Reduction of excess soil and mulch around a young tree trunk. All photos courtesy of the authors.

The information in our first article (published in the March 2026 issue of TCI Magazine) hopefully established biochar as a scientifically supported soil amendment capable of improving degraded urban-soil systems. Urban soils are recognized as fundamentally constrained by compaction, poor structure, limited nutrient availability and reduced biological function. However, it is essential not to identify biochar as a fertilizer, but as a soil enhancer.

The three primary mechanisms of biochar include:

1. Physical
Biochar improves soil porosity, aeration and water-holding capacity, reducing compaction impacts and enhancing root growth conditions.

2. Chemical
Biochar increases cation exchange capacity (CEC), improves nutrient retention and buffers soil chemistry without significantly altering pH when properly sourced.

3. Biological
Biochar acts as a habitat and catalyst for microbial communities, increasing functional diversity and improving nutrient cycling.

Additionally, our first article highlighted environmental cobenefits such as carbon sequestration and improved stormwater management. Biochar enhances soil function over time, particularly when combined with other practices such as fertilization and soil decompaction.

Our second article (published in the April 2026 issue) shifted the focus from why biochar works to how it should be used in practice. It emphasized that biochar performance is highly variable and dependent on correct application, product quality and site-specific constraints.

Key considerations:

1. Variability in results
Outcomes differ based on species traits, soil texture, compaction levels and biochar properties (e.g., particle size, ash content, pH, feedstock).

2. Biochar as a catalyst
It accelerates soil recovery, nutrient exchange, microbial function and carbon stabilization, rather than acting as a direct input.

3. Quality control
The importance of adhering to International Biochar Initiative (IBI) standards and reviewing laboratory analyses (organic-carbon percentage, ash content, pH, H:C ratio) to ensure product reliability.

4. Operational considerations
Awareness of interactions with pesticides (e.g., adsorption of imidacloprid or glyphosate) and implications for treatment timing.

5. System-based application
Biochar performs best when integrated with compost, mulch and organic fertilizers, creating synergistic effects that improve soil resilience.

It is important to manage expectations with proactive and reactive applications when treating the soil environment around trees. These include:

• Strongest results occur in new plantings and early interventions.
• Moderate improvements in stressed but viable trees.
• Limited reversal potential in advanced-decline scenarios.

Additionally, we would like to reinforce that biochar is not a universal solution and must be applied based on diagnosis, not assumption. Supporting case studies demonstrate improved soil quality, microbial activity and tree growth – especially when biochar is combined with other treatments and applied under appropriate conditions.

Application: Newly transplanted trees

Multiple studies demonstrate a positive increase in both tree establishment and young tree growth with the application of biochar at the time of planting (Scharenbroch et al. 2013 / Schaffert et al. 2022). Given the outlined benefits biochar can provide, it’s not surprising it could help with the establishment of roots and improve tree health. The soil area surrounding the root ball in the planting hole is typically seen as detrimental to initial root growth and establishment of a young tree. Outside of proper planting procedures, such as ensuring that the planting hole is the appropriate width, removing burlap/cage materials and planting at the correct depth, adding appropriate amendments to the planting hole is a recommended practice. Since urban soils are typically degraded and already suffer from compaction and poor water retention, amending a planting hole to improve soil characteristics is a wise decision in most soils and a mandatory one in some.

Using biochar in any situation, and especially during transplanting, is a case where “less is more.” While some studies have shown large quantities of biochar being used without negative consequences, others have shown drawbacks. The cost of using more biochar is often prohibitive in real-world scenarios, and to put it simply: A little goes a long way. As we’ve mentioned before in these articles, biochar is recommended to be used in conjunction with other amendments or fertilizers, and that holds true during tree planting in challenging urban conditions as well. For transplanting of trees, we recommend: Incorporate 0.5 to 1 quart, or 2 to 4 cups, of biochar per diameter inch of the tree, mixed with an organic amendment, into the planting hole backfill.

Installation of a mulch ring with a topical biochar and compost application.

There are multiple products on the market already premixed, i.e., biochar mixed with humates. In these cases, talk to the product representative about how much biochar is in the product, and adjust accordingly. The benefit of products like biochar is that there is no pesticide label and no “perfect” amount. Carbon-based products, and especially biochar, have a wide range of appropriate rates. Using quality leaf compost during planting has shown to be beneficial to tree establishment (Mcgrath et al. 2020), and it pairs well with biochar. They’re not the same type of product and do not provide identical benefits, and therefore can be used effectively when paired together.

While liquid or micronized biochar is a widely available product choice, it is our opinion that granular (rice-sized) biochar is the preferred choice for tree planting. When the planting hole is already excavated, incorporation into the backfill soil is practical and efficient. More research is needed on micronized biochar and the possible differences in field results.

