Biochar: Opportunities and Limitations for Soil Health and Water Quality

Biochar is gaining renewed attention in agriculture and environmental management as a tool with the potential to address several pressing challenges at once. In Pennsylvania, where producers are balancing soil health goals, soil moisture retention during extended droughts, nutrient management requirements, and water quality concerns, biochar offers a promising future. This is especially true within the Chesapeake Bay Watershed. Researchers, land managers, and conservation specialists in this region are continually seeking new practices to help meet water-quality goals. Biochar shows many promising attributes, but like many other conservation practices, biochar has limitations and is not yet an additive that can be done for all land or management strategies.

Produced by heating organic materials in a low-oxygen environment, biochar is a carbon-rich material that can be added to soils or used in a variety of agricultural and environmental applications. A variety of organic materials can serve as the base, or feedstock, for biochar, including wood chips, crop residues, and manure solids. Each of these biochar feedstocks gives the resulting biochar unique properties and should be considered before using biochar in water quality or soil health-related situations. Overall, research across the Northeast and beyond suggests that biochar may improve soil structure, increase water-holding capacity, and enhance nutrient retention, while also providing opportunities for long-term carbon storage.

However, outcomes from biochar use are highly variable. Some studies show clear agronomic and environmental benefits, while others report minimal or no measurable improvements. This variability is especially important for Pennsylvania producers and stormwater managers to consider, as they must deal with a variety of soil types, climates, and management systems. This article explores the basics of biochar, covering its potential benefits for soil health and water quality, as well as its limitations.

Biochar History

While biochar is currently considered a new conservation strategy, it does have a long history of use in South America. The concept of biochar is rooted in ancient land management practices. Indigenous peoples in the Amazon Basin created highly fertile soils known as Terra Preta by incorporating charcoal along with organic wastes into the soil. This was essential for them to raise crops to support their population since the soils in the Amazon Basin are very old, weathered over time, and often lacked the nutrient-holding capacity of more fertile soils. These biochar-amended soils remain more productive than surrounding areas even centuries later, sparking modern interest in biochar as a tool to improve soil health and store carbon.

Over the past two decades, biochar has been studied extensively in agricultural, environmental, and climate contexts. Interest has grown in Pennsylvania and the broader Mid-Atlantic region due to increasing attention on soil health, nutrient management, carbon sequestration, and soil water retention.

Biochar Definition

Biochar is a stable, carbon-rich material produced by heating organic biomass in a low-oxygen environment. This process, known as pyrolysis, produces other byproducts, such as bio-oil and syngas. These byproducts can be used for energy production, for heat, or power generation. Unlike ash from open burning, biochar retains much of the original carbon structure of the feedstock, making it resistant to decomposition and capable of persisting in soils for decades to centuries. Further, after the more volatile compounds in the biomass are lost from the biochar during pyrolysis, the remaining structure has a very large surface area that increases water and nutrient holding capacities. One key takeaway from the production of biochar by pyrolysis is that it is not a uniform product. Its characteristics, and therefore its suitability for different applications, depend on how it is produced.

There is no single material called biochar. Instead, there are many different biochars with very different properties, just as each feedstock material has very different characteristics before processing into finished biochar. This affects how they interact with soils, release or hold nutrients, modify microbial communities, remove or release pollutants, and store carbon.

Biochar often has several properties that make it a beneficial soil amendment for agriculture or other stormwater features. These include the following:

• High surface area and porosity:  creates habitat for beneficial soil microbes and helps improve soil water retention.
• Chemical stability:  the carbon in biochar is highly resistant to microbial decomposition, enabling long-term carbon storage in any system to which biochar is added.
• Variable pH:  biochar often has a slightly alkaline (basic) pH. This can help neutralize acidic soils. Therefore, biochar may act like a liming agent in agricultural soils. The extent to which biochar can neutralize acidic soils depends on the application rate.
• Cation exchange capacity (CEC):  biochar enhances nutrient retention and reduces leaching of nutrients like nitrogen and phosphorus.
• Adsorptive capacity:  biochar can bind contaminants such as heavy metals and organic pollutants.

