Unlike corn or soybeans, trees are long-lived woody structures, high in carbon and endlessly complex. Unlike crops, they must continue growing and defending year after year, producing enough energy to get through the sleepy dormant season (or, for evergreens, protect against it).

Unfortunately, trees cannot move locations, so they must rely on the soil they grow in to provide the resources they need. I say resources, because the soil does not simply provide nutrients for trees, nor should we, as arborists, ever think of the soil as just an empty bank of nutrients. The soil is a complex and living organism, full of biological and chemical interactions and modifications by the millions. There are roots, fungi, bacteria, protozoa, worms, wasps and more moving through the soil and consuming, excreting and changing the soil ecosystem. Minerals, vitamins, acids, enzymes, hormones and compounds decompose and change form every second.

Dry humates with biochar. Photos and graphics courtesy of the author.

To say the soil is simply a bank of nutrients is to say a chain saw is just a pile of metal. Yet you breathe life into either of these, and magic happens. When you begin to modify either one of these, you can get a positive outcome – or a negative one. Attempt to modify a chain saw in the wrong way and maybe it just runs poorly – or maybe it blows up mid log. The soil is no different, especially when dealing with the unnatural state of urban soils. Modify it in an improper manner, and you may not get the result you thought you would. (Graphic 1)

There is no shortage of ways to modify soils and, by extension, trees, but when I think of the “safe bets” for soil inputs, I try to think like a forest. What happens in the forest that allows so many trees to thrive?

Simply put, organic matter. Even more simply put, carbon.

Carbon is the catalyst of all biological function and the backbone of all living organisms. For trees, it makes up to 50% of its mass and is, quite literally, the backbone of cellulose, lignin, carbohydrates(sugar), proteins, lipids and almost all of the secondary metabolites and root exudates a tree makes. While carbon is not listed very often as a nutrient above nitrogen, phosphorus and potassium (NPK), it is indeed the single-most important nutrient.

Because trees make their own food, aka glucose, we often forget that there is an entire world below the soil, starving for carbon. In the lush forest, this carbon ecosystem is thriving and diverse. In urban soils, not so much. So how does one add carbon to the urban soils to help quench this thirst? Luke Scheberl’s February 2025 article on organic matter (“Why Does Organic Matter Matter for Trees?” TCI Magazine, February 2025) dove into some of the options we have. Healthy mulch rings, leaf compost and biochar are all possible carbon inputs we can practically use as arborists. For today, though, this article is going to focus mainly on another group of carbon compounds: humic substances.

Graphic 1: The nutrient cycle in soil.

What are humic substances?
When organic material begins to decompose, through microbial and insect action, nutrients and compounds are released into the soil. Further decomposition produces smaller and smaller compounds, and some of these compounds undergo chemical and microbial changes and are reassembled to form humic substances.

In a native forest soil, this fraction of the soil would be called humus, which is a diverse and complex but stable portion of the soil organic matter. Humus is extremely important for soil health and plays a role in many significant physical, chemical and biological processes. Humus can be roughly divided into three categories by molecular weight: humic acids, fulvic acids and humin. (Graphic 2)

Graphic 2: Mineral uptake in soil. A fulvic acid compound chelating (bonding with) an Iron molecule and allowing absorption through the root hair. AI generated image courtesy of the author.

It cannot be understated the importance and power of humus and, therefore, humic substances in the soil. Microbial biomass and diversity, root-to-microbe mutualism, pathogen suppression, nutrient concentration and availability, water-holding capacity, pore space, etc. are all impacted immensely by the quantity and quality of soil humus. Organic matter and the humus fraction are the drivers of soil health and fertility. Here are two examples of this:
Humic acid can create cation bridges and direct bonding to clay particles in the soil, creating larger stable soil aggregates that improve water holding and pore space, while also creating more surface area for cation-exchange capacity.

Fulvic acid can increase the cell-wall permeability of root hairs and latch on to otherwise unavailable nutrients (iron or manganese), making them available for root uptake. This is one way organic matter can “buffer” soil pH.

In soil horizons, the “O” layer is the organic layer, and consists of decomposing organic material and some humus. The “A” horizon is the layer considered “topsoil” and a mixture of mineral soil, some organic matter and the highest concentration of humus. The downside to humus for urban environments is that often, through development, the entire O and A layers often are bulldozed and either taken away or regraded to a separate section.

Humus is not exactly harvestable or scalable as a product, since it is a complex process that must be made and organized naturally. However, the humic substances that make up native humus can be harvested or extracted from a few sources: manures, shale/slate deposits, composted or fermented organic materials, leonardite (pre-coal) and lignite(low-grade coal).

While composted organic materials produce humic substances and are sustainable, leonardite produces the highest yield and quality of humic substances.

Available humic products
Attempting to navigate the myriad products these days can be challenging and confusing. It seems every company has an organic product that may or may not be entirely organic, or is derived from this or that, in concentrations that aren’t always shown. While I cannot go through every product made, I can hopefully go over a few quick things to help clear the mud.