Application: Established trees (young or mature)

Outside of new installations, established trees, whether early established or medium aged, are the prevailing situation where biochar is going to be most beneficial. Remediating or improving soil before severe decline is visible in the tree is always going to be the most beneficial use case and typically will yield fewer secondary issues (insects, decay, deadwood, etc.).

This presents a challenge for some arborists, as identifying very early stress can be difficult and takes years of experience to master. It is well founded that urban soils are degraded and compacted, and lack the proper space below ground required for larger trees (and sometimes smaller trees). We know as trees age, they will begin to experience more stress than their natural forest counterparts, with less soil volume to help alleviate that stress. Proactive soil enhancement or remediation, even around relatively healthy trees, is not excessive or unnecessary, as it simply helps to improve the soil functionality. By ensuring soil quality, it is possible to mitigate or even avoid the common problems typically associated with aging trees in urban settings, or at the very least, create a soil environment that is more resilient and less susceptible to compaction.

Once a tree is well rooted into the surrounding soil and past the initial establishment phase of life (1 to 5 years post planting), incorporation of bulk biochar becomes less practical and more expensive. Arborists have three basic ways to get biochar and accompanying amendments into the soil area and root zone.

1. Incorporation using air excavation or through traditional methods such as auguring small holes in the root zone and backfilling with biochar.

2. Topically spreading biochar across a newly installed mulch ring (under the mulch), or directly over mulch beds.

3. Injection into the soil via liquid micronized biochar (just like fertilization is done).

Incorporation via air excavation

This is by far the most transformative operation an arborist can perform on a tree to improve soil function. Decompaction to 8 to 12 inches of depth, and incorporation of bulk amendments in that entire volume of soil, is undoubtedly a massive positive change and can correct a lot of major issues in a relatively short period of time. This is also, by far, the most expensive treatment type, as it requires a lot of time, more product and additional equipment. Large tree projects can easily climb into the thousands of dollars, but even small trees can be expensive, as the total area of air excavation can still be time consuming.

Most research on biochar use on trees has prescribed the application of biochar at around 5% by volume. That means if you air-excavate a 10-foot-diameter mulch ring, you should be using around 3.5 to 4 cubic feet of biochar, which can get expensive fast. While more research is needed on lower quantities of biochar, our experience has yielded good results with 1-2% biochar application by volume. This lowers the amount needed down to 1 to 2 cubic feet of biochar, lowering price and ease of application.

Also, we want arborists to understand there is no perfect rate, and incorporating some biochar is better than no biochar. This allows the ability to scale the rate of biochar between 0.5% and 5% depending on project budget. We do not recommend going over 5% biochar by volume at this time.

Topical application

Chip-sized biochar being added to a new mulch ring.

There are many ways to implement topical application to the root zone. This includes broadcast spreading, spreading over bare soil under mulch (in a mulch ring or bed),

application over mulch and raking in, etc. The positive aspect to this type of application is cost and ease of application. The downside is the inability to put the same quantity you would apply compared to full incorporation, and positive effects will most likely take longer waiting for a single layer of biochar and amendments to work into and actually modify the soil. This type of application could obviously be repeated, since total volume used is going to be lower.

Unfortunately, there is no set or studied topical rate. Some companies that produce granular biochar have a turf rate, and we think that would be reasonable to follow as a minimum for trees as well. Application under the mulch in mulch rings would necessitate a slightly higher rate, but we would recommend avoiding the creation of a “layer” of biochar where soil is no longer visible. This is a great type of application where a good compost can be used, and biochar is then spread in with the compost.

Liquid micronized biochar (soil injection)

Liquid biochar and biochar mixtures have become increasingly popular, and for good reason. Allowing arborists who already have a spray truck or skid to simply switch products and continue to use the fertilization probes or soil injectors they use daily is very appealing. These products can have micronized biochar ranging from 5 microns up to 200 microns in size, which essentially means it will run through most common pumps and filters that are used in agricultural and arboricultural sprayers.

The massive benefit of practicality and cost with these products is difficult to ignore. While most of these products do cost more than traditional fertilizer, the benefits can far outweigh the mild price increase. Additionally, these products can be applied during any season, including mid-summer, as they do not contain synthetic fertilizers or harsh salt-based nutrients (unless added by the manufacturer).

This is, once again, an area where very little academic research can be found. There are a few papers looking at the differences between powdered biochar and pellet-sized biochar. In general, benefits are similar, with water holding capacity (WHC) being less but still improved with micronized biochar (Bartocci et al. 2017). There may be some indication that micronized biochar could interact with CEC and nutrient availability on an increased level, but that research is still new and unreplicated. Based on current research and best practice, it would be reasonable to recommend these products for repetitive use, but we also want to highlight the obvious difference in particle size and its most likely effect on compaction. Larger particle biochar is going to reduce compaction due to its physical size. If micronized biochar does relieve compaction, it will most likely be through the natural biological processes it can help improve, rather than a physical change.

