What are the basic components a tree needs to be healthy? My son was asked this in first grade last year for Arbor Day, which made me go back to the fundamentals. In general, plants need sunlight, air, water and nutrients. So, we know that nearly two-thirds of what trees need comes from the soil – including 15 of the 18 essential elements! To be good tree managers, we must take a holistic look at those trees and their environments, including below-ground matters.

When talking about trees and soils, we must not overlook the importance of tree roots. All the benefits of great soil don’t translate to a great tree canopy without a healthy root system. To that end, let us explore the life of a root as it interacts with the soil.

Soil texture

Soil provides the physical support trees need. When the tree canopy waves back and forth in the wind, its roots lean on the it for some support. Over time, a tree develops large structural roots that serve as the core strength of that plant. The texture, structure and pore space/density all significantly impact a tree’s ability to grow these structural roots.

Soil texture is the size of the individual particles and is where sand, silt and clay come into play. It is important to note that there is no “good” or “bad” texture. Rather, there are just differences, each of which requires unique management. Coarse soils are those high in sand content, and these have large pores for roots to grow through easily. These are quick draining and often nutrient poor, requiring much more root growth to acquire enough moisture and nutrients to maintain tree health.

Soil pores

On the other hand, fine soils have tiny pores, are slow to drain and are much more susceptible to compaction. Plant roots grow best in an equal mixture of soil particles in soils referred to as loams. These have a good balance between large and small pores, providing a mix of drainage, water holding and nutrient storage.

The pore space is further affected by soil structure. Structure is the arrangement of individual soil particles into aggregates that we give names such as granular and angular blocky. These aggregates form through soil shrinking/swelling and freezing/thawing cycles, and biological activity. Clay and organic matter play an important role in “gluing” together these aggregates. As these structures form, they provide a balance of large and small pores. Ideal soils have large pores for coarse roots to grow through for stability and small pores for fine roots and root hairs to exploit where they can acquire water and nutrients. (Photo 1)

Soils in urban environments

Unfortunately, degraded urban environments result in the compaction of many pore spaces usually found in native soils. Compaction can destroy most of the large pore space, making it difficult for water to drain and provide enough oxygen. Roots struggle to grow and acquire nutrients under these saturated anaerobic conditions. (Photo 3) Reduced root growth leads to unstable and unhealthy trees.

There are many compactive forces, ranging from large machinery to foot traffic. One force that we have control over is the equipment we use during tree operations. Our loaders, lifts and trucks cause most of their compaction in just one to three passes through an area. (Photo 2) We must reduce compaction by using management practices such as working in frozen or dry conditions and using mats or wood chips to protect the soil. By protecting pore spaces, we can maintain healthy tree roots to aid in a plant’s water and nutrient acquisition.

Maintaining ideal pore spaces is just one part of the story for urban tree roots and water. Urban trees are often growing in a non-native-turf setting. Turf requires more water and shallower water than trees, often resulting in high moisture levels in the tree-rooting zone. (Photo 4) If the large pores are filled with water, there is insufficient oxygen for healthy roots. These anaerobic conditions are common in fine-textured urban soils. Saturated conditions often lead to high levels of root rot and disease, creating the potential for a negative feedback loop if we aren’t careful. As the tree loses roots, it will show signs of stress. If we respond by fertilizing or watering without understanding the situation, root issues may further increase, leading to more tree decline.

Assessment tools

Education is key to breaking the water-mismanagement cycle and improving tree-root health. The first step is to assess moisture levels. I challenge anyone to be able to accurately measure soil moisture simply by looking at a tree. Instead, we need to do a quick assessment. My favorite tools are using my fingers to check the moisture of the top few inches or using a soil-sample probe to assess the top 10 to 12 inches.

Another option is a soil moisture probe. There are many different models on the market that can help determine current moisture levels, although performance can be quite variable (Scheberl et al. 2019). It is also important to remember that moisture percentages are only meaningful if we know the texture. (Graphic 1) Different-sized particles are better at holding onto water and can prevent plants from taking it up.

Make recommendations

After assessing current soil conditions, we can make appropriate recommendations. Management options may include changing watering patterns and altering irrigation zones to create separate zones for trees and turf. Sometimes we also may need to dig deeper to address more significant hydrologic issues. Moisture assessments can be crucial in diagnostic services, and are essential when selecting planting sites and tree species. If we make good decisions at the time of the installation, it can prevent many of these headaches before they start.

So, why do trees care about soil? Because it is everything to them. Soil and root interactions are crucial to everything we see above ground. It is where they get most of their water and nutrients to grow. It is their support structure in times of physical struggle. If we don’t have healthy soil, we don’t have healthy trees.

Why do arborists care about soil? Because it is everything to the trees that are in our care. Proper soil management may take some time to assess and understand, but it is worth it to the trees and their owners. It also can be difficult to change many properties, so please test the soil before planting, and make good species selections based on those test results. It may not help you with some of the management decisions today, but it will help you and others in the industry in the future. Consider yourself not just an arborist but also a soil manager for urban trees, and make decisions that promote healthy soils that can support healthy trees.

Resources

Scheberl, L., Scharenbroch, B.C., Werner, L.P., Prater, J.R. and Fite, K.L., 2019. Evaluation of soil pH and soil moisture with different field sensors: Case study urban soil. Urban Forestry & Urban Greening, 38, pp.267-279.

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 presentation on the same subject during TCI EXPO ’22 in Charlotte, North Carolina. To listen to an audio recording created for that presentation, go to this page in the digital version of this issue of TCI Magazine online at tcimag.tcia.org and, under the Resources tab, click Audio. Or, under the Current Issue tab, click View Digimag, then go to this page and click here.