A critical examination of this trending soil amendment
Biochar is marketed worldwide as a climate-smart solution for improving soil management practices and crop production. What could be better for the planet than turning waste into a soil amendment that can enhance soil and plant health while improving crop yields?
For all of its exciting possibilities, biochar can also possess harmful properties that farmers would do well to consider when deciding how to improve their soil health. Crop advisors and farmers should look closer to understand the hidden downsides of biochar, which proponents of the soil amendment tend not to mention. A critical review of recently published research on biochar use offers a more comprehensive insight into the soil amendment.
Biochar generally only benefits specific soil types (e.g., acidic soils). A majority of published research and reviews lead readers to believe it’s a viable solution for all soil types. However, recent biochar trials and published research papers provide data and information that should make experienced agronomists and farmers hesitant to use this soil amendment.
Biochar’s benefits are overshadowed by the one-size-fits-all approach that ignores agronomic principles. The primary purpose of biochar use is to provide a climate solution — i.e., we should all use it because it helps to sequester carbon and reduce greenhouse gas emissions. This universal push ignores the variability of its benefits in different soil types (either acidic or alkaline). While biochar use does sequester carbon in all soils, the claimed soil benefits are not universally observed.
Broad biochar use as a soil amendment is a waste disposal solution, not an agronomic solution for all soils.
What is biochar?
Biochar is a waste-derived, carbon-rich substance that is created through pyrolysis – a process of combusting organic matter at high temperatures (200°C to 900°C) with limited oxygen. Made at different temperature ranges and with a novel branding approach, biochar is a particular kind of charcoal. Unlike certain forms of charcoal, the product’s intended use is as a soil amendment rather than as fuel. Those who support biochar use claim it improves soil health, boosts agricultural crop yields, and facilitates carbon sequestration.
However, meta-studies confirm that carbon sequestration and pH increase (towards alkaline) are the only consistently observed benefits. Biochar elicits benefits in some micro-conditions, commonly found in soils, more precisely acidic soils, but it’s not a wide-reaching solution to our diverse agricultural challenges. What yielded terrific results in Costa Rican soil may not generate the same results in Illinois. When reviewing published papers, it’s notable that the research showing micronutrient tie-up and crop yield decreases fail to make it into the climate-smart headlines.
What are the benefits of biochar in soil?
There are benefits associated with biochar. It’s an excellent waste management tool that converts waste into something potentially useful. According to the USDA, biochar can improve soil health, raise pH, remediate polluted soils, sequester carbon, lower greenhouse gas emissions, and improve soil moisture.
The International Biochar Initiative (IBI) claims that biochar fights global warming, produces a soil enhancer that holds carbon, makes soil more fertile, reduces agricultural waste, and produces clean, renewable energy. Moreover, farmers can get carbon credit and certificates for using this amendment due to its role in carbon sequestration. Other researchers point to how using biochar as a soil amendment can increase nutrient availability and nitrogen retention, boost crop yields, and bind with heavy metals to support soil quality and environmental remediation.Â
These benefits sound pretty great, but as we dig into the research papers and meta-studies, it becomes clear that only a few types of biochar elicit the benefits mentioned above and only work in specific soil types. In several published papers, a significant portion of the attributed benefit comes from biochar’s liming potential, which raises soil pH. According to a U.S. Biochar Initiative promotional brochure, applying biochar increased wheat production by 280%. The brochure author failed to highlight that the soil pH in the trial was 3.95, making the addition of ag lime or any product with liming potential to this soil beneficial. Unlike the severely acidic soil in the reference biochar trial, most of the world’s arable soil is near neutral or alkaline and will not react significantly to items that raise soil pH.
Concerns about biochar
The primary issues with biochar are the notable differences in composition between the feedstocks used in the pyrolysis process and the byproducts produced throughout the process. Additionally, the research doesn’t demonstrate the agronomic (soil management and crop production related) benefits we see in the media hype.
This point is emphasized in the UC Davis Biochar Database, a resource aiming to provide open access to the ever-evolving biochar data, which 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 simply means that we should take caution while utilizing biochar, even though it may still have a role in agriculture and carbon sequestration. Using biochar shouldn’t be dictated by climate initiatives, disregarding agronomic principles.
One review of 259 studies conducted by researchers at Mendel University in Brno, Czech Republic, painted a less pleasant picture than the current biochar narrative. The study revealed inconsistent findings. It showed that biochar reduced soil fertility and increased soil salinity in some soils, while increasing erosion and particulate matter emissions in others. They also found that biochar causes adverse impacts on the reproduction, growth, and DNA integrity of earthworms, and negative results in the soil microbiome, such as a shift in the fungi-to-bacteria ratio.
Charging the char
After a quick search for biochar-related reviews online, you see a consistent mantra: “You gotta charge the char.” The fundamental idea is that biochar is sterile due to the pyrolysis process, and combining it with other amendments or soil inoculants would be more advantageous. Some researchers have tested blending biochar with other organic materials, like humates.
Humates are decomposed plant matter that supports nutrient absorption and soil health. They have also been shown to support heavy metal elimination in the soil when blended with biochar.
In this research trial, both the vermicompost and the humate blended with biochar considerably lowered the human health risk index of heavy metals compared to biochar alone. However, the study did not test humate and vermicompost independently from the biochar, suggesting that further research is needed to isolate the specific contributions of each amendment and assess whether the observed benefits are derived from the biochar or the additional materials.