Biochar Production Impacts Its Characteristics

Feedstock composition strongly influences biochar characteristics because different biomass types contain varying proportions of lignin, cellulose, hemicellulose, and mineral constituents (Tomczyk et al., 2020; Li et al., 2022). Wood-based feedstocks generally produce biochars with higher carbon content and specific surface area, but lower nutrient and ash content compared to crop residues or manure-derived biochars. The higher lignin and cellulose content in woody biomass promotes the formation of more condensed aromatic carbon structures during pyrolysis, thereby increasing stability and sorption potential. In contrast, herbaceous and manure-based feedstocks tend to yield biochars with higher nutrient concentrations and higher ash content, reflecting the greater mineral content of the original biomass.

Feedstock type also influences physical structure and porosity. For example, switchgrass-derived biochar often exhibits greater pore volume due to retention of its original plant structure, whereas wood biochar typically develops a higher specific surface area due to lignin-driven pore formation during pyrolysis. Additionally, non-woody feedstocks such as crop residues generally produce biochars with higher pH and cation exchange capacity, making them more suitable for nutrient retention and soil amendment applications. Overall, feedstock selection defines the baseline chemical and structural properties of biochar, which are then further modified by pyrolysis conditions.

Pyrolysis temperature is one of the most critical determinants of biochar physicochemical properties because it governs the extent of biomass decomposition, carbonization, and structural transformation. Across studies, higher pyrolysis temperatures (generally >500 °C) consistently increase biochar carbon content, aromaticity, pH, ash content, and specific surface area, while decreasing hydrogen-to-carbon (H/C) ratios and nitrogen content (Ippolito et al., 2020; Tomczyk et al., 2020; Li et al., 2022). This occurs because higher temperatures promote the volatilization of more easily degradable compounds, leaving behind a more condensed, aromatic carbon structure that is more resistant to microbial degradation. Lower hydrogen-to-carbon (H/C) ratios at higher temperatures indicate greater aromaticity and stability, often corresponding to long-term persistence in soils, with potential residence times ranging from decades to centuries. Furthermore, high-temperature pyrolysis enhances pore development by driving off volatile compounds, thereby increasing pore volume and specific surface area, which are essential for sorption processes.

Conversely, biochars produced at lower temperatures (≤500 °C) retain more oxygen-containing functional groups such as carboxyl and phenolic groups, which contribute to greater cation exchange capacity (CEC) and surface reactivity. However, these low-temperature biochars are generally less stable. They contain more volatile matter that can be more readily decomposed in soil. Thus, pyrolysis temperature reflects a tradeoff between chemical reactivity and long-term stability. In general, higher temperatures favor persistence and sorptive capacity, whereas lower temperatures favor nutrient exchange and short-term reactivity.

Potential Uses of Biochar

Improving soil properties

In general, application of biochar to soils can improve soil aggregate stability, raise pH, increase buffering capacity, increase soil carbon storage, increase cation exchange capacity, increase available nutrients, improve water retention, and increase microbial activity, but results can be quite variable depending on the type of biochar, the application rate, and the soil characteristics.

Soil aggregation is one of the most important physical properties for soil health and function. Soil aggregates are soil clumps of varying sizes and shapes that allow gases and water to be stored and to move through soil, provide microhabitats for a variety of soil organisms, reduce soil erosion, and form a protective structure that prevents soil organic matter from breaking down rapidly, resulting in soil carbon storage.

Research has demonstrated that adding biochar at low to moderate rates can improve soil aggregation and carbon storage, with varying effects across soil types and biochar types. The highly porous structure of biochar improves soil aggregation by providing numerous small, sheltered spaces for microbes, protecting them from desiccation and predation, supplying them with carbon, and promoting the formation of glue-like secretions and the development of fungal hyphae that help bind soil particles and biochar together. In general, biochar application is less effective in improving aggregation in sandy/coarse textured soils, which tend to be more vulnerable to carbon loss.

Biochars are typically alkaline, particularly from wood-based feedstocks. Application of biochar generally exhibits a liming effect, increasing soil pH more slowly than lime but lasting longer. This can be particularly beneficial in amending acidic soils and soils with low cation-holding capacity.

The porosity and high surface area of biochars, along with the attendant improvements in soil structure, typically also significantly reduce soil bulk density and increase infiltration, water holding capacity, and saturated hydraulic conductivity, the ability of water to move in wet soil. Applying biochar can increase water infiltration and reduce runoff and soil erosion. In some cases, this can substantially reduce runoff, soil erosion, and nutrient loss, especially nitrate. Some research has suggested that no-till incorporation of biochar, along with other soil amendments such as nitrogen fertilizer, is more effective at preventing erosion than tilled incorporation. Higher biochar rates are required to have a significant impact on runoff volume. Runoff reduction results vary, even when using the same type of biochar at the same rate. However, the largest impact is often seen in compacted soils with an initially low surface water infiltration capacity.

