A large-chip biochar, showing the carbon framework of tree rings. Photo courtesy of Zack Shier.

Urban forests are critical to the resilience and sustainability of our cities and towns, offering ecosystem services such as temperature regulation, air and water quality, carbon sequestration and stormwater management. However, trees in urban settings often grow under hostile soil conditions characterized by compaction, contamination, limited rooting volume and poor nutrient availability (Scharenbroch et al., 2022). These constraints to healthy trees necessitate innovative soil-management strategies to enhance establishment and long-term vitality. Among emerging amendments, biochar, a carbon-rich byproduct of pyrolyzed biomass, has shown substantial promise in improving urban soil function and tree performance.

This article is part of a series that synthesizes peer-reviewed evidence on the application of biochar in tree planting and management, emphasizing mechanisms of action and best practices for field implementation. Additionally, it aims to inform urban-forest managers and arborists, encouraging the application of biochar-based strategies to enhance tree health and performance.

Direct benefits to urban trees
In urban forestry and arboriculture, biochar has been investigated for its physical, chemical and biological impacts on urban soils. When integrated into tree planting or used as a post-planting amendment, research indicates biochar improves soil porosity, water-holding capacity and nutrient retention (Schaffert et al., 2022). Field trials have demonstrated enhanced growth responses in species such as Acer saccharinum and Quercus rubra, especially when biochar is combined with composts or biofertilizers (Sifton et al., 2023).

Biochar improves soil aeration, structure and water infiltration, which are vital for root growth in urban environments. Its porous, low-density nature increases soil porosity, aids oxygen delivery to roots, reduces compaction and supports biological activity (Schaffert et al., 2022). Biochar’s high surface area also enhances soil structure and water retention, benefiting typically compacted, stressed urban soils.

Urban trees are often nutrient deficient, and supplemental fertilization is necessary. Using biochar with fertilizers enhances nutrient efficiency and soil health by serving as a nutrient reservoir, reducing leaching and promoting gradual nutrient release (Schaffert et al., 2022). This combination improves nutrient uptake, supports beneficial microbes and fosters sustainable fertilization for better tree growth. While the application of biochar in urban forestry is still growing, studies have begun to bridge the gap between laboratory findings and field applications. For example, Scharenbroch et al. (2022) documented improvements in soil organic matter and basal-area growth in suburban street trees following a combined treatment of biochar, fertilization and air tillage.

Applying biochar with a pH of 9 into soil with a pH of 7.5 during air spading and soil reduction. Photo courtesy of Zack Shier.

Environmental and ecosystem co-benefits
Biochar offers environmental benefits for urban forests by storing carbon long term and lowering greenhouse gas emissions, while recycling urban biomass. Its stable structure from pyrolysis makes it an effective carbon sink, with soils amended by biochar retaining carbon for centuries! Also, biochar has been found to improve stormwater management through better soil infiltration, water retention, and contaminant filtration, supporting healthier vegetation and more resilient urban infrastructure. By preventing biomass decomposition and interacting with native organic matter, biochar boosts soil carbon stability. Lifecycle analyses suggest biochar systems using renewable feedstocks and energy recovery can achieve net-negative emissions, making biochar a viable climate-mitigation tool.

Properties and mechanisms 
The unique properties of biochar make it especially useful in the poor, degraded urban soils that arborists and urban foresters work with every day. Biochar products of good quality can be useful in many different types of soils and species of woody plants. While recent research has shown a myriad of positive results, there is still much to learn about how biochar reacts in all types of soils and the benefits it can provide. This section aims to outline the known physical, chemical and biological characteristics and properties of biochar, as well as provide context for the differences between granular (>0.5mm) and powdered/micronized (<0.5mm) biochar – whether dry or in liquid solution.

Physical properties
There can be many sizes of biochar. The most common sizes seen on the market are of the granular and powdered sizes, but larger “chip” biochar and sizes in between can be purchased.

