February 1, 2025

Foliar and Soil Testing for Tree Health: How to Get Started

Soil and foliar sampling are not exactly a new science, nor are they new ideas in the world of arboriculture. However, it is my opinion that they are still underutilized or underemphasized in the field. As practicing arborists, we must use all the tools at our disposal to not only provide good service to our customers, but also to do our best to improve tree health.

Soil Testing for Tree Health

Photo 2: Soil sample using a shovel. Two to three small samples can be taken from one large chunk of soil.

Purpose of testing for tree health
As our field improves and modernizes, there is a realization that the trees we care for cannot be judged by comparison to other living organisms, such as turf or annual crops. If we understand that basic principle, then we also must realize that, to progress our understanding of what drives tree health and nutrition, we need an objective way of measuring that data for trees specifically. One of the best ways to achieve this is through laboratory testing of the soils that surround trees and the foliage of the trees themselves.

Through the combination of this testing, over time, we can better understand what reasonable nutrition levels are for urban trees in different types of urban soils (not forest soils). We also can assess different metrics of testing and develop more accurate thresholds for urban conditions. This allows us to more easily understand how much work, or what treatments, may be involved in correcting known issues.

Lastly, and the reason I decided to write this article, is that the implementation of testing seems to be the number-one hurdle for arborists. I’ve talked to hundreds of arborists across the country after presentations, and the question of how to implement testing within their company seems to be the largest obstacle to getting started. The goal of this article is to help outline how one can do that, whether for a large or small company.

Different types of testing available
If you look at the most recent ISA “Best Management Practices (BMP) for the Soil Management of Urban Trees (2nd Edition),” you will see that it recommends both soil and foliar nutrient analysis to understand what is happening in the soil and in the tree. I think this is extremely wise advice, and every arborist who currently does testing should absolutely be doing both types of tests when possible or applicable.

Soil Testing for Tree Health

Photo 1: Brix Refractometer.

It is my opinion that foliar nutrient analysis should be the first, or primary, test done for urban trees. Fertilization for trees should not be based off soil testing alone, as you have no way of understanding whether deficiencies in the soil also will be deficiencies in the tree, and vice versa. A foliar analysis is the only way to truly know what may be deficient in the tree. Since we are dealing with hundreds of different species of trees in an endless variation of degraded urban soils, we really need to understand what is happening in the tree first.

Outside of nutrient analysis via laboratories, there are a few types of testing you can do on your own that may help bridge the gap between a lab test and nothing at all. Those include, but are not limited to:

  • Soil plant analysis development (SPAD) meter – tests chlorophyll content of leaves.
  • Brix Refractometer –measures the sugar levels (Degrees Brix, symbol Bx) in the leaf/petiole sap. (Photo 1)
  • Soil microbe testing – lab-based testing that can indicate different types of soil bacteria, fungi and protozoa.
  • Testing can be very detailed and can show how different types of soil microbes are influencing your soil carbon and nutrient availability. (Image 1)
  • Microbe testing labs include, but are not limited to: Earthfort Labs (Oregon), RegenAg labs (Nebraska), Soil Food Web (various locations) and
  • BiomeMakers (California). (Image 1)

How to get started
One of the hardest parts of implementing soil and foliar testing, at least for me, was simply starting the process. If you are a plant-health-care (PHC) manager or technician, you also may need to get “buy-in” from upper management or ownership. If you are starting to test at a larger company, you may have to deal with a large sales team or different managers in different sections as well. For myself, it was simply trying to figure out how to take samples, where to send them and what to do with them once I received the results.

One of the best pieces of advice I give arborists is to just start testing, regardless of whether you feel you have everything figured out. No matter what, you’ll have lessons to be learned, confusing results and things to figure out, so you might as well start as soon as you can. In my opinion, the three biggest obstacles to starting are determining:

  • Who is paying for the testing.
  • Where to send your samples.
  • How to practically take samples.
Soil Testing for Tree Health

Image 1: Soil microbe test from Earthfort Labs, including aerobic and anaerobic fungi and bacteria and three types of protozoa. Courtesy of the author.

Generally speaking, the client is most likely going to pay for the testing, which means it would have to be a line item or part of a line item that you need to propose to them. In my experience, I have found that most customers are happy to pay for legitimate testing of their tree, often remarking that they didn’t realize you could do that. You will also have clients who ask to take that off the proposal, and that’s fine, too. One thing arborists need to remember is that you won’t test trees if you don’t propose the test in the first place. This comes down to a talk with your sales arborist(s) and an active effort to suggest or educate them about the benefits to clients of testing.

Another option is to pay for the tests yourself, as a way to gain more information for yourself and your client. I do not recommend doing this often, or for no reason, but it can be a great tool and a professional look if you were to offer a few free tests with your larger or more loyal clients.

Where to send your samples
Where to send your samples depends on where you are in the country, as well as your understanding of how to interpret the samples themselves. Some laboratories are focused more on agriculture and may not have the best information regarding trees, even if their testing methods are great. Other labs may have minimal testing but a good understanding of trees and nutrient needs.

This brings me to another good point, and that is to establish a relationship with whatever lab you do choose and allow the agronomist, soil scientist or lead lab technician to assist you. The agronomist I talked to, and still talk to today, was instrumental in helping me understand and interpret and simply giving me ideas around what I was seeing on the tests and in the field.

