The Plight of Urban Soils

A Combination of Stress Factors

by Zack Shier


What is Soil?

There are few things I always try to keep in the back of my head when I think about soil. One of the most important characteristics that is often overlooked with soil, is the fact that soil is alive. It’s not just alive in the sense that living things are some byproducts of soil or just simply occupying the space in the soil, but these living organisms are quite literally the only way plants can even exist on land. If soil is the car, 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 more are the engines behind making weathered rock into a useable, sustainable, and regenerative giver of life.

Forest Soils vs Urban Soils

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

Let’s first look at what traditional forest soils are like, because after all we are concerned with growing trees in the urban area, so we need to understand where trees naturally grow in the wild, which is forests. 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 increase in their ability to sustain life the closer you get to the surface. The first layer of any forest floor is the O layer, which is where most of the organic matter is. This layer in forests, is usually quite thick and has decomposed, decomposing, and fresh organic material in it. Every year trees die and fall over, leaves drop, and animals die; decomposed by microorganisms and turned into fresh nutrients and a myriad of chemical compounds benefiting the soil. The tree roots, earth worms, and fungal mycelium penetrate the soil deep, creating beneficial relationships and improving the physical soil structure. Trees are grown naturally, from seed, and adapt to their often lush environment, making any needed hormonal changes based on their consistent and diverse soil environments.

Figure 1: Proper Forest Soil Horizons

The urban soil we use however…is not so similar. When homes and neighborhoods are built, soil is dug up, spread out, mixed up, ran over a million times by large equipment, and displaced repeatedly. Topsoil 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, lower soil horizons to the top layer, and a myriad of foreign materials like rocks, wood, trash, etc.

This all creates a soil environment that is usually very poor, and that is purely from a physical and chemical perspective. The biological toll is often heavy on sites like these, and while microbes are almost always still present, the diversity and population levels are low. The large amount of turf used also changes the composition of microbes to more bacteria and less fungi, which is a disadvantage to trees and their roots, which prefer fungi.

Figure 2: Small, Compacted Urban Root Zone

All these issues culminate into 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 much in our cities.

Figure 3: Mixed Urban Soil Layers

Soil Related Stress Factors and Their Effect

While weather events and invasive pests do play a serious role in the overall health of urban trees, they generally do not have the day-to-day impact that our soil-based issues do. The reason the average age of an urban tree is 17 years old is not because of a few wet springs, or Emerald Ash Borer. The day-to-day soil conditions are what drives this entire downward spiral for urban trees and is also the one thing we can attempt to change and/or correct, even if that is only a short-term correction.

These stress factors impact all the physical, chemical, and biological characteristics of soil whether directly or indirectly. For this article I am also only listing the soil issues, which does not even consider things like poor nursery practices, lopping off 70-90% of a tree’s root system before transplanting, improper planting (leaving on burlap/cage, depth, 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 manmade)

-Foreign material (rock, trash, oils/fuels)

-Low microbe population and diversity

-Low Organic matter

-High/low pH

-Poor soil aggregation

-Poor water holding capacity

Figure 4: Dry, Compacted and Rocky Urban Soil

Some of these stress factors are very much linked in their decline, 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 can all 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, while also reducing the food that microorganisms need to thrive.

Most of these stress factors are intertwined with one or more other factors, and as soil health declines, they all tend to decline in some manner. Now it should be mentioned of course that different regions of the U.S. have different specific problems, but in general, 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, while some metrics you cannot directly change, but instead will improve if soil health in general is improved.

A classic example is changing soil pH, where the studies will show you 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 to necessarily change the pH of the soil forever, but instead allow better nutrient availability by changing the pH for a short time. Now that is something that is achievable, and realistic.  Since most tree companies who do plant healthcare do renewals for clients, it’s not out of the norm to go back to a clients tree every year and do work. If you can modify soil pH for a short time, every year, then I feel like we are accomplishing our goal. We have to remember we are not modifying pH on a 500-acre farm, but instead an often small 50 to 100 square foot root zone area.

Another thing to think about as we work through possible corrective measures, is how a healthy forest soil works. We talked about how forests don’t have these same issues that our urban soil does, yet we often don’t discuss what it might take to get our urban soils to mimic our forest soils. Instead, a common practice is to do a soil sample, and 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?

That brings a difficult question though, which is how do we get urban soils to act like forest soils? Out of all the stress factors listed, which ones can we even attempt to change, let alone actually change?

When I look at the list of differences between forest soils and urban soils, I think one of the easiest and most realistic ways to help urban soils is through organic matter. Organic matter (OM), with continued use, can improve microbial population and diversity, can retain water while also creating more pore space (less compaction), improves cation exchange and nutrient availability and can also buffer pH issues. Now 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 soil, compaction and low microbial populations seems to be the main issues regarding the physical soil itself, and 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 that a tree growing in the city is going to fair worse than a forest tree, even if their soils were identical, solely because of how much less area the tree has to spread its roots.

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 plant healthcare plans. With a foundation of OM, you can build and customize what else is needed for a specific situation, whether that be lime or sulfur for adjusting pH, extra iron or manganese for chlorotic trees, or even an NPK fertilizer for soils that are low in nutrients. OM also has the benefit of improving fertilizer uptake and reduces leaching of nutrients like nitrogen and phosphorous. In fact, some popular NPK fertilizer 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.

Figure 5: Humate and Biochar Organic Fertilizer

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 quality organic matter, or the logistics involved in getting it put into the ground has been challenging. The myriad of organic matter choices can be daunting, which only leads to using it less and relying on the much more standardized synthetic fertilizers instead. The good news is, there are more and more options every year, and the general trend is to produce a liquid humate, sometimes accompanied by some form of bio stimulant. This really changes the game when it comes to the quality and consistency of OM you’re getting and puts OM products closer to the standardization of synthetic fertilizers. It’s still important to understand the concentration and make up of the product, but its quickly reducing the guessing game and logistical problem associated with OM of 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 apply to help the trees you’re managing, always remember to understand why you are treating and how the treatment will affect the tree and soil. If you do not know why you chose the product you did, I implore you to do more research and educate yourself on the effects of different treatments on our urban  trees and environment.