The Plight of Urban Soils: A Combination of Stress Factors

What is soil? One of the most important characteristics of soil that is often overlooked is the fact that soil is alive. It’s not just alive in the sense that living things are some byproducts of soil or are simply occupying space in the soil. These living organisms are, quite literally, the only way plants can even exist on land. If soil is the car, then the living organisms are the engine that keeps it going. They cycle the nutrients, break down the organic matter, extend root systems, deliver nutrients and improve both the physical and chemical makeup of the soil.

These living organisms are what make sustained plant life possible. Fungi, bacteria, nematodes, ants, bees, worms and many other organisms are the engines behind making weathered rock into a usable, sustainable and regenerative giver of life.

Forest soils versus urban soils

Living organisms and microbes aren’t the only things that matter when talking about soils and their ability to facilitate plant life. I think it’s important to explain what happens to our urban soils that causes the “plight” I refer to in the title of this article.

Let’s first look at the traditional forest soil composition because, after all, if we are concerned with growing trees in an urban area, then we need to understand where trees naturally grow in the wild ‒ in the forest. Forest soils are layered, just like any other soil, with horizons of different layers originating from the parent material far below the surface we walk on. These layers, or horizons, have a greater ability to sustain life the closer the layer is to the surface. (Diagram 1)

Diagram 1: Proper forest-soil horizons. A soil horizon is a layer parallel to the soil surface with physical, chemical and biological characteristics that differ from the layers above and beneath. Graphic courtesy of

The first layer of any forest floor is the O horizon, where most of the organic matter is located. In forests, this often-thick layer is comprised of decomposed, decomposing and fresh organic material. Every year, trees and animals alike die, then decompose with the help of microorganisms. This matter becomes fresh nutrients and myriad chemical compounds that benefit the soil. The tree roots, earthworms and fungal mycelium penetrate the soil, creating beneficial relationships and improving the physical soil structure. Trees grow naturally from seed, and adapt to their often-lush environment, making any needed hormonal changes based on their consistent and diverse soil environments.

The urban soil we use, however, is not so similar. When homes and neighborhoods are built, soil is dug up, spread out, mixed up, compacted by large equipment and displaced repeatedly. Topsoil often is scraped off and either dumped or sold, and “clean fill” is brought in for grading or filling purposes. This introduces different types of soil, often from lower soil horizons, to the top layer, along with an array of foreign materials like rocks, wood, trash, etc. (Photo 1)

Photo 1: Small, compacted urban root zone. TCIA staff photo by Susan DiPietro.

This all creates a very poor soil environment from both a chemical and physical perspective. There also is a heavy biological toll on sites like these. While microbes are almost always still present, the diversity and population levels are low. The use of turf also changes the composition of microbes to more bacteria and less fungi, which is a disadvantage to trees and their roots, as trees prefer fungi. (Photo 2)

All these issues culminate in a list of stress factors that affect urban trees in a negative manner and result in the rampant tree death and pest issues we see so often in our cities.

Photo 2: Mixed urban-soil layers. Photo courtesy of Susan D. Day.

Soil-related stress factors and their effects

While weather events and invasive pests do play a serious role in the overall health of urban trees, typically they do not have the day-to-day impact soil-based issues do. The reason the average lifespan of an urban tree is just 17 years is not because of a few wet springs or invasive insects, such as the emerald ash borer. The day-to-day soil conditions are what drive this entire downward spiral for urban trees and are the one thing we can attempt to change and/or correct, even if it is only a short-term correction.

These stress factors impact all the physical, chemical and biological characteristics of soil directly or indirectly. For this article, I am speaking only to the soil issues; this does not include things such as poor nursery practices, lopping off 70-90% of a tree’s root system before transplanting, improper planting (leaving on burlap/cage, planting depth, amount of water, etc.) or things like improper or over fertilization.

Some of these stress factors are:

  • Mixed soil layers (lower horizons on top).
  • Compaction/low pore space (both natural and man-made).
  • Foreign material (rock, trash, oils/fuels).
  • Low microbe population and diversity.
  • Low organic matter.
  • High/low pH.
  • Poor soil aggregation.
  • Poor water-holding capacity.

These stress factors are very much linked to a tree’s declining health, as well as having other factors outside of soil also diminishing their capacity. Both compaction and low organic matter can reduce microbe population and diversity, while also having a negative effect on soil aggregation and water-holding capacity.

Low pore space reduces oxygen, which can reduce microbe population levels while also reducing the ability of water to infiltrate soil. Compaction, low fungal population and foreign material all can make it harder for roots to grow and establish new roots. Low organic matter can make pH issues worse, lowering nutrient availability and the chelating/buffering ability of soil, as well as reducing the food microorganisms needed for trees to thrive. (Photo 3)

Photo 3: Dry, compacted and rocky urban soil. Photo courtesy of the author.