Humic products can come in both dry and liquid form, and they can come in both “whole” or complete humate products, or they can be individual humic substances that have been extracted. For reference, humates are prehistoric lake or river beds that have built up immense amounts of organic material that over time have become decomposed and rich in humic substances. This is referred to as leonardite, or you may also hear the word humalite.

Personally, I would recommend making sure the product you choose is a complete or whole humate product, if your goal is to modify and condition the soil. A whole humate product guarantees it contains the entirety of its natural diversity and synergistic effect of having all humic substances. I wish I could tell you that every product label states this clearly, but I cannot. I would recommend talking to vendors about their products and asking where the humic substances are sourced from, whether they are a complete humate product and if there was any extraction or processing done to them.

Extraction of humic substances is generally done in a chemical way, using acid and alkaline agents to pull individual substances out of solution. While I do not recommend this for a single product, if you are buying something like a biostimulant or combination product, it’s not the end of the world. Often, products that have a combination of compounds, in an attempt to produce more benefits, may have extracted humic substances to either cut down on total cost or to elicit a specific effect, i.e., fulvic acid in a biostimulant foliar spray.

Generally speaking, a product that contains a whole humate product in a higher concentration would be a better choice for soil conditioning, but a product containing smaller concentrations of organic acids, B-vitamins, hormones and trace minerals may be better for abiotic stress defense (just an example). In the perfect world, a combination of different compounds, all in higher concentrations, would be great, but cost is absolutely a limiting factor, and in order to keep application practical and economical, we often must choose one or the other. This gets confusing, of course, because both products will most likely help both the soil and the tree, but it’s possible they achieve the goal in different ways. I wish I could give more clear instructions, but this is an area where we desperately need more research to understand what pathways and actions are happening in a given time.

Not all liquid products have extracted humic substances. Often, liquid products can be in a suspension, as opposed to a solution. A suspension simply means the products and water have not been fully dissolved and often require a little shaking before tank mixing.

With dry products, you do not have to worry about this. Adding dry humates to air-excavation projects or mulch-ring installations is a great idea for poor urban soils.

Air excavation can be used to clear out existing soil before adding a layer of humus and other organic matter.

How to implement humic products
Implementation of humic products can be a little daunting, especially if your PHC section already has established protocols, which may include the general use of traditional synthetic fertilizers. It may help to organize these recommendations into two categories: new or young PHC sections and pre-
existing or tenured PHC sections. You can take information from one or both to apply to your company structure.

New or young PHC sections: This is undoubtedly the easier section to apply these products to, since you may not have entrenched protocols or veteran sales staff who have been selling PHC the same way for years. Simply adding these products in and developing a way to educate or sell them to clients comes quicker and with fewer obstacles.

I always recommend young companies start simply and choose two or three products that may have different benefits or costs associated with them. With any company, I recommend having a dry product for when you do air excavating, soil amending, planting or mulch-ring installations. A whole dry humate and possibly a biochar product added in is a good starting point.

If you already have a tank system you use for fertilization, sometimes simply switching out your traditional fertilizer for a humate-based soil amendment is an easy switch that can be used with the same equipment, and sometimes the same price (this varies). I would not recommend ridding your entire stock of fertilizer, as synthetic nutrients can and do play a useful role in tree health. I just don’t recommend using synthetic fertilizers across the board without properly utilizing foliar and/or soil-nutrient testing first.

Also, the beauty of humic substances is that they can work synergistically with synthetic fertilizers and make them more available and useful. Note: Reduce synthetic-fertilizer rates by one-quarter to one-half when using in conjunction with humates.

A final option is to have a product that would benefit trees more quickly for abiotic stress conditions, such as drought, flooding, extreme heat, etc. A product in the “biostimulant” class can be useful here, and I recommend finding one that does contain humates or, at minimum, humic and/or fulvic acid by themselves.

Existing or tenured PHC Sections: I am empathetic to the challenges that come with major change to seasoned companies. As arborists, our world is always changing, and we have to continue to challenge ourselves and improve our companies if we are to build a better industry.

In a lot of places in the country, running a tree care business is tough, with high overhead and high risk. Having a modern PHC section that provides modern approaches can improve profit levels and reputation at an exponential level, easing the burden of running a business.

I always recommend starting slowly as a tenured company, and having education sessions to teach sales staff, technicians and managers about the changes and why the products selected will help improve the company. Recognize that there will always be a few hurdles to get over (anyone moved from paper to digital?), but in time, the process and protocols will become smooth and yield better results.

Closing thoughts
Changing from traditional fertilizers in tank mixes to liquid humic products can be as simple as switching the product, adjusting price if needed and educating sales staff and technicians on the product itself. Other humic product additions may require some field testing or technician trials to make sure they fit with the program and don’t cause issues, either logistically or with equipment. Either way, stay motivated and take the change slowly.
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. He has a bachelor’s degree in Forest Ecosystem Science from The Ohio State University.

This article was based on his and Luke Scheberl’s presentation, “Navigating the Interwoven Web of Soil and Tree Nutrition,” presented during TCI EXPO ’23 in St. Louis, Missouri.