The Dark Side of Biochar: A critical examination of this trending soil amendment [VIEWPOINT]

Biochar is marketed across the world as a sustainable solution to improving waste management practices and plant yields. What could be better for the planet than turning waste into fuel that improves plant health? Unfortunately, like all things that sound too unbelievable to be true, digging into biochar research presents a different picture. This waste-derived carbon product may have particular benefits in specific environments, but overall, the research is contradictory and doesn’t present data that makes experienced agronomists confident about this amendment. In truth, biochar’s benefits are overshadowed by the complexity of its application and the variability of its benefits in various soil types.

What is biochar?

Biochar is a carbon-rich substance created through a process of decomposing organic matter at high temperatures with limited oxygen called pyrolysis. Many point to biochar as a possible solution for enhancing soil health, increasing crop yields, and sequestering carbon. However, its impacts are not so cut and dry. Biochar elicits these benefits in some environments with some soil types and some crops, but it is not a wide-reaching solution to all agricultural problems. In fact, in many soil types, it presents more damage than benefits.

Benefits of biochar

Biochar has many benefits. For starters, it is an excellent waste management tool in that it converts waste into something potentially useful. According to the USDA, biochar can improve soil health, raise soil pH, remediate polluted soils, sequester carbon, lower greenhouse gas emissions, and improve soil moisture. The International Biochar Initiative claims that biochar fights global warming, produces a soil enhancer that holds carbon and makes soil more fertile, reduces agricultural waste, and produces clean, renewable energy. Farmers can also get carbon credits and certificates for applying this amendment. Other researchers point to how biochar as a soil amendment can increase nutrient availability and nitrogen retention. It can also boost crop yield and bind with heavy metals to support soil quality and environmental remediation.

This all sounds pretty great. However, as we dig into the research, it becomes clear that only some types of biochar elicit these benefits in some soil environments. Plus, the long-term implications of biochar use in agriculture are unknown.

Concerns About Biochar

Biochar’s main concerns are significant composition variation and how it may impact different soil environments. Additionally, the research does not demonstrate the benefits we see in the media. As the UC Davis Biochar Database, a resource aiming to provide open access to the ever-evolving biochar data, states, “this deficit in basic biochar science makes it difficult for biochar end users to make informed decisions regarding the specific biochar properties to consider when selecting a particular biochar for their use.” It doesn’t mean that biochar may not have a place in agriculture and carbon sequestration, it just means that we should approach its use with a critical mind.

One review of 259 studies conducted by researchers at Mendel University in Brno did not paint a favorable picture of the marketing message. Their review found variable results, including that in some soils biochar decreased available water content, in others it increased erosion and particulate matter emissions, and in others it increased soil salinity and reduced soil fertility. They also found that biochar can cause adverse impacts on earthworm reproduction, growth, and DNA integrity and negative results in the soil microbiome, such as a shift in the fungi-to-bacteria ratio. Other studies point to the greater need for nitrogen-based fertilizers with biochar use due to a higher carbon-to-nitrogen ratio.

What About the Mixing and Biochar?

Some researchers have tested blending biochar with other organic materials, like humates. Humates are decomposed plant matter that supports nutrient absorption and stress resilience. When combined with biochar, they also support heavy metal elimination in the soil. However, in another study, researchers mixed wood biochar with arbuscular mycorrhizal fungi. They concluded that the mixture inhibited phosphorus and nitrogen uptake and possibly had phytotoxic effects on potato growth.

The USDA vaguely recommends mixing biochar with other soil amendments “to address a wide range of environmental, agricultural, and forestry challenges.” There is potential for amendments blended with biochar to improve soil health. However, research into the long-term benefits of mixing biochar with humate or compost is limited. The benefits depend on the biochar feedstock and the soil environment in which it is applied.

A Breakdown of Specific Concerns About Biochar Feedstock Variability and Regulatory Concerns

Not all biochar products are equal. Biochar products vary significantly depending on their composition. Biochar is made from a variety of feedstocks, including wood debris, poultry litter, switchgrass, and walnut hulls, to name a few. Different biochar compositions understandably produce very different products, which have various impacts on soil environments. For instance, biochar manufactured from wood debris or crop residues does not contain significant amounts of plant nutrients like nitrogen and phosphorus, making it less impactful as a soil amendment.

Because different feedstock origins create different nutrient content, pH, and adsorptive capacity, we face regulatory and labeling challenges. The Association of Plant Food Control Officials (AAPFCO) is working to establish definitions and standards for biochar products that focus on clear labeling and the elimination of toxic products from the supply chain. AAPFCO recently approved an official definition of biochar for labeling. This emphasizes the need for growers to understand precisely what is being incorporated into their soil amendment:

Biochar is a solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment (pyrolysis) containing at least 60% carbon. Feedstocks may be crop residues, wood or other forest waste, and animal manure. Materials transported in salt water, painted, or treated with preservatives are prohibited. When listing biochar in an ingredient statement, the feedstock shall be designated by prefixing the term biochar with the feedstock from which it was produced. This includes, for example, poultry litter biochar, green waste biochar, and paper mill biochar. When more than one feedstock is involved, all feedstocks greater than 10% of the total volume are to be listed in decreasing order of volume.

Biochar and alkaline soils

One of the main concerns with biochar in the United States is that it is not effective in alkaline soils. This is like those found throughout the western part of the country. Biochar’s liming effect leads to micronutrient deficiencies, soil health damage, and decreases in yield in these environments. Notably, much of the research touting biochar’s benefits was conducted in acidic soils where biochar’s alkalinity helps neutralize soil pH. This is similar to Brazil, Japan, and Costa Rica.

Environmental and Health Risks of Biochar

Biochar is usually lauded as an environmental superstar, but this image may not present the whole picture. This amendment has a dark side. Biochar may contain toxic compounds such as dioxins, chlorinated hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs), and its disposal may lead to secondary pollution,11 potentially harming human health. Additionally, biochar’s production — the high heat burning of organic material — still emits carbon into the air.

Many in the media have embraced biochar as an environmental savior. Recently, Microsoft announced that it would be buying 95,000 tons of carbon removal credits generated by a biochar production facility.12 But we must ask ourselves: What is the actual impact of these credits? Is this an example of virtue signaling rather than a solution reached by listening to expert agronomists’ insight?

A Call for Further Biochar Research

While biochar has shown some promising benefits in certain situations, many experienced agronomists agree that we need to approach this soil amendment with caution. They also agree that much more research, labelling, and regulation must take place before it becomes a widespread input that agricultural systems depend on.

Biochar is either useless or damaging for Western soils. However, the scientific community will continue to conduct and review research pointing to particular use cases in which it can be beneficial. We particularly need more research around specific feedstocks and their impact on crop health, nutrient absorption, and yield. Until then, proceed with caution.