I recommend using a third-party laboratory over a university. While I have not personally used more than two universities, in my discussions with other arborists from around the country, the same opinion is usually held.

Soil Testing for Tree Health

Photo 3: Custom made foliar sampling kit with 3- to 4-foot poles that can fit into a car’s trunk.

It has been my personal experience that the state or local universities typically took longer to get samples processed, in the neighborhood of two to four weeks versus one week or less for third-party labs. Also, while the pathology labs usually have a good response and explanation, the soil labs have left more to be desired, as they are not accustomed to foliar analysis in conjunction with soil analysis. The third-party labs usually have someone who readily can talk to you about both samples and help guide you based on those samples.

That being said, we use a university (The Ohio State University) for pathology/disease testing. I also would encourage you to try your local university if you already have a good rapport with them, or if they have a known specialty (Cornell Soil Science).

A few third-party labs I personally have used for foliar and soil nutrient testing include:

  • Spectrum Analytics – Ohio
  • Waypoint Analytical – Virginia
  • A&L Great Lakes Lab – Indiana
  • Midwest Laboratories – Nebraska
  • New Age Laboratories – Michigan

Creating a portable sample kit
How to practically take samples is something I have taken seriously over the years, as working for a small business usually means you don’t have a whole team of people to assist you. For all of us, though, just getting out to take samples can prove to be difficult sometimes. That is why you need to make the process as smooth and easy as possible.

Soil Testing for Tree Health

Photo 4: Red circles depict areas of a tree where quick foliar samples can be taken.

One easy way to make sure you and your sales arborist(s) or technicians can take foliar samples when on site is to make portable kits that can fit in the trunk of a Honda Civic or on any truck you have. (Photo 3) The kit pictured was made using 8-foot poles from our tree crews that broke while working. We cut the broken poles down and made them into 3- and 4-foot-long mini poles. I grabbed an older pruner head that the crews didn’t want any more and cleaned the blade up. Each kit has three to four short poles, a bag of gallon plastic or paper bags and a pruner head.

I’ve made a few of these kits so our sales arborists, myself or our senior PHC technician can take samples without having to come back to the site. This greatly improves efficiency in getting samples taken and submitted. Soil-sampling kits are also easy to make and require less equipment overall.

Making the process scalable and practical
No matter how you want to physically sample trees, doing so in an efficient manner is imperative to continue the testing operations and increase the number you can do annually. Just like any additional operation at your company, communication and support is going to be the driving factor behind success. Developing a clean and efficient process allows that support to come easily and communication to be simple. Because samples must be taken and also mailed, it may require more than one person in the process. However you choose to mix this into the daily workflow, make sure you are evaluating your process to see where you might introduce greater efficiencies.
One example might be to schedule all your approved foliar/soil samples on the same day, so you can create an efficient driving route and send the samples all at once, saving on time and shipping costs. Another example might be having your PHC technician take the samples while applying a treatment, allowing for only one trip instead of two. This may not be applicable if the test is required for the treatment, but if not, it makes quick work of getting samples taken.

While soil sampling can be a little bit more involved training-wise, most PHC technicians have had some form of pruning training in the past. This allows you to easily take, at minimum, foliar samples with one visit using your technician(s). When sampling foliage, taking fast and accurate samples is imperative. While I would love to get 25 clippings from all over the canopy, that is not practical for the everyday arborist. In Photo 4, you can see the general area where I will take four to eight small cuts of 1- to 3-year-old growth, ensuring that the leaves you sample are the “average”-looking leaf for the whole canopy. Sampling the worst or best leaves will give you an inaccurate or misleading result.

Soil Testing for Tree Health

Photo 5: Slate rock in soil, preventing any soil probe from being used.

Lastly, sampling soil in urban areas can be challenging, with rocky, compacted and polluted soils. This has generally left us without the practical use of soil-sampling probes. A good idea is to use a soil auger, a spade or trench shovel or a bulk density probe that uses a sliding hammer to push through small rocks or compaction. Small samples can then be taken from a larger chunk of soil to reduce the total number of times you need to dig. (Photos 2, 5 and 5a)

Soil Testing for Tree Health

Photo 5a: “Most shovels can dig 6-8 inches pretty easily, and I sample at a depth of 4-8 inches for trees,” says Zack Shier.

Summary
If you are just starting to take foliar and soil samples, or if you are looking to scale your testing to do more, reading and understanding the ANSI A300 ISA Best Management Practices is the first place I would start. Start taking samples as soon as you can and develop a relationship with a third-party lab. Over time, you will discover how to utilize your testing to best suit you and your clients’ needs.

No matter how you are using testing, the arboriculture industry will be improved if we can educate arborists about the benefit and necessity of foliar and soil analysis.

Zack Shier, Board Certified Master Arborist (BCMA), ISA Tree Risk Assessment Qualified and Ohio certified applicator, is plant-health-care manager with Joseph Tree Service LLC, an accredited, 12-year TCIA member company based in Dublin, Ohio. He has a bachelor’s degree in Forest Ecosystem Science from The Ohio State University.

This article is based on his presentation on the same topic at TCI EXPO ’23 in St. Louis, Missouri. To view a prerecorded video made for that presentation click here.

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