Application rates for liquids are also not set or standardized. If liquid forms are being applied, we would recommend buying a combo product that has other amendments in it already. There are products that combine biochar and humates, biochar and biostimulants, etc. This increases practicality, and applicators can simply run these combo products through fertilizer probes. Rates can be all over the place, but in general it seems that most labels are in the 2 to 4 fluid ounces of product per DBH or gallon of water applied. The good news is, since these are not fertilizers, rates can scale up and down as the budget allows. Keep in mind, though, that the “1-5% by volume” we talked about with air excavation is never going to be practically hit in one application. While there are gains in practicality with liquids, there is a sacrifice with volume, so hitting that 1-5% by volume would be outrageously expensive from a product perspective.

Application: Declining trees

Unfortunately, there is little research on the use of biochar for actively or severely declining trees, as the nature of tree decline makes studying them difficult and impractical. This section aims to help guide arborists in understanding the limitations of biochar, or any treatment, and the realistic expectations when deciding to treat a declining tree.

As with all biological phenomena, change occurs as a gradual process. Tree decline is not a singular, acute event, but a biological freight train going in the opposite direction we want it to. For trees, which never stop growing, it is the exact opposite direction from the #1 energy priority of all trees … growth (Herms 2002).

In order to correct this, we must first stop that freight train of tree decline and reroute it in the right direction. This, of course, requires patience, time, heavy inputs and some luck (environmentally). This challenge should not be taken lightly, and it is imperative that arborists communicate with their clients about how rare it is to bring trees in severe decline back from the brink of death. While we are not opposed to the idea of attempting to save a declining tree, especially a large, mature tree of value, both clients and arborists need to understand that no one can assume success in these situations and that, for lack of a better term, it can be a gamble.

In our opinion, on a mature tree, once the crown gets to approximately 60% of severe chlorosis and/or dieback, correcting these issues to an acceptable level of recovery is highly unlikely. In our experience, the decline is usually faster than the response to treatments, and that’s assuming an intensive treatment program. We wrote this section so that arborists do not assume that biochar can do something that no other treatment has been able to do. It is a unique organic product, and we would recommend biochar for most situations, but we don’t want arborists to read these articles and assume biochar can now make these situations noticeably more successful in every application.

Also in 2025, this photo shows a healed canker wound.

The same tree in 2025, revealing wound wood development.

A mature oak pictured in 2023, with bacterial cankers and decay.

That said, there are times when intensive treatment is possible financially and desired by both the client and arborist. We wanted to at least show an example of a more serious case that, as of early 2026, has been successful. The first photo is of a mature bur oak in late 2022, infected with bacterial cankers, most likely associated with two-lined chestnut borer and cambial death/rot. The canopy was not heavily chlorotic, but it did have minor tip dieback and thinning. In early 2023, air excavation was implemented to reduce excess soil levels on the root zone, decompact soil and incorporate amendments. Granular biochar and humates were applied at 2% by volume, with 4 cubic feet of leaf compost added as well. An 8- to 10-foot mulch ring was created where the air excavation and decompaction were done. The oak was additionally treated for borer infestations, and potassium phosphite was systematically applied to the bark via spraying. A liquid soil injection of micronized biochar and humates was applied in spring, summer and fall for two years. Potassium phosphite was applied via bark spray in spring and fall for two years.

Conclusion

The intention of our series on biochar was primarily to move the common discussions we hear of biochar from theory to arboricultural practice. Hopefully we’ve established biochar as a legitimate, science-based tool for improving soil function in urban environments. Furthermore, we sought to emphasize the importance of maintaining appropriate rigor when utilizing this tool effectively.

Biochar’s real value depends on how it is used – and it’s most effective when matched to site needs, combined with suitable amendments and chosen for proven quality. When used proactively, biochar can meaningfully improve soil structure, nutrient efficiency and biological function, leading to more resilient urban trees. When used reactively or without proper consideration, results become inconsistent and expectations misaligned.

We like to think that urban-soil management is not input driven, but diagnosis driven, because soil limitations vary on every site. Constraints such as compaction, texture, nutrient availability and biological function all require different interventions. Effective treatment depends on identifying the primary constraint and selecting inputs that address that specific limitation, rather than applying amendments generically. Biochar represents a powerful addition to the arboricultural plant healthcare toolkit, but only when applied with the same level of precision, restraint and intent that defines any other PHC intervention in professional tree care.

Zack Shier, Board Certified Master Arborist (BCMA), ISA Tree Risk Assessment Qualified and Ohio Certified applicator, is plant-healthcare 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.