In another study, researchers mixed wood biochar with arbuscular mycorrhizal fungi, concluding that the mixture inhibited phosphorus and nitrogen uptake and possibly had phytotoxic effects on potato growth. This finding is consistent with meta-studies on biochar, demonstrating a wide range of beneficial or detrimental microbial population variations observed in biochar experiments.
The USDA vaguely recommends mixing biochar with other soil amendments “to address a wide range of environmental, agricultural, and forestry challenges.” The NRCS Conservation Practice Standard Code 336 details that users should “use manure, compost tea, or compost to inoculate biochar to help an application by stabilizing pH, balancing nutrients and their interactions, and improving the moisture content of changes.”  Â
Unfortunately, using beneficial soil amendments that provide advantages independent of biochar is not the same as recommending blending those amendments with biochar to increase soil health.
Future research is needed to demonstrate the long-term benefits of blending biochar with humates, microbes, and compost. Current knowledge is limited, and the benefits depend highly on biochar feedstock and the soil environment (rhizosphere micro-climate) in which it is applied.
Additional biochar concerns
Feedstock Viability
Not all biochar products are equal. Biochar products vary significantly depending on their composition. Biochar is made from a wide assortment of feedstocks, including wood debris, poultry litter, switchgrass, and walnut hulls, to name a few. Naturally, varied compositions of biochar result in highly distinct products, each of which has a different effect on soil ecosystems. For instance, biochar manufactured from wood debris or crop residues does not contribute significant levels of plant nutrients like nitrogen and phosphorus. Unlike most biochar products, manure-derived biochar is a source of minor nutritional value.
Regulatory Concerns
As a result, biochar faces regulatory and labeling challenges because varied feedstock origins produce different nutritional content, pH, and adsorptive capacity. The Association of American Plant Food Control Officials (AAPFCO) recently established definitions and standards for biochar products that emphasize accurate labeling is necessary to help remove potentially hazardous goods from the supply chain.
In 2016, AAPFCO approved an official definition of biochar for product labeling, underscoring the need for growers to know precisely what is going into their soil amendment.
“Biochar is a solid material obtained from thermochemical conversion of biomass in an oxygen-limited environment (pyrolysis) containing at least 60% carbon. Feedstocks may be composed of crop residue, wood or other forest waste, and animal manures. Materials transported in salt water, painted, or treated with preservatives are not permitted. 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, i.e., poultry litter biochar, green waste biochar, papermill biochar, etc. When more than one feedstock is involved, all feedstocks greater than 10% of the total volume are to be listed by decreasing volume. Their uses include soil amendments.”
Coal can also be used to create char amendments, and research shows that coal char and biochar produce similar results in terms of soil health. In a recent study, biochar was inferior to coal char in terms of many soil health indicators. However, the benefits of using fossil fuels (e.g., coal) to make biochar don’t fit the climate-smart narrative and didn’t make headlines.
Biochar and alkaline soils
One of the main issues with biochar use as a soil amendment is that it may adversely affect alkaline soils, which are prevalent in many regions worldwide. The liming effect of biochar leads to micronutrient deficiencies, soil health damage, and decreased yield in the environments mentioned above. Notably, much of the research touting biochar’s benefits was conducted in acidic soils where biochar’s alkalinity helps neutralize soil pH, such as in Brazil, Japan, and Costa Rica. The following statement related to pH and micronutrient binding is found on the International Biochar Initiative (IBI) website:
“One study that compared the effect of adding biochar to an acidic and an alkaline soil found greater benefits on crop growth in the acidic soil, while benefits on the alkaline soil were minor. In another study, adding biochar to soil caused increases in pH, which had a detrimental effect on yields, because of micronutrient deficiencies which occur at high pH (>6). Care must be taken when adding any material with a liming capacity to alkaline soils.”
Another study conducted by researchers at Delaware State University and Sichuan Normal University indicates that the best practice for biochar is to prescreen soils based on pH being less than 7.5 to avoid micronutrient availability problems caused by biochar.
Environmental and health risks of biochar
Biochar is credited as an environmental superhero, but the picture painted doesn’t give farmers the whole story. This amendment has an environmental dark side. According to the IBI, many testing parameters, including known carcinogens, are present and must be below acceptable thresholds. Biochar may contain toxic compounds such as dioxins, furans, chlorinated hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs), and its disposal may lead to secondary pollution, potentially harming human health.
Germination inhibition assays are one of the pass/fail tests needed to obtain IBI accreditation. A farmer who grows crops from seed should be highly concerned about the potential for biochar to hinder and reduce crop germination.
Many media outlets have jumped on the hype of biochar as the environmental cure to carbon removal. Microsoft announced it would buy 95,000 tons of carbon removal credits generated by a biochar production facility.
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 the insight of expert agronomists? Will this biochar be applied to soils solely to achieve carbon credits without thorough consideration of their agronomic merit?
A call for further biochar research
Although biochar use in agriculture has demonstrated some encouraging results in specific circumstances, many experienced agronomists agree that we must approach this soil amendment cautiously. A growing body of research confirms that biochar can be ineffective or harmful for most western soils. The scientific community will continue to conduct and review research on specific use cases in which biochar can be beneficial.
Perhaps the solution that serves farmers and environmentalists equally well is out there waiting to be realized, but we need more research on particular feedstocks, biochar variability, and their impact on specific crops’ health, nutrient absorption, and yield. Crop experts and growers should wait patiently, ignore the media hype, and proceed cautiously.
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