Have you heard of biochar?
Biochar is charcoal that you add to your soil, and it’s gaining in popularity among farmers and gardeners across the planet because it help build healthy soil and increases production yields. It’s made by superheating biomass in an environment with little or no oxygen. This causes the original biomass to transform into a black, light, porous, carbon-rich charcoal, instead of simply burning to ash.
While its popularity is currently on the rise, biochar has been integral to building healthy soil for a long, long time. When I was a Peace Corps Volunteer serving in West Africa, I participated in the centuries-old annual tradition of setting fire to the farmland. My community knew that if they increased the amount of charcoal in their soil, it would help make their land fertile for the next growing season. It’s just how they’ve been doing things for a long time, and it’s worked.
Now, I want to pull back the curtains on biochar so we can see the simple, yet ongoing, science underlying it all. Let’s dig in!
Biochar is mostly empty space
If you check your kitchen, there’s probably a sponge resting near the sink. Sponges are great at soaking up spills, and so have earned their rightful place as an essential tool for cleanliness. Sponges excel at this job because they have pores, which are empty spaces liquid can flow into and fill up.
In many ways, biochar resembles and behaves like a sponge. When you look at it through a microscope, you’ll see a vast, intricate network of empty spaces. In fact, about 85% of biochar is empty space. That means it’s exceptional at holding onto water. As all growers are aware, it’s comforting to know that if you’re going through a dry spell in the season, or watering your plants simply slipped your mind, your plant’s can still find the water they need stored in the biochar.
However, biochar isn’t limited to quenching your plant’s thirst. Biochar plays a crucial role at helping your plants and soil breath because its pores can also be occupied by air. Some soils hold onto too much water, caused either by compaction or by excessive amounts of clay. These aren’t ideal growing conditions because they cut down on the amount of oxygen in the soil. When there’s not enough oxygen in the soil, harmful, oxygen-hating microorganisms take over. However, when oxygen is abundant plants can breath, and beneficial, oxygen-loving soil bugs thrive. All this makes for a healthy soil food web, and of course, happy plants!
Biochar is pure carbon
Let’s say you added nitrogen fertilizer to your lawn to make your grass grow greener. Your lawn would only consume a small percentage of the fertilizer because plants only take in the nutrients they need. This creates a problem. There’s really two places for excess nutrients to go. They can either stay in the soil to be used by plants later, or they can leach into the soil water. More often than not, it’s the latter.
Agricultural runoff is a problem encountered all too frequently, and it has devastating and long-lasting consequences. In fact, you can see this happening right now in Florida, where there are massive Red Tide algal blooms off the coast spurred in large part by agricultural runoff.
The measure of a soil’s ability to hold onto nutrients rather than let them leach into the soil water is called cation exchange capacity (CEC). Soil with low CEC is like a game of musical chairs where each round a seat is removed, and one player is kicked out of the game. Soil with high CEC, on the other hand, resembles an arena where there are always enough seats for everyone. To solve the runoff problem, we need to think about building an arena in the soil. That’s where we turn to biochar.
Remember, biochar is mostly empty space. We can quantify it, too. One gram of biochar has the surface area of over 1000 square yards! Impressive as that is, that’s not all. Biochar’s surface is actually made of pure carbon, or graphite, which has the special property of binding organic material to its surface. Biochar can even bind with many pesticides and heavy metals in the soil, rendering them inert.
That sounds crazy, but really those are just the effects of a graphite material with lots of surface area. How can we quantify biochar’s ability to bind with organics in terms of CEC? While a typical, healthy, organic soil might have a CEC around 60, biochar clocks in at a whopping 270! Whether we’re an industrial size farm worried about agricultural runoff, or growing some tomatoes on the porch, that’s a real, game changing difference.
Biochar is permanent
When the starting biomass is heated, it goes through several chemical transformations before it turns into biochar: excess water in the starting biomass evaporates, and the inorganic compounds, the things that make up ash, largely stay intact.
The real tour de force, though, is what happens to the organic starting material at a molecular level. It is transformed into a black, lightweight, porous carbon, which is an entirely different chemical structure, that boasts an increased resistance to biological and chemical breakdown in the soil.
The Terra Preta soils of the Amazon are a perfect example of how permanent biochar really is. These soils have manmade amounts of charcoal which date back over two thousand years. For all intents and purposes, this kind of longevity makes biochar a permanent soil amendment when compared with common fertilizers and compost, which break down in a matter of days, weeks, and months. With biochar, you put it down once and see the benefits year after year.
Biochar’s longevity in the soil also hasn’t gone unnoticed by those in the international community developing climate change mitigation strategies. The upcoming United Nations Framework Convention on Climate Change is to have panels that discuss how biochar can be used to help forests and land absorb more CO2, and improve water management.
Biochar is really just charcoal. It’s not like the charcoal you grill with, though. The process of making biochar, called pyrolysis, transformed the starting biomass into a form of charcoal that is highly optimized for water and nutrient retention, not for use as a fuel.
To get an intuitive sense of how it works, there are two things to remember. First, biochar is mostly empty space, which allows both water and air to flow into it, and provides an ideal habitat for beneficial soil microorganisms. Second, biochar’s surface is pure carbon, and has the special property of binding with organic material.
These two superpowers, if you will, combined with its longevity in the soil, make biochar a critical tool not just in any grower’s toolkit, but also an important strategy to consider as we attempt to heal our ailing planet and mitigate climate change. I hope that now the next time someone asks you if you’ve heard of biochar, you’ll be able to say, “Yes!”