Soil organic matter (SOM) is one of the most critical components of a healthy urban soil. While it is often a small component (<8%) of soil, it influences nearly every aspect of that soil’s ability to support tree health.

Photo 1: This photo depicts an excellent example of a soil profile, showing the abrupt change from the high-organic topsoil to the low-organic subsoil. The urban topsoil pictured is about 10 inches thinner than the nearby, undisturbed, wooded topsoil. All photos courtesy of the author.

What is organic matter?
So, what is organic matter? Anything in the soil that is living or was once living makes up soil organic matter. The living component would include plant roots and organisms such as earthworms, bacteria and fungi. The nonliving component is made up of dead material in various stages of decomposition.

Most SOM was plant material at some point. Fresh organic matter that breaks down quickly (days to years) is known as labile organic matter. After the labile material breaks down, what is left is known as humus (pronounced hyoo-
muhs). This is not to be confused with hummus, the delicious source of plant protein and fiber for humans.

Humus is a stable form of organic matter that often gives productive soils their dark, rich brown color. It is made up of modified lignin and amino acids creating a 3-D, net-like structure. While humus is considered a stable form of organic matter, humus will still decompose at a rate of about 2-5% per year, depending on climate and soil conditions.

Now that we know what organic matter is, we can get to the root of its benefits for trees.

Image 1: The University of Illinois Extension color chart for estimating soil-organic-matter content in grass-dominated soils. These percentages may not be exact to your location, unless you are in Illinois. Courtesy of the University of Illinois.

Organic matter’s effects on soil
Cation exchange occurs on the surfaces of humus. The 3-D, net-like structure of humus gives it a remarkably high surface area, or capacity. Cations (positively charged ions) are attracted to the humus. Not only are many of these cations essential plant nutrients, but they affect a soil’s acidity, salinity and aggregate stability. Humus content is important when it comes to a soil’s
cation-exchange capacity (CEC). The ability to store plant nutrients in the root zone leads to lower fertilization costs, reduced potential for water pollution and healthier plants.

Aggregate stability is improved by humus due to its ability to attract the mineral components of soil and bind them together. Labile organic matter also can improve soil aggregation by increasing microbial activity. These aggregates are what we see when we put a scoop of soil in our hand and let it fall apart naturally. Our most productive soils have a nice granular structure that looks like a bunch of small marbles in our hand. Unfortunately, most of our urban soils do not look like that.

Compaction and urban development destroy aggregation, but low organic matter also plays a significant role. Soil aggregation is critical for building and maintaining pore space. Water infiltration and aeration are both often lacking in urban soils. Pores allow water to infiltrate and drain away, providing plant roots area to breathe. An increase in water infiltration and pore space also results in an increase in water storage.

No discussion of SOM would be complete without talking about soil microbes. There is no soil without microbes. However, soil fertility influences the variety, total populations and activity levels of microbes. Microbes play a critical role in energy cycling, nutrient availability and decomposition (aka humus formation). Most microbes do best under conditions where our trees thrive. The biggest difference is that they cannot make their own food and need organic residues for their source of energy. The good news is that when we create the best habitat for our plants, we also create the best habitat for our microbes that support those plants.

How can an arborist manage organic matter?
Before making management recommendations, arborists should assess current soil conditions. There are several ways to estimate organic-matter content, but in some cases, laboratory analysis may be needed. Total organic-matter content analysis can be performed by many commercial providers. Most often a visual assessment to estimate organic-matter content is sufficient. These estimations are made using a color chart such as the AG1941 – Color Chart for Estimating Organic Matter in Mineral Soils of Illinois (Image 1) or a Munsell Soil Color Chart. (Image 2) Typically, the darker brown the soil is, the more organic matter it contains. (Photo 2)

Once we have an idea of current SOM levels, we can make management recommendations using several tools in the arborist toolbox.

Image 2: A Munsell soil color chart can be used to quantify color characteristics of soil. Generally, the lower the soil’s value, the higher the organic-matter content. Image courtesy of the author.

