Gardening Soil

How Does Biochar Work to Improve, or Even Decontaminate, Soil?

Biochar made from fruitwood chips.

Fruitwood pyrolized into biochar. Photo: Jeff Novak, USDA-ARS.

An article by Jim Ippolito, Colorado State University

Biochar – as the name implies – is made from formerly living substances. Source materials can be plant materials like leaves, stems and woody tissue. Biochar can also be made from manure or even biosolids (the solids left after wastewater treatment). Biochar is the material left over after burning one (or more) of the items above in a low- or no-oxygen environment.

The finished material itself can look like charcoal or ashes left over after a campfire. Biochar can be made in several different types of furnaces. The source material as well as the temperature at which biochar is made makes a difference in how the biochar functions.

When biochar is made at relatively low temperatures, some organic materials remain in the biochar. Some of the organic matter is still usable for plants and soil microbes to use as nutrient sources. In this way, applying biochar improves soil nutrients, and can help plants grow better. Adding biochar to soil can improve its texture, too, allowing for more water filtration during rainstorms and snowmelt.

Bowl of fruitwood chips being put into a pyrolyzer.
Fruitwood chips before firing into biochar through a process called pyrolizing. Photo: Jeff Novak, USDA-ARS.

In addition, the organic materials of biochar can form associations with chemicals in soils. Lower temperature biochars contain “functional groups.” These chemical portions of the biochar molecules can trap other chemicals in soils. Functional groups can trap both organic chemical contaminants – like pharmaceutical waste – and heavy metals. This protects plant life from the harmful effects of these chemicals.

You may not know that soil typically has a negative charge – depending on how much clay the soil contains. (For a review on how soil has a charge, visit this Soils Matter blog:  Soil: Largest Reactor on the Planet?). Similarly, biochar is often negatively charged, and when added to soil, this can enhance and chemically attract positively charged chemicals. Biochar made at a low temperature has a structure that can hold, but not change, the toxic metals. This reduces the amount of heavy metals that can get inside plants.

Another feature of biochar is its high surface area. Think of it like a charcoal sponge. This large surface area can hold a lot of chemicals and toxic metals, making it quite valuable.

When biochar is made at relatively high temperatures, most functional groups are lost. This type of biochar can still improve and decontaminate soil, but in different ways. What remains are mostly carbon ash and some helpful chemicals such as carbonates, oxides and hydroxides. These materials can chemically react with heavy metals in soil. This changes the metals into forms that are less soluble and therefore less available to plants. Addition of high-temperature biochar helps reduce metal toxicity and allows plants to grow in areas where they typically couldn’t thrive.

Scientists are researching uses of biochar to help reclaim heavy metal contaminated soils in former mining sites. Plants have difficulty growing in soils found near some historic mining areas due to excessive soil metals. Additions of biochar have successfully improved soils so plants can grow.

So, how do biochars work to improve or decontaminate soil? Adding biochar to soil can add organic nutrients that plants and soil microbes need as food source. It improves the physical structure of soil, improving water movement. And materials and compounds in biochar can even help contaminated soils by holding onto or chemically reacting with heavy metals and other soil contaminants.

This blog was adapted from an article appearing on the Soil Science Society of America (SSSA) website, a progressive, international scientific society that fosters the transfer of knowledge and practices to sustain global soils. Its members are dedicated to advancing the field of soil science. It provides information about soils in relation to crop production, environmental quality, ecosystem sustainability, bioremediation, waste management, recycling, and wise land use.

Larry Hodgson is one of Canada’s best-known garden communicators. After studies at the University of Toronto and Laval University where he obtained his B.A. in modern languages in 1978, he succeeded in combining his language skills with his passion for gardening in a novel career as a garden writer and lecturer. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He is a regular contributor to and horticultural consultant for Fleurs, Plantes, Jardins garden magazine and has written for many other garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening, Rebecca’s Garden and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 50 other titles in English and French. He can be seen in Quebec on French-language television and was notably a regular collaborator for 7 years on the TV shows Fleurs et Jardins and Salut Bonjour Weekend. He is the President of the Garden Writers Association Foundation and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. An avid proponent of garden tourism, he has lead garden tours throughout Canada and to the gardens of over 30 countries over the last 30 years. He presently resides in Quebec City, Quebec.

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