The Pros and Cons of Biochar in Horticulture

— Written By Nancy Power
en Español / em Português

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The biggest handicap to growing plant crops in the Sandhills is the sandy soil. Acidic and low in organic matter, it does not hold water or nutrients. These ills can be corrected by adding organic matter in the form of compost, cover crops, or organic mulch, but the organic matter breaks down and needs to be reapplied every year. What if something could enhance the soil permanently? That is where biochar comes in. Biochar is a type of charcoal that is used as a soil amendment.

To make biochar, organic materials such as timber waste or poultry litter are heated to a high temperature in the absence of oxygen. Such decomposition is called pyrolysis. The heating removes water and gases and leaves a stable, mostly-carbon substance. For use in farming, the char is ground into a powder that is applied to the soil. It provides many of the same advantages as compost, such as water and nutrient retention and a better microbial community. However, whereas compost breaks down within one year in a hot, humid climate, biochar can last for hundreds or thousands of years. The most famous example is in the Amazon basin, where biochar added as far back as 6000 years ago still renders the soil darker and much more fertile than the surrounding, low-quality soil.

To test the efficacy of biochar in a Sandhills peach orchard, NC State University’s Dr. Mike Parker is conducting a 10-year study at the Sandhills Research Station in Jackson Springs. Four different treatments include biochar applied via the drip line annually, and biochar added to make 5% or 10% by volume of the soil to a depth of 1 foot in a 10-foot-wide strip. For comparison, another group of trees, the “control,” have no biochar in the soil. Four years into the study, the biochar trees have not shown significantly different growth, survival, or fruit production than the control group. However, the CEC (Cation Exchange Capacity) and potash in the soil have increased. CEC is a measure of nutrients available to plant roots, so higher is better. Potash is a necessary nutrient that is low in our sandy soil because it leaches out easily. The biochar itself usually does not supply enough potash for the plant crop; it mostly helps the soil to retain the potash that is applied. In this fertilizer retention role, biochar acts like compost, but biochar is more stable. Whereas compost breaks down within a year in our climate, biochar lasts for thousands of years even in the hotter, wetter climate of the Amazon basin.

In a study with common beans, biochar persisted longer and had a higher CEC than other forms of soil organic matter. It also increased bean yields and N fixation by beans.

Besides field studies, biochar is being tested to replace Perlite in the substrate for container plants. The porosity of biochar is similar to that of Perlite, but biochar has better water retention.

From an environmental perspective, the pyrolysis of organic matter produces heat and gases that can be used for energy production. Biochar reduces fertilizer runoff and potentially reduces fertilizer needs. Most importantly, biochar stores carbon for centuries. Biochar is thus a tool that, if used on a massive scale, could reverse climate change.

With all these benefits, why isn’t everyone using biochar? On the down side, as seen in the peach study, biochar can take years to start improving plant crop production or the soil chemistry. Furthermore, biochar has not been standardized. Different feed stocks and processing temperatures and times lead to products with different properties. Research is ongoing to find the combinations that make the best biochar for specific applications, and the best percentage of biochar in the soil to maximize production. The biggest downside so far, though, is the cost. For garden-scale applications, prices start at about $4 per gallon, enough to add to 4 square feet of garden space or to 16 gallons of soil. I could not find any biochar for sale in field quantities in our area.

Research on biochar and kiln design continues at NC State University and other universities. It also continues on a small, informal scale at Flow Farm in Moore County. The owner, Mark Epstein, has the goal of finding a design by which a kiln can be made for $50,000 – $100,000, and to have 5,000 kilns spread across the U.S. that can sell biochar for $10/ft3, rather than his current price of $35/ft3 (about $4.67/gallon). Time will tell whether all the research can make biochar economically viable and available for commercial growers.

For more information on growing horticulture crops, or opportunities for learning at the Demo Farm, contact the N.C. Cooperative Extension, Richmond County Center at 910-997-8255. Visit our website and follow us on Facebook.

  1. Rondon, M.A., Lehmann, J., Ramírez, J. et al.Biological nitrogen fixation by common beans (Phaseolus vulgarisL.) increases with bio-char additions. Biol Fertil Soils 43, 699–708 (2007).
    A tractor spreads biochar in a field.

    A tractor spreads biochar in a field. Photo by Mike Parker.