Wednesday, May 13, 2015

Biochar for Soil Mix

Buzz Durham (from Grace Covenant PC in Asheville, NC) in Léogâne, providing some ideas to workshop participants about using charcoal dust to create a biochar mix for vegetable tires. Buzz recommended starting with adding about 1 part charcoal powder to 6 parts "regular" soil mix. Our normal soil mix is 3 parts soil, 2 parts dried crushed manure (cow, horse and burro) and 1 part sand. So now we recommend 3:2:1:1, soil:manure:sand:charcoal powder. The exact proportion of soil and sand will depend on how heavy the soil is (how much clay it has) and also how fine the sand is. For example, I have a sand that is so fine, I just use it as soil, mixing it with the dried animal manure.  The next step is to fill the vegetable tires with the mix, wet them thoroughly, cover them with some type of mulch and leave them for a week to ten days. This give the charcoal time to convert into biochar--biologically active charcoal. Photo by Mark Hare
Biochar is charcoal used as a soil amendment. Like any charcoal, biochar is created by burning biomass of some type in the absence of oxygen (pyrolysis). Biochar is under investigation as an approach to carbon sequestration. (From Wikipedia: Wikepedia: Biochar).

As a soil amendment, we have started using charcoal powder in the soil mix we make for our vegetable tires. Because charcoal particles are extremely porous they can absorb water and nutrients. The tiny holes in each particle also create good places for beneficial microbes to live and develop.

All charcoal is not equal in terms of benefits, it turns out ("Gardening with Biochar" Gardening with Biochar). Some types of charcoal provide more nutrients ("bio-oil condensates") for the little beasties. More beasties (micro-organisms) means more life in the soil. Healthy soil means healthy plants. In addition to making nutrients in the soil more available to plants, a healthy soil micro-biology also provides plants a better defense from diseases caused by viruses and other pathogens.

In the mini-workshop that Buzz Durham led in Léogâne, September 2014, we used charcoal powder that had accumulated in the area within the local marketplace where charcoal merchants sell their product. The powder comes from the bits of charcoal that fall out of the sacks and then get crushed into powder by the weight of the sacks of charcoal sitting on it and by people constantly walking on it. The coordinator for farmer organization ODEPOL, Luxène Sommervil, collected two sacks of the powder there for free, and I brought one of them home to use in my family's vegetable tires here in Barahona (Dominican Republic).

If you don't have a local marketplace that sells charcoal, there are ways you can make charcoal in your backyard. Here is a youtube.com demonstration of a simple way to do this, developed by Amy Smith from MIT: Amy Smith: Agricultural Charcoal. Commercial charcoal briquettes are not good because the material used to bind the charcoal may not be good for the soil.

One of the most important characteristics of the charcoal added as a soil amendment is that it is durable. We can add it once to our soil mixes and it will keep providing benefits for a very long time. The charcoal residues in the terra preta soils of the Amazonian rain forest are hundreds, possibly thousands of years old (Terra preta soils in the Amazon).

In terms of carbon sequestration, what this means is that by using a biochar system, we are capturing the carbon in the carbon dioxide (CO2) and storing it in ways that there is a net decrease in CO2 in the atmosphere.

Turning charcoal into biochar is something like learning to put the petroleum back in the ground, and then growing food with it.
 
Buzz detailing the benefits of using charcoal dust in soil mix. Photo by Mark Hare.


An experiment with biochar. The pot to the left has no biochar, the one in the middle has about 10% and the one to the right has about 20%. Photo from PowerPoint presentation by Larry Sthreshley, PC(USA) mission co-worker in the Democratic Republic of Congo. Used by permission.

A tire with tomatoes at our family's home in Barahona. The tire is filled with a mix of sandy soil, manure, compost and charcoal powder from Léogâne. Photo by Jenny Bent.

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