Biochar Drawbacks

Like all things in the gardening/ farming world, biochar has benefits as well as potential drawbacks. It’s critical to understand the potential cons and how to counter them should they occur.

“Depending upon the feedstock (the original organic material used to make the biochar), some biochar can alter the soil’s pH, shifting the pH either up (making the soil more basic) or down (making the soil more acidic),” says USDA researcher Ariel A. Szogi. “Plants tend to have a particular

‘pH window’ in which they thrive, so biochar additions can shift that pH far enough in a direction to hamper that plant’s growth.”

Some biochars — based on their feedstock and pyrolysis conditions — can bind and then hold onto micronutrients. “Suppose the soil is low in a particular micronutrient,” he says. “In that case, adding biochar can exacerbate the issue and result in plant deficiencies.”

Have a comprehensive soil test performed by a local extension office before biochar application and then again at the end of the growing season — or if abnormalities in plant growth are observed — after the biochar is applied. This is an excellent way to see how the biochar is influencing the chemical composition of the soil.

Soil scientist Kurt Spokas’ research found that the main negative effects were initially observed from biochars that contain a high amount of sorbed organics (bio-oil) from the pyrolysis process itself or biochars that haven’t been preconditioned (i.e., ‘fresh’ biochar). The recommendation currently would be to co-apply biochar with compost or, even better, apply biochar at the beginning directly in the compost pile. Although, exact application rate guidelines are missing for this use, as well.

Spokas warns of the potential human health implications resulting from handling biochar and possibly inhaling the generation of fine particulate matter. However, inhalation risk could be reduced by handling moistened biochar.