Crop nutrient management

Biochar has a high cation exchange capacity, allowing it to attract and retain minerals and organic matter. This can directly improve nutrient availability to plants. Biochar's impacts on nutrient management are mixed, depending on soil, crop, timing, and biochar type. Biochars can enhance or reduce the availability of nitrate and phosphorus. Biochar selection must therefore be based on the specific crop and soil type, as well as the desired results. A general rule of thumb is that biochar made from crop residues, or manures, can improve soil fertility and even serve as a better nutrient amendment than woody-based biochar. The tradeoff being that the biochar made from these feedstocks have less carbon and can be more easily decomposed.

Biochar addition has shown a significant increase in crop yield in some cases, but in others, biochar application has had no impact or even reduced crop yields, suggesting that nutrients already meet or exceed crop demand. Low-nutrient biochars, such as those made from woody biomass, applied without fertilizer may have no impact on yield. Biochar application with fertilizer may increase yields beyond the initial application periods, suggesting residual benefits over multiple years. Excessive rates of biochar application can reduce yield, typically by immobilizing nutrients and making them unavailable to plants. In general, sandy or low-nutrient soils show more benefit from biochar application, particularly when applied with fertilizer.

Many biochars can mobilize and slowly release nutrients from binding sites over time. Biochar-coated fertilizer is an option to slow the release of fertilizer nutrients, allowing plants to use them efficiently, and reducing leaching.

Biochar's porosity, surface functional sites, and high adsorption capacity can improve plant nutrient availability by reducing in-field nutrient loss from runoff. Biochar may decrease runoff volume and the concentration of nutrients present in the runoff, including total nitrogen, total phosphorus, and nitrate.

Biochar in soil can release or store nutrients, depending on its nutrient composition, which is largely determined by feedstock and production methods. When applied superficially rather than incorporated into the soil, biochar can lose nutrients through runoff. Biochar generally has a greater impact on reducing nitrate leaching in coarse or degraded soils. In some cases, biochar application may increase losses of phosphate and ammonium, particularly immediately after initial application, whereas in others it decreases losses.

In general, manure-based biochars have the highest nutrient content, particularly those derived from poultry and swine manure, making them a desirable choice for crop nutrient management. They can also contain higher salts and sometimes other unwanted pollutants, so it is important to verify that any biochar used in food crops has been tested and meets safety standards.

Remediation/contaminant removal

Because of its large surface area, biochars can bind with heavy metals and other soil pollutants, making them less bioavailable. This can be beneficial in some contaminated soils to prevent crop uptake of harmful substances, but it must be evaluated for continuing effectiveness. Micronutrients needed by crop plants may also be immobilized in the same way, so additional fertilization may be required.

Stormwater applications

Studies indicate that biochar may be a valuable tool to enhance the effectiveness of stormwater systems and improve the health and resilience of plants within stormwater infrastructure due to its adsorption capacity and porosity. When integrated into bioretention basins, rain gardens, green roofs, grassed swales, and permeable pavers, biochar can increase the water holding capacity and retention time, as well as reduce chemical oxygen demand, remove pollutants, such as nitrate, lawn chemicals, sodium, metals, pesticides, road and vehicle chemicals, organic contaminants, and some bacteria. Phosphate removal is variable because untreated biochars generally do not adsorb phosphate efficiently or uniformly, so modified or activated biochar may be needed.

Biochar use for stormwater treatment should be based on characterization of the influent and prioritizing the constituents that need to be removed. Woody or plant feedstock biochars generally perform more effectively in stormwater applications. Manure- or sewage sludge-based biochars should be avoided because of their higher nutrient content, which could lead to nutrient and pollutant leaching. Competition for binding sites can impair nutrient and pollutant removal, especially when organic matter or compost is high in the media. The service life or exhaustion period of biochar in stormwater systems can range from months to years and should be monitored.

Wastewater applications

Biochar has the potential to improve wastewater treatment in similar ways to its benefits in stormwater treatment. For maximum effectiveness, this typically requires some form of modification to enhance the biochar's surface chemistry and improve functionality. This can involve chemically or biologically treating the feedstock or the biochar so it more readily removes or recovers nutrients such as phosphate and ammonium and efficiently traps pollutants.

