How does biochar work to improve, or even decontaminate, soil?

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 above items in a low- or no-oxygen environment.

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

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.

Fruitwood chips before firing into biochar through a process called pyrolizing. Credit: 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 post https://soilsmatter.wordpress.com/2013/12/02/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 less available to plants. Addition of high-temperature biochar helps reduce metal toxicity and grow plants in areas where plants typically cannot 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.

Jim Ippolito, Colorado State University

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