Photo by Rene Notenbomer/iStock

In Part I of this series on the application of biochar into urban soils (published in the March 2026 issue), we discussed how research science clearly supports biochar as a powerful soil enhancement. However, successful implementation depends on understanding what distinguishes a high-quality, arboriculturally appropriate product from a generic carbon material, as well as constraints.

As biochar adoption increases, arborists need to move beyond curiosity and into specification and application. The profession must shift from asking, “Does it work?” to asking, “Under what conditions, in what combinations and with what expectations does it perform best?”

This article, the second in this series, examines International Biochar Initiative (IBI) standards, laboratory analyses, quality-control parameters and real-world case studies to provide practitioners with a usable framework for selecting and applying biochar in urban-tree systems.

Biochar could be considered as the “coral reef of the soil,” or a “tiny carbon sponge.” That is a great way to think about biochar, because it’s more like the frame and body of a car than it is the engine. It provides the support and framework for everything to interact and work properly. Biochar is a scientifically supported soil amendment capable of addressing the chronic limitations of urban soils – compaction, limited rooting volume, nutrient instability and biological dysfunction. It could be said that it is primarily a catalyst for accelerating structural recovery, facilitating nutrient exchange, amplifying microbial function and stabilizing natural carbon sources.

Mixed results

Biochar use in urban soils demonstrates mixed outcomes with various applications, because biochar should not be considered a single input. Its impacts depend on tree species, biochar chemistry and soil constraints.

Biochar activity depends on:

• Tree-species traits and symbioses.
• Biochar chemistry (feedstock, pyrolysis temperature, ash content, pH, surface area and particle size).
• Urban-soil constraints (compaction, texture, pH, salts, contamination, low organic matter and disrupted hydrology).

As we have said previously, and will say again, there are no silver bullets in plant-health-care treatments, and that is true with biochar applications as well. The last thing we want is an arborist going out and buying biochar in heaps and dumping it into the soil, expecting a miraculous resurrection of a severely declining tree.

Across the literature, the most consistent pattern is that biochar helps trees most when it addresses the site’s main limitation (e.g., structure, water-holding capacity, nutrient retention) but can be less effective or neutral when it doesn’t match the constraint or is applied at unsuitable rates/particle sizes, or when urban soils already have excessively high pH values.

There are three primary reasons for mixed results, which begin with dependency on tree species. More specifically, on their root/microbial partnerships. Urban-tree responses can diverge, because species differ in drought sensitivity and rooting strategy. Biochar-driven gains in plant-available water are more likely to benefit drought-sensitive species (especially in coarse, sandy soils). Applied research emphasizes matching biochar properties to the rooting environment as critical for improved outcomes in disturbed urban soils. Biochar should be viewed as a tool whose success depends on diagnosing the limiting constraint, as mentioned above, as well as species needs, rather than treating it as a universal amendment.

This leads to a second reason for mixed results in application, which includes dependency on soil texture and physical condition, especially compaction levels. Urban soils are frequently compacted and structurally degraded. These conditions make them suitable candidates for applying biochar. The soil’s texture and level of compaction influence how effective biochar is at improving water retention – coarse-
textured soils typically see the most consistent benefits, whereas fine-textured soils may respond differently due to variations in pore spaces. Benefits depend on incorporation depth and particle size of the product. Larger particle size is recommended for any opportunity to improve structure.

The efficacy of biochar

Biochar lab analysis from Control Laboratories. All images courtesy of the authors, unless otherwise noted.

Biochar’s effectiveness starts with quality and consistency. The International Biochar Initiative, active for more than 20 years, sets the standards and offers information for biochar production, testing, and use. IBI continually updates lab testing protocols to ensure reliable results. Labs such as Control Laboratories, located in California, specialize in biochar analysis. Request an analysis report when buying biochar to confirm threshold values.

Although these analyses often provide more data than an arborist requires, it is important to focus on several essential metrics. As long as there are no elevated levels of heavy metals found in the analysis, we suggest paying close attention to the percentage of organic carbon, the hydrogen-to-carbon ratio, ash content and pH value. Obtaining a biochar with a higher organic-carbon content ensures a quality source that is carbon rich. Organic-carbon percentage is recognized as higher level if it is more than 60%, but that number is generally more than 80% in the higher quality products.