Managing labile organic matter
Fresh organic residues such as mulch and leaves increase the labile organic-matter content of the soil. Remember that the labile portion of organic matter serves as food for our living soil helpers. When we provide them with a food source, they break it down and release plant-available nutrients. Decomposition also builds soil aggregation and reduces soil compaction. Because of all these benefits, labile organic-matter content is often used as a soil-quality indicator. Unfortunately, urban soils are generally lower in organic matter and have a shallower layer of topsoil or A horizon. (Photo 1) Managing urban soils for urban trees often requires rebuilding and maintaining appropriate organic-material levels in the soil.

There are commercial labile organic-matter products that can be used to give microbial communities a boost. Long-term management should focus on the establishment and maintenance of surface mulches under the entire tree canopy. Often, clients won’t want to make that large of a mulch ring. I always tell clients the tree will take as much area as you can give it, and some is better than none.

Some may disagree here, but rocks do not count as mulch. Depending on who you ask, “mulch” has origins in potentially many different European languages. But those origins all have roots with meanings like soft, ripe and moist. None of those words describe rocks. In contrast, they do describe decomposing organic matter.

Part of an arborist’s job is education and outreach on behalf of the trees we work with. This is particularly true when it comes to soil- and root-related issues, as all too often they are forgotten. In our busy, chaotic world, it is essential to remember the importance of building and maintaining organic matter in the soil through natural cycles. Some of the most overlooked “products” to improve soil health are grass clippings and tree leaves. The best sources of these are the lawns around the tree and the tree itself. We do not need to go burning fossil fuels to create and ship materials when nature had a way of doing that before we came along.

Managing humus
When fresh organic materials break down, they also form beneficial humic substances. Adding fresh organic matter is the best way to maintain site quality over time. It is how trees have evolved to live and thrive, and the more successful we are at replicating that, the more successful our trees will be. Sometimes, however, we may need to provide an additional boost to the soil. There are several commercial products available that have a humic-substance component.

You may have seen humic substances mentioned in products but might not fully understand them. Humic substances is a term that includes humin, humic acid, fulvic acid and ulmic acid.

Photo 2: Side-by-side comparison of one hand (left) of dark soil that is rich in organic matter and one hand (right) of light soil that is low in organic matter.

Humin is insoluble and is the largest molecule in this group. It is very stable, and increases water-holding capacity and improves soil structure. Humic acid is a heavier molecule that settles out of suspension. It has a dark-brown to black color with a carbon content of 50-60%. Fulvic acid is a lighter molecule that stays suspended in solution. It has a tan-to-brown color with a lower carbon content and a greater cation-exchange capacity compared to humic acid. Ulmic acid is a smaller, less-stable humic fraction and is derived from the initial stages of organic-matter decomposition. It is brown in color and can aid in soil aggregation and cation-exchange capacity.

Many forest soils, where trees grow naturally, are high in fulvic acids. The high-fulvic-acid content is partially responsible for the brownish-yellow
water of many forest streams. Grass-dominated systems like prairie soils are often higher in humic acids. Humic and fulvic products are an area of focus, and there are new products every year at the TCI EXPO trade show. Most products contain just one of the humic substances mentioned here. They also contain very little humin if they have undergone any processing. The most common humic-substance products contain humic acid, but
fulvic-acid-containing products also are available. Not all products are created equal, as different manufacturing and extracting processes have a big impact on the final product.

Explore your options and stay informed
There is much to learn about using humic products in tree care, and we hope you will continue to educate yourselves on them and their use. We encourage you to test some products, being sure to include nontreated plants for a comparison. It is also important to stay up to date on the latest research.

There are many great industry options, including scientific journals such as Arboriculture and Urban Forestry (which is open access), and periodical materials such as TCI Magazine and ISA’s Arborist News. Another great way to stay informed is by attending industry conferences like TCI EXPO, the ISA Annual International Conference and your local ISA chapter conferences.

I also encourage you to watch out for part two of this article series in a future issue of TCI Magazine, in which Zack Shier, my co-presenter on this topic at TCI EXPO ’24 in Baltimore, Maryland, will provide more information on these humic products and how to integrate them into your tree-health-management programs.

Being an arborist means being a life-long learner. The more we learn about trees and the soils they grow in, the better we can manage both!

Luke Scheberl is an ISA Certified Arborist and an instructor at Mid-State Technical College in Wisconsin Rapids, Wisconsin. This article was based on his and Zack Shier’s presentation on the same subject during TCI EXPO ’23 in St. Louis, Missouri. To watch a video recording created for that presentation click here.