University of Arizona Extension recommends starting with a mix of 1:10 (biochar to compost) to first see how plants tolerate the addition. Add as you would normal compost applications for your area and incrementally increase the biochar in future years as needed.
Avoid top dressing with biochar-compost mixtures or using biochar in beds where tilling will take place due to the dusty residue.
Build your soil's capacity for a thriving garden
Now when we consider that lush greenhouse filled with tropical plants, we imagine what our own greenhouse could look like as we build our soil’s capacity to nourish a wide range of plants. When applied in moderation, biochar is a powerful tool for improving soil health, sequestering carbon, and reducing waste. Together, we can also use biochar to play a key role in regenerative agriculture, climate resilience and a thriving garden.