Biochar pH is linked partially to ash content, but is not entirely dictated by the ash content. The feedstock and method of pyrolysis all matter in creating a suitable biochar for plant health care. Most high-quality biochar products exhibit pH values between 8 and 9.5 in their unmodified state, indicating that no additional substances have been incorporated. This pH range is acceptable for use, including in alkaline soils, as confirmed by numerous biochar studies conducted on urban soils. Biochar with pH values higher than 9.5 and/or with ash content above 10% should be avoided if working in alkaline soils with a pH above 7.5. Ash content drives up not only the pH value, but also increases the “liming effect” of the biochar. While not all ash is created equal, the higher the ash content, the more likely the biochar is going to raise the pH of the soil. Ash content should be as low as possible, especially when being used in alkaline urban soils, so keeping ash content below 10% is recommended.

Producing high-quality biochar requires specialized equipment, knowledge and expertise. Although it is feasible to create “homemade” biochar, it is advisable to procure the product from a reputable manufacturer who can consistently deliver a reliable product and provide analytical verification of its quality.

Compost, mulch and organic fertilizers

Chip biochar being incorporated into compacted soil.

While biochar alone offers measurable benefits, the most effective results occur when it is integrated with other soil amendments. Biochar interacts positively with many of the products we are already familiar with, allowing relatively easy introduction into protocols. Urban soils rarely fail our tree’s ability to not just survive but thrive due to a single deficiency or variable. It’s often a combination of compaction, low organic matter, disrupted soil biology, nutrient imbalance or limited rooting volume, which coexist as silent killers. As such, biochar should be considered one component of a systems-based soil strategy rather than a standalone solution. A growing body of research demonstrates that biochar performs most effectively when paired with complementary amendments.

• Compost supplies labile organic matter, microbial inoculum and nutrient sources that “charge” biochar’s pore space and enhance biological activation.
• Organic mulches support biological cycling, extending the functional impact of biochar deeper in the profile.
• Organic fertilizers and biostimulants provide nutrient forms that interact with biochar’s exchange sites, improving nutrient-use efficiency and reducing leaching losses.

Instead of acting as competing inputs, these materials operate in a synergistic manner. Compost and organic fertilizers enhance the chemical and biological activity of biochar, while biochar contributes to nutrient stabilization and improved structural durability. Collectively, this integration results in a more resilient soil system that supports sustained tree growth even under challenging urban conditions.

Interaction with pesticides

What often does not get discussed is the interaction of biochar with pesticides. Most notably, biochar can adsorb (adhere to tightly) glyphosate and imidacloprid, a commonly used herbicide and insecticide in arboriculture and landscape services. While it is not as exact as biochar simply stopping these pesticides from working at any quantity, it is still a possibility given the common use cases for some of these products.

If an arborist performs an air-spade excavation and incorporates biochar, followed by a soil injection of imidacloprid, it is possible that some of that insecticide can be absorbed to the biochar and not be taken up by the tree. While this is not a lethal issue for the tree, it is something arborists need to be aware of if treatment protocols for insects are to be as effective as possible.

Managing expectations

A critical issue in the expanding adoption of biochar is expectation management. Biochar does not “revive” severely declining trees in advanced physiological collapse, nor does it override fundamental site constraints, such as restricted rooting volume or chronic root damage.

Outcomes vary depending on tree condition and site context.

1. New tree installations:

• Newly planted trees in compacted soils show improved establishment when biochar is incorporated during installation.
• Reduced transplant stress.

2. Proactive applications:

• Stress may be minimized with applications, leading to improved soil aggregation, increased infiltration and supplemental irrigation.
• Long-term improvements in soil organic matter and nutrient cycling are more likely when biochar is applied before significant decline occurs.

3. Soil rehabilitation (initial decline):

• Trees experiencing increased or moderate stress from compaction or nutrient deficiency may demonstrate improvements in vitality and health when biochar is combined with air excavation and compost incorporation.
• Improvements occur over multiple growing seasons rather than immediately with replenishment of compost/mulch and reducing activity over the critical root zone.

4. Reactive applications (advanced decline):

• In trees with significant crown dieback, chronic root damage or hydraulic dysfunction, biochar may improve soil conditions and reduce further decline advancement, but likely will not reverse decline.
• In these cases, the goal shifts from restoration to slowing progression rather than full recovery.

Understanding these distinctions is essential for ethical expectations and client communication.

Case studies

An Eastern redbud before it was treated with liquid humates and biochar.