Limitations and Tradeoffs

Despite its well-documented benefits, biochar application introduces several limitations and tradeoffs that must be carefully considered for sustainable use. A recurring theme across published scientific studies is the high variability and uncertainty associated with biochar performance. The effects of biochar are strongly context-dependent, varying with soil type, crop system, biochar feedstock, and production conditions. For example, while biochar can enhance crop productivity in degraded or nutrient-poor soils, its effects in fertile or temperate soils may be negligible or even negative, with reported cases of reduced yields and inconsistent responses across regions (Tisserant & Cherubini, 2019; Xu et al., 2025). Similarly, studies assessing global biochar usage show large variability in outcomes, with crop yield increases ranging widely depending on environmental and management conditions, underscoring the challenge of generalizing results (Li et al., 2024). This variability highlights the need for site-specific evaluation prior to application.

One major limitation stems from the dependence on soil properties and application conditions. Biochar can improve water retention, structure, and fertility in coarse-textured or degraded soils but may produce limited or adverse hydrological effects in clay-rich or already fertile systems (Xu et al., 2025). In some cases, biochar addition can alter soil hydraulic properties, reducing infiltration or disrupting the existing structure. Excessive application rates further exacerbate these issues. High biochar application rates have been shown to increase soil pH beyond optimal levels, particularly because many biochars are alkaline, thereby reducing nutrient availability and potentially lowering crop productivity (Tisserant & Cherubini, 2019; Xu et al., 2025). Additionally, high application rates can influence nitrogen cycling, including increased leaching or altered gaseous emissions, demonstrating that application rate is a critical factor influencing both agronomic and environmental outcomes (Li et al., 2024).

Biochar also presents tradeoffs in nutrient dynamics, particularly through nutrient immobilization processes. Its porous structure, high surface area, and strong sorption capacity enable it to retain nutrients such as nitrogen and phosphorus, but this same property can reduce short-term nutrient availability to plants (Li et al., 2024). For example, biochar can adsorb ammonium and nitrate, altering nitrogen transformation pathways and temporarily limiting plant access to these nutrients (Li et al., 2024; Xu et al., 2025). Furthermore, interactions with soil microbial communities can increase nitrogen immobilization, especially when biochar introduces labile carbon that stimulates microbial uptake of available nitrogen (Xu et al., 2025). Although these effects may diminish over time as equilibrium is reached, they represent an important short-term tradeoff in biochar-amended systems.

Another significant limitation involves chemical interactions with agrochemicals. Biochar’s high adsorption capacity allows it to bind organic compounds, including pesticides and herbicides, through mechanisms such as pore filling, electrostatic attraction, and hydrophobic interactions. While this property can enhance contaminant remediation and reduce environmental pollution, it can simultaneously decrease the effectiveness of agrochemicals in agricultural systems (Tisserant & Cherubini, 2019). Reduced bioavailability of pesticides may limit their efficacy and require higher application rates, introducing economic and environmental tradeoffs. Additionally, biochar’s sorptive properties may limit the biodegradation of organic compounds by restricting microbial access, further complicating its role in soil chemical processes (Tisserant & Cherubini, 2019).

During its operational life, biochar can reach a maximum capacity for holding nutrients and contaminants. When binding sites are filled, the biochar may no longer have the capacity to remove or hold additional materials. If the biochar becomes saturated, pollutants can pass through without being removed, unless there is a chemical that is more competitive and can displace what has already been captured.

Finally, contamination risks and system-level impacts must be considered. Biochar composition varies widely depending on feedstock and pyrolysis conditions, and some biochars may contain potentially harmful substances such as polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds, and heavy metals (Tisserant & Cherubini, 2019). While biochar can immobilize certain contaminants and reduce their bioavailability, it may also alter the mobility and behavior of pollutants in unpredictable ways. For example, changes in soil chemistry may increase the mobility of some elements under specific conditions, highlighting the complexity of biochar–contaminant interactions (Tisserant & Cherubini, 2019). Furthermore, life-cycle and system-scale considerations, including feedstock sourcing, transport, and production emissions, introduce additional tradeoffs that influence the overall sustainability of biochar systems (Tisserant & Cherubini, 2019).