One of the best properties of biochar leads to one of the best benefits it provides to soils: pore space. The porosity and surface area of biochar is extremely high, resulting in a relatively non-compactable carbon framework. Biochar’s structure looks and acts almost like a sponge or a coral reef, where tiny holes and tubes create space between the framework of pyrolyzed carbon. Pyrolyzed carbon is the carbon-rich solid produced when organic biomass is thermally decomposed under low-
oxygen conditions, resulting in a chemically stable and porous material with extensive micro- and macro-pore structure resisting biological decay. When biochar is made of woody material, this space is quite literally the vascular and vessel system of wood. This immense amount of porosity and surface area reacts to the soil environment in a similar way that soil pore space does, allowing space for water, air, microbes, roots, etc.

This physical structure is the basis for all the physical, chemical and biological benefits biochar provides for soils. Thinking back to soil science 101, it is the combination of soil humus, organic matter and soil biology that “glues” soil together, creating macro and micro pores within the soil, allowing water, air and soil organisms to better utilize the space. While we are not saying biochar works identically to this, it does provide and improve pore space in a carbon-based structure, positively interacting with the soil. By providing more pore space, gas exchange and water-holding capacity improve, thereby reducing compaction – attributes most desirable for degraded urban soils.

Research indicates biochar made from woody materials provides superior porosity and improved surface area (Schaffert et al, 2022). This makes sense, considering woody materials already have a carbon framework (Photo 3) in their wooden structures. Consequently, biochar made from other materials, like sewage or animal waste, lacks that wooden structure which contributes improved porosity. With more arborists using powdered or micronized biochar, the question of porosity comes up, specifically whether powdered biochar still provides benefits physically to the soil. Initial research has indicated that while powdered biochar is not as physically large, it still provides some of the physical soil benefits. Bartocci et al., 2017 revealed that granular or pellet-sized biochar improved water-holding capacity by 23%, and powdered biochar improved water-holding capacity by 4% in the soil, but this is an area of research that needs more attention.

Powdered biochar may be more practical to apply, given that it can be pumped through tanks and soil probes into the soil profile, potentially allowing a more affordable price for the client, as well as easier application. We believe it is clear that powdered biochar is not a substitute for granular biochar when the physical soil characteristics are a priority; however, it does appear that powdered biochar is still improving the physical attributes of soil, whether directly or indirectly, through biological changes that improve soil carbon stability and soil aggregation. Expected soil transformation includes macro and microporosity, improved aeration and enhanced water retention.

Biochar particle sizes ranging from up to 1″ in diameter down to 5 microns. Photo courtesy of Biochar NOW.

Chemical properties
The chemical properties that biochar influences may be one of the most misunderstood areas in regard to soil processes. This may be due to the diverse composition of biochar itself, depending on how it’s made, or because the interactions of biochar and soil differ than those of fertilizer and soil, the latter of which most arborists are far more familiar with.

The biggest question often asked about biochar, chemically speaking, is about the pH value of biochar and whether it can be used in alkaline, high-pH, urban soils. Typically, biochar from a reputable manufacturer has a pH ranging between 8-9, which makes it alkaline. That said, making your own biochar or buying from a manufacturer who does not test for pH could yield pH levels as high as the 10-11.5 range. This has made arborists who work in already high-pH urban soils nervous, as increasing pH creates a problem for nutrient absorption and chlorosis. If you are an arborist who works in acidic soil conditions, this may not apply as much; however, pH is not the only characteristic to consider when sourcing a quality biochar. So, can biochar be applied on higher-pH soils? The informed response is yes, reputable sources of biochar can be effective in high-pH soils.

A Scanning Electron Microscope photo of the surface of a biochar particle. Photo courtesy of Zack Shier.

This article focuses on the chemistry behind this answer. The second article in this series, to be published in an upcoming issue, will dive into the International Biochar Initiative (IBI) standards. This will address the multiple analyses of biochar, so arborists can understand what makes a quality biochar and what metrics are applied in a biochar analysis in order to make an informed decision when talking to biochar vendors.

The main scientific explanations for why alkaline biochar typically does not harm soil health include its ability to buffer pH, enhance cation exchange capacity (CEC) and promote beneficial biological changes in the soil (Percival, 2023). Multiple studies using biochar in urban soils have been examined, most notably by Scharenbroch et al. 2013, Scharenbroch et al. 2022 and Percival et al. 2023. All three studies noted no significant change or difference in pH compared to controls when used in neutral to alkaline urban soils. The pH of the biochar used in these studies was 9.18 and 8.17, respectively.  This shows that when pH levels of biochar are moderately alkaline, any change in soil pH is either irrelevant or the positive effects far outweigh the change in pH. While soil pH is an important metric, it is not the deciding factor in whether a tree can thrive in a given location.

