Pure Carbon

Biochar

Using charcoal as a soil amendment has numerous benefits.

What is biochar?

Biochar is simply charcoal used a soil amendment. Although it sounds high tech, it has actually been used by humans for over 2000 years. Amazonian tribes used it to make their famous terra pretta (“black soil” in Portugese).

How does it work?

Very simply, charcoal is extremely porous. This porosity creates lots of nooks and crevices which have two major benefits. Firstly, this porous structure creates a lot of habitat for microbes to live, keeping them from leeching out of the soil when it rains. This is invaluable for creating a healthy soil food web, which is important for proper nutrient cycling and a host other benefits.

The other big benefit also refers to the porosity. Charcoal has immense water holding capacity because it can act like a sponge. The scientific principle behind charcoal’s ability to hold water so effectively is primarily based on capillarity, also known as capillary action. Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. This phenomenon occurs due to the adhesion of water molecules to the charcoal surface being stronger than the cohesion forces among the water molecules themselves. The extremely porous structure of charcoal, with its myriad of tiny pores and channels, amplifies this effect by providing an extensive network for capillary action to take place.

Additionally, the porous structure increases the surface area within charcoal, which enhances its adsorption capacity—the process by which a solid holds molecules of a gas or liquid on its surface by physical or chemical means. Adsorption contributes to charcoal’s ability to retain nutrients and water, making them available for plant uptake over time. These combined properties—capillary action and adsorption—explain why charcoal, and biochar by extension, are so effective at holding water and improving soil moisture retention.

Consequently, the use of charcoal in soil amendments significantly contributes to improved soil structure, increased moisture retention, and a richer, more active microbial community, leading to healthier and more productive soil ecosystems.

Biochar benefits

  • Helps nutrient and microbe retention in the soil.
  • Biochar can sequester up to 50% of the carbon that would otherwise be released.
  • Helps water retention in the soil.
  • Also helps hold nutrients that may otherwise be washed away.

How we make biochar

We use the simplest method possible. It creates an acceptable quality, although more sophisticated methods would surely produce a higher quality product with less wood oils and organic matter residues* and could reduce carbon dioxide output into the atmosphere.

We create a trench about two feet deep, 3-4 feet wide and 8-12 feet long.

We then get a fire going with branches of up to 2-inch diameter. Get the fire burning hot to start the pyrolysis process. You ideally shouldn’t see any smoke. Smoke is a byproduct of incomplete combustion, indicating that the organic material is not being fully converted into biochar. To achieve a clean burn, maintain a high temperature that facilitates the thermal decomposition of wood material in the absence of oxygen. Watch the fire continually and keep adding material to keep it burning effectively.

Once you’ve exhausted your material or you start to see signs of ash beginning to form quickly quench the fire with lots of water. You will need to continue to apply water longer than you may think. If the fire looks put out, it may reignite and you come back to a pile of ash, not charcol. This process transforms the biomass into biochar, a stable form of carbon that not only enriches soil fertility but also acts as a significant carbon sink, reducing the amount of carbon dioxide released into the atmosphere.

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    December 12, 2022

    Keeping a large amount of biomass (straw, grass, wood chips, etc) on top of the soil retains moisture, suppresses weeds and slowly breaks down building soil with lots of organic material. We also grow edible mushrooms in the mulch of all our garden beds.

    Mimicking how a forest works, we aim to produce food using woody perennials and agroforestry principles. Functioning food forests have been discovered that are over 150 years old.

Simple DIY

Passive Solar Coldframe

Using packed earth tires as a thermal mass, this simple coldframe extends our season by 1-2 months in the spring and fall.

What is a coldframe?

A coldframe is essentially just a mini greenhouse. It protects plants from cold temperatures and can help growers in northern climates increase the length of their season.

Thermal Mass Wall

This design adds a passive solar element. By having a large, heavy wall (on the north side) made of tires filled with dirt, we are capturing sunlight and heating the tires. At night this heats is radiated back into the growing space keeping the plants warm.

The build

This was a very simple (and not very elegant) build. We used all recycled materials (even the hinges), and so it only cost a couple dollars for screws.

Planting

We decided to make a mini hugelkultur bed because we were low on compost and soil. We laid down a few logs in a trench and then covered them with leaves, sticks and some unfinished compost.

As an experiment to see how lazy we could be, we didn’t add any finished compost or soil and just threw seeds directly on top of these leaves and unfinished compost.

You can see kale seeds strewn randomly in this rough unfinished compost.

It seemed like this couldn’t possibly work because we weren’t even planting into soil. But the plants sprouted and took root, their roots snaking down to the moist and rapidly decomposing logs below. We had fresh salads and baby greens by the end of April, barely after the snow had melted. Overall this was a massive success. The lessons from this coldframe will be applied to our full size passive solar greenhouse.

Results

In the spring nighttime temperatures would routinely reach -5C, but the coldframe never had a killing frost, and didn’t even appear to freeze at all. The cold hardy plants (kale, mustard, lettuce, swiss chard) all established themselves from seed and grew well. This cold frame allowed us to significantly extend our growing season and have fresh greens earlier.

Update 2024. We decided to dismantle this and focus our efforts on consolidating our garden space, but this was a cool experiment showing the power of thermal masses collecting and reflecting heat into a space.

View More Projects

Check out some other stuff we’ve been working on.

    June 29, 2022

    Using cedar posts and recycled wood, this garden shed took 4 days to build and cost under $340.