Most of these stress factors are combined with one or more other factors, and as soil health declines, the tree’s health will decline in some manner. It should be mentioned that various regions of the U.S. have different but specific problems. However, no matter where you are, when you have mixed-up urban soils, your traditional metrics for soil health go down. When soil health goes down, tree health suffers.

Corrective measures

Attempting to correct some of these metrics may prove challenging, depending on the situation. Some metrics cannot be directly changed, but will improve if general soil health is improved.

A classic example is changing soil pH, where studies will show that long-term soil-pH change is difficult. While those resources aren’t incorrect, I always challenge arborists to look specifically at what we’re trying to accomplish and how we operate. The goal isn’t necessarily to change the pH of the soil forever, but instead to enable better nutrient availability by changing the pH temporarily. A short period of pH change is achievable and realistic.

Most tree companies that do plant-health-care (PHC) work and renewal visits for customers will go back to a customer’s tree every year for work. If we can modify soil pH for a short time every year, then we will accomplish our goal. Remember, we are not modifying pH on a 500-acre farm, but rather often it’s a small, 250- to 1,000-square-foot root-zone area.

Something else to consider when thinking through possible corrective measures is how a healthy forest soil operates. We talked about how forests don’t have the same issues urban soils do, yet it’s not often we discuss what it might take to get urban soils to mimic forest soils. Instead, a common practice is to do a soil sample, then fertilize with synthetic nutrients accordingly. Is that getting to the core problem? Is that the way forest soils work? Or are we attempting to mask the real problem by treating only the symptoms we see on paper?

How do we get urban soils to act like forest soils? That’s a difficult question to answer. Out of all the stress factors listed, which can we effect change on?

When looking at the list of differences between forest soils and urban soils, I think one of the most realistic ways to help is through increasing organic matter. (Photo 4) Organic matter (OM), with continued use, can provide the following benefits:

  • Improve microbial population and diversity.
  • Retain water while also creating more pore space (less compaction).
  • Improve cation exchange and nutrient availability.
  • Potentially buffer pH issues.
Photo 4: Humate and biochar organic fertilizer. Photo courtesy of the author.

Soils do not have to be high in organic matter to be successful, but in my experience urban soils have a unique set of issues where increasing organic matter helps alleviate some of these concerns. For urban soils, compaction and low microbial populations seem to be the main issues regarding the physical soil itself. These two issues are only exacerbated by the fact that most trees in the city have a smaller root-zone area to call home. This means a tree growing in the city is going to fare worse than a forest tree, even if their soils are identical, solely because of how much less area the tree has to spread its roots. (Photo 1)

Organic-matter applications can improve almost every negative aspect of urban soils. There are plenty of situations where other products can and should be used alongside OM. But, as a general health treatment for urban trees, organic matter should be the starting point for most PHC plans.

With a foundation of OM, you can build and customize what else is needed for specific situations, whether that be lime or sulfur for adjusting pH, extra iron or manganese for chlorotic trees or even an NPK fertilizer (nitrogen, phosphorus and potassium, the nutrients that compose complete fertilizers) for soils that are low in nutrients. OM also has the benefit of improving fertilizer uptake and reduces leaching of nutrients, such as nitrogen and phosphorous. In fact, some popular NPK fertilizers have added humates for improved absorption.

Building a synthetic fertilizer plan using OM as the foundation can reduce synthetic-fertilizer rates, lower environmental impact and improve overall success long term. Breaking up compaction using an air-excavation tool is a great way to make a big impact in a short period and a great way to introduce dry or wet OM products directly into the soil. You can change the OM percentage of a tree’s root-zone area quite drastically using an air-excavation tool and modern OM products, essentially “replanting” a tree into improved and more forest-like soil in a few hours.

Traditionally, the biggest hurdle to using organic matter is the form it comes in. Usually in the form of compost or a self-brewed compost tea, the huge spectrum of what is considered quality organic matter and the logistics involved in getting it into the ground have been challenging. The multitude of organic-matter choices can be daunting, which only leads to using it less and relying on the standardized synthetic fertilizers instead.

The good news is, there are more options every year. The general trend is to produce a liquid humate, sometimes accompanied by some form of biostimulant. This really changes the game when it comes to the quality and consistency of OM you’re getting and puts these products closer to the standardization of synthetic fertilizers. It’s still important to understand the concentration and makeup of the product, but the increased options are quickly reducing the guessing game and logistical problems associated with OM in the past. Modern humate-based, liquid OM products can be mixed in a tank just like fertilizer and applied via fertilizer probe.

No matter what you choose to utilize when treating the trees you’re managing, always understand why you are treating and how the treatment will affect the tree and soil. I implore you to research and educate yourself on the effects of different treatment types on our urban trees and environment.

Zack Shier, Board Certified Master Arborist (BCMA), ISA Tree Risk Assessment Qualified and an Ohio certified applicator, is plant-health-care manager with Joseph Tree Service, LLC, an accredited, 10-year TCIA member company based in Dublin, Ohio. He has a bachelor’s degree in Forest Ecosystem Science from Ohio State University

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