Due to biochar being carbon based and highly porous, soil nutrients and soil microbes can interact with biochar immediately. Not only can biochar participate in CEC, which is the soil’s ability to hold and swap positively charged nutrient ions with plant roots, but it can generally hold both positively and negatively charged nutrients, reducing nutrient leaching of important, positively charged nutrients like phosphates, and also allows more nutrients to be readily available for both soil microbes and tree roots. This helps to buffer the existing pH, allowing more nutrients to be available in a high-pH soil than would otherwise be available. This mechanism is the same “buffering” action used to calculate necessary fertilizer rates based on soil texture. If your soil has more clay, your CEC goes up, and therefore more fertilizer needs to be applied. However, the difference when using biochar is that the application allows for reducing compaction, increasing nutrient holding and availability and improving soil microbe interaction at the same time. This is the exact reason the recommendation exists to lower fertilizer rates when used in conjunction with biochar applications. Biochar improves nutrient utilization and allows more nutrient uptake, regardless of soil pH. Additionally, this is why it is recommended to use an accompanying amendment or fertilizer with biochar. When soil or foliar testing is not available or nutrient deficiencies are unknown, use organic products or biostimulants in conjunction with biochar.

When comparing granular biochar to micronized or powdered biochar, initial research would indicate that powdered biochar possibly has a greater ability to influence CEC and nutrient availability (Thomas, 2021). This is because cation exchange is a function of both surface area and electrical charge. Clay particles have a large surface area and are generally negatively charged, which attracts positively charged nutrients like Iron (Fe3+). Since biochar can participate in CEC, this means it can “act” like clay with regard to nutrients, and therefore its surface area influences this as well. This is why micronized, or powdered biochar could have an advantage regarding speed or capacity for nutrient availability.

Bartlett Study on Biochar vs Plain Soil. Photo courtesy of Dr. Glynn Percival of Bartlett Tree Experts.

Biological interactions
Being carbon based, the interaction between nutrients and organic materials (to include organic products and biostimulants) can improve biological interaction and function in the soil. Biochar serves as a biological catalyst in soil by not only offering a physical habitat for organisms, but also by facilitating nutrient and carbon exchange. This increases both the total population and functional diversity of soil microbes, ensuring balanced microbial roles and maximizing benefits to the soil and tree.

Another often overlooked role of soil microbes is nutrient availability. Microbes play a major role in almost all nutrient cycles in the soil, modifying or changing nutrient forms, making them both available and unavailable to tree roots. While some nutrients can be modified chemically through organic matter or root exudates, it is primarily a biologically driven process through soil microbes. When functional diversity is high, the nutrient cycling and availability that microbes influence also increases. This is also a major reason why improving soil organic matter and functional diversity of soil microbes can buffer the pH of a soil, allowing an urban soil with a pH of 8.0 to not “act” like a soil with a pH of 8.0. This also applies to acidic soils as well, allowing the problems associated with a soil pH of 5.0 to not influence nutrient availability and root health the same way. This buffering effect is one of the many ways biochar is showing to be a very useful tool in urban soils with regard to the biological and chemical characteristics of soil.

In closing, we think it is important to provide well-rounded information to arborists to better help our industry make responsible decisions. Look out for the next article in this series, when we review case studies from urban locations where biochar was implemented, and we’ll dive into what makes a quality biochar product and how biochar interacts with other common products.

Zack Shier is an ISA Board Certified Master Arborist (BCMA), ISA Tree Risk Assessment Qualified and an Ohio-certified applicator. He serves as vice president of the Ohio Chapter of the International Society of Arboriculture (ISA) and is the PHC manager at Joseph Tree Service LLC, an accredited 14-year TCIA member company based in Dublin, Ohio. Lindsey Purcell is an ISA Board Certified Master Arborist (BCMA), an American Society of Consulting Arborists (ASCA) Registered Consulting Arborist (RCA) and principal of Lp Consulting Group in Cortez, Florida. He serves as executive director of the Indiana Chapter of ISA.