Biochar: An Old Idea Resurfaces - March 11, 2009
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

A vegetable gardening friend told me about a recent article in Mother Earth News (February-March 2009) on the use of “biochar” as a soil amendment. Biochar is charcoal produced by heating organic materials (such as wood and yard waste) using high heat and a limited amount of oxygen to drive off all the compounds contained in the waste except for the carbon. The resulting charcoal, called biochar, is almost pure carbon and very resistant to decomposition in the soil environment

The literature about biochar begins with “terra preta” (“dark earth” in Portuguese) found in prehistoric Amazonian farmlands. Twenty five hundred years ago, the people living in the Amazon Basin grew cassava, maize, and tree fruits on these dark soils. With arrival of European colonials and the devastating diseases they brought with them, farming in these terra preta areas was largely abandoned. However, the carbon content and fertility of these soils persists today after 500 years of lying fallow under tropical climatic conditions. Biochar is one of the major components that appear to have contributed to these highly desirable soil conditions in the Amazon Basin.

Since the discovery of the terra preta, scientists have worked to unravel the biological processes in these systems. Microbiologists have discovered relatively large populations of fungi and bacteria in terra preta soils. However, the presence of abundant carbon makes the microorganisms live and reproduce at a reduced pace. This results in a reduction in the turnover rate of organic matter in the soil, so composts and other soil-enriching forms of organic matter last longer. This should get vegetable gardener’s attention. Regular additions of organic matter (compost, manures, peat, etc.) are a yearly ritual for vegetable growers where our alkaline soils and high temperatures act together to quickly break them down.

Researchers have shown that biochar in soil increases productivity by making nutrients already present in the soil better available to plants in field trials with corn, rice and other crops. Results have been especially dramatic when biochar is added to productive soils that already contain ample minerals and plant nutrients. Here the addition of biochar effectively reduces losses of nitrogen, phosphorus, calcium and magnesium from leaching and/or volatilization to the atmosphere. When biochar is incorporated into soil, it sequesters (stores) carbon for long periods. This carbon would otherwise be released to the atmosphere where it would otherwise become a greenhouse gas contributing to global warming.

Mother Earth News noted several methods that people can use to make biochar. The easiest is to dig a deep trench in your garden, fill it with brush and monitor the color of the smoke. As it turns from white to yellow to gray, this signifies the driving off of water, then resins and sugars. As the smoke thins to blue gray, cover it with an inch or so of soil and leave it to smolder. When you can still see charcoal chunks, douse the fire with water, allow the soil to dry for a few days and dig it in. Biochar can also be made in burn barrels, barbeques, and airtight wood stoves. Make sure to avoid charring or burning chemically treated wood scraps and poisonous plants such as oleander. The smoke from these and other toxic materials can be very poisonous.

While I have not made any biochar intentionally, I was just looking at the area where I dump the ashes from our wood stove. I saw many chunks of charcoal and will try to separate the chunks from the gray ashes to gather some for experimentation in my garden. I’ll let you know how it goes. If any readers have had positive or negative experiences with biochar, I’d like to hear from you. In the meantime, you can learn more about biochar online at or

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: July 16, 2009
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