The advantages and disadvantages of biochar in horticulture


Photo courtesy of Dr Mike Parker

A tractor spreads biochar in a field.

The biggest handicap to growing Sandhills is the sandy soil. Acid and low in organic matter, it retains neither water nor nutrients. These ailments can be altered by adding organic matter in the form of compost, cover crops, or organic mulch, but the organic matter breaks down and must be reapplied annually. What if something could permanently improve the soil? This is where biochar comes in. Biochar is a type of charcoal that is used as a permanent soil amendment to improve the soil’s water and nutrient holding capacity.

To make biochar, organic materials such as wood waste or poultry litter are heated to high temperatures in the absence of oxygen. Such a decomposition is called pyrolysis. Heating removes water and gases and leaves a stable, mainly carbonaceous substance. For use in agriculture, char is ground into a powder which is applied to the ground. It offers many of the same benefits as compost, such as water and nutrient retention and better microbial community. However, while compost breaks down in a year in a hot, humid climate, biochar can last for hundreds or even thousands of years. The most famous example is in the Amazon Basin, where biochar added 6,000 years ago still makes the soil darker and more fertile than the surrounding soil poor in nutrients.

To test the effectiveness of biochar in a Sandhills peach orchard, North Carolina state fruit tree specialist Dr. Mike Parker is conducting a 10-year study at the Sandhills Research Station in Jackson Springs. Four different treatments include biochar applied annually to the soil surface and biochar incorporated into the soil to form 5% or 10% by volume of the soil, to a depth of one foot in a 10 foot wide strip. For comparison, another group of trees, the “controls”, have no biochar in the soil. Four years after the start of the study, the biochar trees did not show significantly different growth, survival or fruit production compared to the control group. However, CEC (Cation Exchange Capacity) and potash in the soil have increased. CEC is a measure of the nutrients available to plant roots, so more is better. Potash is a necessary nutrient that is low in our sandy soil because it seeps easily. Biochar itself generally does not provide enough potash for crop cultivation; it especially helps the soil to retain the potash which is applied. In this fertilizer retention role, biochar acts like compost, but is more stable. While compost breaks down in a year in our climate, biochar lasts for thousands of years, even in the warmer, 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 nitrogen fixation by beans.

In addition to field studies, biochar is being tested to replace perlite in the substrate of potted plants. The porosity of biochar is similar to that of perlite, but biochar has better water retention.

From an environmental point of view, the pyrolysis of organic material produces heat and gases which can be used for energy production. Biochar reduces fertilizer runoff and potentially reduces fertilizer requirements. Because biochar stores carbon for centuries, it has the potential as a tool to fight climate change.

With all of these benefits, why isn’t everyone using biochar? In contrast, as the peach study shows, biochar can take years to start improving crop production or soil chemistry. In addition, biochar has not been standardized. Different raw materials and temperatures and processing times lead to products with different properties. Research is underway 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 to date, however, is the cost. For garden-scale applications, prices start at around $ 4 per gallon, enough to add up to 4 square feet of garden space. I have not been able to find any biochar for sale in large quantities in our area.

Research on biochar and furnace design continues at universities like NC State and USDA. It is also continuing on a small scale at Flow Farm in Moore County. The owner, Mark Epstein, aims to come up with a design that can make a furnace for $ 50,000 to $ 100,000, and have 5,000 furnaces spread across the United States that can sell biochar for $ 10 / cu. rather than its current price of $ 35. / ft3 (approximately $ 4.67 / gallon). Time will tell if all the research can make biochar economically viable and available to commercial growers.

For more information on growing horticultural crops or learning opportunities at the demonstration farm, contact the NC Cooperative Extension, Richmond County Center at 910-997-8255. Visit our website at and follow us on Facebook.

Nancy Power is the Commercial Horticultural Officer for the Richmond County Co-op Extension.


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