Assessing Soil for Tree Species Selection

OK, but why soil?

Take a minute to picture a tree. Maybe even grab a pen right now and draw one out in the margin – I’ll wait. Some things you might note are the canopy shape, leaf-on or leaf-off, flowers or fruit and branching structure. Did you include the root system in your picture? If you did, great job! If not, well, why not?!

Half of healthy trees exist below ground, and this is the more important half. Why is it important if most people won’t even see it? Because it is the foundation for everything above ground.

If you have ever bought real estate, you know how important a good foundation is. If you have ever had to repair a foundation, unfortunately you know how expensive it can be to fix. To use another analogy, we focus so much on birth-to-three programs for our children because we know just how important they are for development. In both cases, it is exponentially harder and more expensive to make repairs or effect changes rather than set things up for success. The same is true for trees.

When planting a tree, when was the last time you thought about the soil that tree is going into? That must be the first place to start when a client says, “I’m thinking about planting a tree.” Think about it, how can you put the right tree in the right site if you have not assessed the site? Assessment is more than looking around and noting exposure, overhead wires, etc. We need to get below ground.

Soil assessment

Pre-planting assessments are crucial, but don’t need to be expensive, time-consuming endeavors. Every current tree site or potential planting site should be evaluated separately. “Why?” you might ask. Well, urban soils are extremely variable. If a client has ever asked how come the tree in the backyard is fine but the one in the front yard is struggling, then you have experienced some of this variability. My last pitch for why soil assessment is important is in the ANSI A300 Part 2, Soil Management flowchart. That should be enough to convince you right there!

So you want to perform a soils assessment but are unsure how to start. It’s actually very easy. All you need to do is get in there. My favorite assessment tool is widely available, and most (not all) arborist’s have 10 of them – your fingers! Fingers can tell us a lot about how it looks and feels.

To dig deeper and collect a sample, consider a soil core sampler (Photo 1). With this tool, you can feel any penetration resistance, collect subsamples for further analysis and offer a free core aeration to your clients. (Photo 2). Samples also can be collected with a shovel, garden trowel, soup spoon from lunch – whatever you have to collect 10 to 15 random subsamples from within the tree dripline. We want to sample what the tree experiences, so stick to the top 6 to 8 inches.

Now we have our sample for further investigation. What should we assess this soil for? It depends! Ultimately, this is up to you, but I recommend performing measurements that directly tie to management actions. If we look to research, Scharenbroch et al. 2017 provides several recommendations.

Physical properties

Texture: Soil texture is the relative proportions of sand, silt and clay. It is based on the size of these particles. These different sizes affect it’s ability to hold water and nutrients. Coarse (sandy) soils have reduced water and nutrient-holding abilities, which often results in low nutrient and water availability. Fine (clay) soils hold more water and nutrients but are more likely to be poorly drained and hold too much water for some species. Texture can be measured using the “feel” method. With a little practice, you can consistently get textures down to groups such as sandy, loamy and clayey.

What is a good texture? It is best to match tree species to their preferred soil texture. High-sand soils may not supply enough water and nutrients, while high-clay soils won’t properly drain and may easily become compacted. How can soil texture be managed? It is difficult to alter texture. In sandy soils, amendments such as organic matter can be added to increase water- and nutrient-holding abilities. In clayey soils, amendments should encourage root and other biological life to increase aggregation and improve drainage.

Structure: Soil structure is how the particles are arranged. Different structures affect the soil’s ability to hold water and nutrients. Granular structure increases pore space (roots love pore space), resulting in higher levels of water and nutrient availability and biological activity. Blocky soils are common in urban areas and have a decreased ability to support tree health.

Structure is visually assessed from a sample that is dug with a shovel or trowel. Structure can be improved through management, including protecting soils from compaction and practices that encourage biological life.

Penetration: Soil penetration is the ability of a tool to penetrate the soil. Usually this is your sampling tool, such as a core sampler or shovel. Chaining pins are another great tool. A chaining pin is slightly sharpened and is painted in increments to help quickly assess penetration depth. Plant roots need to be able to penetrate through the soil to get more water and nutrients.

