Bocashi fertilizer at Don Chico's farm, contains biochar.
When I was recently in Honduras, my organic farmer friends kept talking about using crushed charcoal as a soil amendment. They mixed it with worm compost to make a soil mix for a tree nursery, and they mixed it with other ingredients to make bocashi fertilizer. My friend talked about the discovery of black dirt in the Amazon, known locally as “terra preta,” which was discovered to be a human-made soil that has persisted for thousands of years without losing fertility. The studies revealed that ancient civilizations mixed charcoal with pottery fragments to create a soil resistant to the weathering effects of rain and heat. I came back from Honduras curious about soil charcoal, and I discovered that there is a lot of excitement here in the States about using charcoal in the soil to sequester carbon to reduce greenhouse gases and also secondarily to improve the soil quality. They call it “Biochar.”
As an organic farmer I am aware that there are many “miracle soil amendments” out there. For $59.99 I can buy a bag of this or that microbial mix or mineral mix. You can buy fish meal, and bone meal, and fish bone meal. It is seductive to think that all that is necessary to grow incredible vegetables is to find the secret magic short-cut formula. The list of odd soil amendments is long and varied. In BioDynamic farming there is a preparation that involves burying a cow horn with manure in it and then waiting a year and then using that potentized horn manure to inoculate some rain water for spraying on your fields. At our previous farm, we tried using horn manure and actually had very good results. I don’t begin to know why. There are many mysteries in soil health and in human health.
As a person who was trained as a scientist, I am skeptical of things that I can’t explain; I am also drawn to them. Charcoal seemed to be another such thing. The magical dust for the soil? The missing ingredient in our soil health recipe? Maybe magic is real. My interest was piqued, so I did some internet searches. Sadly, there is no magic here. It is actually pretty straightforward. Charcoal is carbon from organic sources that is stable in the soil; because it has been cooked, it doesn’t have any food value for soil organisms. This means that it does not decompose. This is a bonanza for the soil structure and fertility.
Carbon is what makes soil black: the blacker the soil, the more carbon. Carbon in the soil is always the result of the decomposition of dead plants, insects, and animals. As these plants decompose, the nutrients are released through the action of the soil microbes and soil fauna. These ionic nutrients are available to plants to help growth and vigor. As the dead material decomposes and releases nutrients, some of the carbon is released in the form of the gases carbon dioxide and methane and some of it stays in the soil as humus. Over time, if no carbon is returned to the soil in the form of new dead plants or manure, then the soil will slowly lose its microbial life and soil fauna. The soil will go from black and crumbly to gray, brown, or red. This is bad for the soil and for the planet. An agricultural soil without much carbon in it requires more and more chemical fertilizer to make up for the lack of nutrients. To make matters worse, the stable carbon in the soil (humus) on the cellular level looks like a convoluted honeycomb with lots of charged surface area for retaining ionic nutrients. Without such carbon, the soil does not retain nutrients as well. To further complicate matters, the humus particles in the soil improve the soil structure, because they are relatively large and contribute to the crumbly texture that allows air and water to penetrate. Without carbon, the structure disintegrates and water is more likely to run off and the depleted soils can become crusty and hard and vulnerable to erosion.
In organic farming, one of the main ways the farmer amends the soil is by adding sources of carbon. Cover crops, straw mulch, manure, compost, dead leaves, and other sources are all added to the soil. Of course, because the microbial activity is always “burning up” the carbon, the organic farmer is always thinking about how to add more and more carbon. However, too much carbon (like sawdust) can tie up nutrients (especially nitrogen) while the microbes are decomposing the dead plant material, so it is a balancing act. But generally, if a person is adding more carbon than is being burned up each year, then the soil is improved structurally and in nutrient holding capacity and carbon is being “sequestered” in the soil, at least temporarily. The promise of Biochar for organic farmers is that by adding charcoal to the soil, all of the benefits of adding carbon will be realized without all the losses to decomposition. The carbon in the Biochar will also be sequestered for a very long time. Of course, non-charcoal organic matter will also have to be added for the available nutrients, but when those nutrients are available, the Biochar will act like humus and hold on to the ionic nutrients.
The prospect of adding Biochar to the soil is exciting for us in this part of Wisconsin, but the real promise for Biochar is in the warm tropical climates. As air temperature rises, it increases the biological activity of the soil. Decomposition of organic matter is much faster in the tropics. Also heavy rainfall in the rain forest area means that water-soluble nutrients that are not bound in the soil are quickly washed away. This means that in tropical ecosystems, most of the carbon is not usually in the soil, instead it is tied up in the vegetation. This means that the amount of carbon that can be sequestered is limited to the living vegetation. In a functioning old growth rain forest, that is still a lot of carbon, but in a pasture or farmed fields, it is not very much. Also, because of the temperature and fast decomposition, tropical organic farmers have a hard time adding enough carbon to improve the soil. If people are going to continue to live in the tropics, then the long term soil health is important. Without soil health, farmers are forced to clear more and more forests to find new soils that have not been totally depleted. This clearing can have a disastrous effect on the local micro-climate, drying up springs, increasing temperature swings, and decreasing rainfall. In the global climate, the clearing of tropical forests is even worse, releasing all of the carbon stored in the vegetation and contributing to global climate change. If Biochar could be widely applied in the tropics, then less arable land would be needed to feed the same number of people. Then less forest would need to be cut down and therefore carbon could be sequestered in the forests and in the soils.
There are complications and questions. Where would the carbon source come from to make charcoal on a large scale? Could we ever add enough charcoal back into the soils to make up for all of the coal, gas, oil and wood that we are currently burning up? To impact the climate, we will actually need to sequester MORE carbon than is being added to the air. I have been talking Biochar to anyone and everyone lately. A friend (Thanks Peter!) mentioned to me that in his understanding, Biochar may be the ONLY way to really sequester carbon, because the traditional way to sequester carbon by planting forests only sequesters for as long as the trees are alive, then they decompose and release much of their carbon back to the atmosphere. Another question is whether we could sequester carbon fast enough to halt the wheels of climate change.
Here's an article from someone who made her own biochar.
Video promoting biochar.