Prepare Your Garden Soil:
Making Biochar
Around 1870 large areas of "Terra Preta de Indio" or Indian Black Earth soils were discovered in the Amazon basin. These are extremely stable, fertile soils up to 2 meters deep with high levels of organic matter, something almost unheard of in the tropics. They were created by thousands of years of indigenous farmers mixing the charred remains from their fires along with organic wastes into the soil.
The discovery of these Amazonian soils has spurred interest in “biochar”, or charcoal that has been inoculated with beneficial microorganisms.
Biochar has two intriguing properties:
- It can resist breaking down in the soil for a thousand years or more.
- It has an extremely high surface area. An ounce of biochar may have more than two acres of surface area. This provides excellent habitat for beneficial bacteria and fungi. This great surface area can hold essential nutrients in a way that makes them available to both plants and diverse soil life.
The first of these properties offers long-term benefits. Climates that are changing rapidly because of too much carbon in the air are a major threat to all of us. But the threat is most immediate to low-income growers in the tropics, like the millions of campesinos trying to make a living in Central America’s rapidly worsening “dry corridor.”
Plants use photosynthesis to take carbon out of the air, but it cycles back into the air too quickly to provide a long term solution. Biochar’s capacity to hold that carbon in the soil for millennia makes it potentially valuable in the struggle to stabilize our climates.
The second of biochar’s properties makes it immediately useful for growing food. The value of biochar is greatest when the charcoal is broken into small pieces, primed with plant nutrients, and then mixed with compost to inoculate it with a wide variety of beneficial soil bacteria and fungi. Its benefit is the greatest where it is needed most; on thin or sandy tropical soils with low fertility and low organic matter. Adding biochar to the soil can improve the efficiency of both water use and nutrient use while reducing soil erosion, runoff, and contamination of surface water.
How to Make It
Biochar can be bought at a few farm supply stores or through the internet in the US, Europe, and Japan. But even where it is available, it is far too expensive for low-income families to use in their home gardens. A more realistic option for them is to make some of their own.
There are a lot of ways to make biochar at home. Here are two very simple ways and two somewhat more complicated methods. They all use a top-lit updraft (TLUD) fire to create the biochar.
The simplest method is to make a stack of dry brush, wood, or bamboo 3 or 4 feet high and light it from the top. You'll probably need some newspaper or very light kindling to get this top-lit fire going. Add wood as it burns down. As the fire changes from bright flames to hot coals, douse it good with water. Make sure the fire is completely out.
A second simple technique for making biochar is similar, but the fire is largely contained in a pit. This can be a cone about 3 feet deep and 3 feet across. We use a pit roughly 4 feet long by 2 feet wide and 2 feet deep. This allows slightly longer wood to be charred. The pit contains the fire more than the open pile method and allows for the fire to be quenched with either water or by piling dirt on top of the coals.
The third method uses a 55-gallon metal drum to hold the wood. It has a secondary combustion chamber with a smokestack on top of the barrel to burn off escaping gasses. This method burns very clean and puts fewer pollutants into the air. The top of the combustion chamber can be used to cook on to make use of heat that is otherwise wasted. The Internet has several good plans for modifying metal drums to produce biochar.
The fourth method is also more complicated than the pit but also has the major advantage of using the heat generated by producing the char to cook food. Since about 1/3 of the people on earth cook with open fires, a high-efficiency stove that also produces biochar for the garden has great potential. There are dozens of variations on the biochar cookstove. This one, called Estufa Finca (or farm stove), is from an NGO in Costa Rica.
It is cleverly designed from a 5-gallon metal bucket and sheet metal. The bucket is filled with dry wood, or in this case dry bamboo. Then it is lit from the top. A second metal bucket supports the cooking pot. When the food is cooked and the flames have gone down the coals are emptied into a metal bucket. This bucket can then be covered to block off oxygen or doused with water.
However you make your char, the next steps are pretty much the same.
When the pile is completely cool, we rake it and pick out any big sticks that haven't charred. They can be set aside for the next biochar run.
We then sift the remaining char through a ½ inch mesh. Next, we sift that through a ¼ inch mesh.
We smash whatever doesn't pass through the mesh with a bar or hammer, then sift it again. We usually use the tamping end of a 4-foot long steel digging bar like a harpoon. It’s a bit of a workout. There is a lot less dust to breathe if you do the smashing and sifting while the char is still damp from dousing. There is always some char that doesn't sift easily. We just toss that in the compost.
The 1/4-inch sifted char seems about ideal for use in the vegetable garden. Before incorporating it into the garden soil, we prime it with plant nutrients for at least 2 or 3 days. You can use fish emulsion, manure, or any organic fertilizer. Urine works well charging the char with plant nutrients and the biochar absorbs the smell of the urine. Some people may find this unpleasant but the use of urine as fertilizer is becoming more accepted and the combination of urine and biochar has a lot of environmental advantages.
After the char has absorbed the plant nutrients we mix it in with at least 2 parts of compost for every part of treated char. We leave this for at least ten days to enable the beneficial bacteria and fungi to find homes in the millions of microspores in the char.
If you can incorporate 1-3 inches of this char-compost blend into the top 6 inches of your garden your soil and your plants will be happy. Biochar tends to be somewhat alkaline but the compost usually will buffer that unless you are using a lot of char and your pH is already high. If you only have a small amount of bio-char, adding a bucket of char to 3 or 4 buckets of potting soil will help get your seedlings off to an excellent start.
Because biochar is stable in the soil for hundreds of years, you can add a small amount every year, gradually accumulating the optimal benefit of soil health. So your soil and your plants will be happy for a long time.
This is gardening for the long term.
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