Poor penetration (less than 6 inches deep) is an indicator of heavy compaction that may be root limiting. Ideally, penetration is 6 to 8 inches or more with minimal effort, indicating roots can more easily grow through in their quest to support tree health. Soil penetration can be improved by decreasing soil compaction and keeping proper levels of soil organic matter and moisture.

Chemical properties

Soil pH is the measure of hydrogen-ion concentration in the soil. It influences many other chemical and biological reactions and properties. One of the most important of these properties is nutrient availability. Most essential elements are present at a pH level of 6.0 to 7.0. Unfortunately, urban soils are often alkaline (greater than 7.0) due to the weathering of building materials (cement and limestone) and topsoil removal during construction.

There are several tools available to measure pH. Field assessments include color-indicator methods and portable glass-electrode sensors. Soil pH can be difficult to alter, as it generally has a very high buffering ability to resist change. Attempts to lower pH can be made with sulfur, iron or inorganic fertilizers. Raising pH using liming materials has shown short-term improvements.

Soil electrical conductivity is the measure of salt ions in solution. Some of these salts are critical essential elements; others may be harmful contaminants. But high levels of either can cause plant stress by limiting water uptake. Field glass-electrode sensors are available to measure conductivity, and many also measure soil pH. Soils with high levels (greater than 600 µS/cm, or microsiemens per centimeter) or low levels (less than 50 µS/cm) should probably be more closely assessed by a lab to get readings of individual elements. Soils with high salt levels should not be fertilized, as that may cause further moisture stress.

Biological soil properties

Color can provide a quick check of soil moisture and organic-matter contents. The color brown is linked to organic matter, and, in general, the darker the brown the higher the organic-matter content. Light-tan colors indicate low organic-matter levels. These soils should have organic matter added to help increase site quality. There can be too much organic matter, which is indicated by black soils, especially if associated with a rotten smell. Lastly, gray soils are an indicator of too much long-term water (Photo 3). These soils also may have a rotten, sulfur-like smell. Soil saturation will often lead to root rot and other root diseases that reduce tree health. In these soils, efforts should be made to improve drainage by breaking up compaction, changing the slope of the soil surface or altering irrigation.

Topsoil depth tells us how deep the soil’s A horizon is (Soil is made of layers, or horizons,. i.e., O, A, E, B, C, R). The topsoil is where we find the majority of the water and nutrient-acquiring root system. We can use soil color to assess topsoil depth and measure from the soil surface down until we see a color change (Photos 4 and 5). Ideally, trees will have greater than 6 inches of topsoil that will supply adequate levels of water and nutrients to maintain tree health. If you find a thin topsoil before planting, you may wish to add soil to the site before planting. However, it is important not to bury roots of already-established trees. In those cases, surface amending or air tilling can improve the critical rooting zone.

Take home

Trees need soil. Urban soils are highly variable, so site assessments should be performed to investigate any potential planting site. Sampling also works well for assessing existing plant health and investigating any pest or pathogen issues. There are many parameters that can be measured, and it is important to know why you are measuring them before you do. Some great resources to learn more about soils include other TCI Magazine articles, Arborist News articles, ISA’s Best Management Practices (BMPs) – Soil Management for Urban Trees, USDA’s Natural Resources Conservation Service (www.nrcs.usda.gov) and published research.

Building a strong testing program will help both you and your client know that what you are doing is working and necessary. The assessment process also can be a new revenue stream while setting you apart from your competition. Finally, working holistically to improve trees will make our whole industry better and provide more resilient, healthier plants.

Luke Scheberl is an ISA Certified Arborist and an instructor at Mid-State Technical College in Wisconsin Rapids, Wisconsin.

This article is based on his presentation on the same subject during TCI EXPO ’21 in Indianapolis, Indiana. To listen to an audio recording of 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.