Percival et al. 2023 and Scharenbroch et al. 2022 tested biochar in urban soils alongside air spading and either compost or fertilizer. Biochar worked best in combination with these amendments and enhanced air tillage, compared to its use alone or with just fertilizer. These field studies, conducted on urban trees rather than in lab conditions, showed minimal or no significant changes in soil pH, supporting the safe use of quality biochar without major pH shifts or negative effects.

Two peer-reviewed studies (Schaffert & Percival 2016; Scharenbroch et al. 2013) evaluated biochar’s effects on newly planted trees and soil properties. While Scharenbroch et al. compared biochar to other amendments, Schaffert and Percival tested it with various products. Both found that biochar enhanced soil quality and tree growth, either alone or combined with other treatments.

Notably, biochar alone boosted young tree growth as much as high-nitrogen and potassium fertilizer. The findings suggest organic products like biochar can quickly improve growth without the risks of synthetic fertilizers, making them suitable for tree planting, especially since they do not alter soil pH in alkaline conditions.

To give an example outside the realm of academic papers, Zack Shier performed a one-year controlled field trial in Dublin, Ohio, on an existing urban soil. The average pH of the tested soil was 7.7, and the organic matter was around 3%. The plots were soil injected with a fertilizer probe using a mixture of liquid humates and micronized biochar. There were six total humate + biochar plots, three true control plots and three NPK fertilizer control plots. Samples were taken from all 12 sites pretreatment and twice after treatments. Microbial biomass and fungal-to-bacterial ratios were measured.

An Eastern redbud five years after it was treated with liquid humates and biochar.

After 18 weeks, the humate + biochar groups increased microbial biomass by 215%, compared to 73% in the control groups. The fungal ratio in the humate + biochar groups averaged 25%, compared to only 9.6% in the control groups.

Stay tuned! The third article in this series will be published in the June/July issue of TCI Magazine.

Zack Shier, Board Certified Master Arborist (BCMA), ISA Tree Risk Assessment Qualified and Ohio certified applicator, is plant-health-care manager with 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 with Lp Consulting Group LLC in Cortez, Florida. He spent many years as an urban-forestry specialist and teacher in the Department of Forestry and Natural Resources at Purdue University and serves as the executive director of the Indiana Chapter of the International Society of Arboriculture.

References

Sifton, M. (2025). Biochar for Urban Forestry Applications: Improving Tree Establishment in Urban Soils. (Thesis).

Ruogu, Z. (2026). Urban Soil Health and Tree Performance: A Diagnostic Framework and Biochar – Inactivated Yeast Case Study from Toronto.

Kumar, K., & Hundal, L. S. (2016). Soil in the city: Sustainably improving urban soils. Journal of Environmental Quality.

Dong, X., Chu, Y., Tong, Z., Sun, M., Meng, D., Yi, X., … & Duan, J. (2024). Mechanisms of absorption and functionalization of biochar for pesticides: A review. Ecotoxicology and Environmental Safety, 272, 116019.

Yuan, X., Jiang, W., Zhang, H., & Shen, X. (2025). A mechanistic study on removal efficiency of neonicotinoids by biochars with various fabrication methods. Journal of Environmental Chemical Engineering, 13(3), 116875.

Scharenbroch, Bryant C., et al. “Biochar and biosolids increase tree growth and improve soil quality for urban landscapes.” Journal of environmental quality 42.5 (2013): 1372-1385.

Scharenbroch, Bryant C., Kelby Fite, and Michelle Catania. “An arboriculture treatment of biochar, fertilization, and tillage improves soil organic matter and tree growth in a suburban street tree landscape in Bolingbrook, Illinois, USA.” Arboriculture & Urban Forestry (AUF) 48.3 (2022): 203-214.

Schaffert, Emma, and Glynn Percival. “The influence of biochar, slow-release molasses, and an organic N: P: K fertilizer on transplant survival of Pyrus communis ‘Williams’ Bon Chrétien’.” Arboric. Urban For 42 (2016): 102-110.

Percival, Glynn C., Sean Graham, and Emma Franklin. “The influence of soil decompaction and amendments on soil quality.” Arboriculture & Urban Forestry 49.4 (2023): 179-189.

Schaffert, Emma, et al. “The influence of biochar soil amendment on tree growth and soil quality: a review for the arboricultural industry.” Arboriculture & Urban Forestry 48.3 (2022): 176-202.