In summary, biochar application involves important tradeoffs between improving soil properties and introducing risks related to variability, nutrient dynamics, chemical interactions, and potential contamination. Its effectiveness depends heavily on site-specific conditions, biochar characteristics, and management practices. Consequently, careful evaluation and context-specific application are essential to maximize benefits while minimizing unintended consequences.

Effective Use and Safe Handling

Before even considering a biochar for use on your land, it is important to obtain its analytical test results. These can be requested from your biochar supplier and is essential to ensure you are not adding contaminants, such as trace metals, to your system. All tests should be done according to the American Society of Agricultural and Biological Engineers (ASABE) standard S668 (ASABE, 2025). These reports often list the safe threshold for each contaminant, so you can decide whether it is the right biochar for you. Knowing what is in any soil amendment that you purchase will help keep you, your land, and your water safe.

Safe storage and handling of biochar are essential because, despite being a stable carbon material, it presents dust, fire, and self-heating hazards if not properly managed. Biochar can generate airborne particulate matter during handling activities such as sieving, transfer, or packaging (Gelardi et al., 2019). These particles can fall within the Particulate Matter (PM) PM₂.₅ and PM₁₀ size ranges, which are associated with respiratory risks due to deep lung penetration or upper airway irritation. Exposure levels can significantly exceed recommended occupational limits if controls are not in place. To mitigate these risks, operators should implement dust suppression strategies such as lightly wetting biochar prior to handling, minimizing drop heights during transfer, and ensuring adequate ventilation or dust extraction systems in enclosed environments (Miles, 2023). Personal protective equipment (PPE), including N95 respirators, gloves, eye protection, and full skin coverage, is also recommended to protect workers from inhalation and contact hazards.

In addition to dust concerns, biochar presents a unique fire and storage hazard due to its potential for spontaneous combustion, particularly when freshly produced or very dry. Fresh biochar can rapidly absorb oxygen, generating an exothermic reaction that may lead to self-heating and ignition without an external heat source. This risk is heightened when biochar contains residual volatile compounds or when fine particles accumulate in confined spaces, where dust explosion hazards may occur. To safely store biochar, it is recommended to maintain appropriate moisture levels.  Typically, 30 to 40% moisture is appropriate for safe bulk handling (Prabha and Harfield, 2025). This is because wetter material is less prone to ignition. Storage areas should be cool, dry, and well-ventilated, and biochar should be kept away from heat sources and oxidizing agents. Flexible bulk containers are preferred because they allow heat dissipation, and continuous monitoring of stored material temperature is advised to detect early signs of self-heating. Overall, effective biochar handling requires a combination of moisture management, ventilation, safe storage design, and worker protection practices to minimize both health and fire risks.

Sustainability

When considering introducing biochars as soil amendments or water quality media, it is critical to evaluate the economic and environmental sustainability of the biochar, from feedstock through end-of-life disposal. The American Biochar Institute issued draft guidelines in 2011 that can serve as a foundation. In general, biochar feedstocks should be sourced from waste biomass, rather than uniquely grown for biochar production. Biochar production should not be based on land use change from higher-quality use. Production should minimize energy and water use, emissions, and harmful byproducts, and avoid degrading natural resources and biodiversity. Biochar should be manufactured locally/regionally and as close as possible to the point of use and transported using the lowest emissions options.

Biochar production can be an effective way to recycle waste products that might otherwise be burned or landfilled. Additionally, some uses of biochars, such as filtration, have the capacity to recover spent biochar and re-charge or re-activate it for new purposes using heat or chemical treatment. In other cases, when biochar becomes saturated with hazardous contaminants, it must be disposed of safely. This is typically done by burying the biochar, which does not remove the hazardous materials but does provide continued carbon sequestration.

Economics, local availability, and benefits of appropriate biochar for a specific use should be evaluated by determining goals, researching the best biochar recommendations for the particular use, and comparing tradeoffs and benefits. In some cases, biochar applications result in soil health improvement without any yield benefits, or the goal may be carbon capture, but the available carbon incentives don’t cover the cost of the biochar. In general, biochars produced at low to moderate pyrolysis temperatures contain more plant nutrients but less stable carbon, while those produced at higher pyrolysis temperatures have lower nutrients, but more stable carbon for long-term sequestration. Each use case for biochar may benefit from combining different biochars for multiple desired benefits, combining biochar and fertilizer application to maximize efficacy, or integrating activated or enhanced biochars for greater and more targeted function.