If you go to a local landscape supply store and look on the fertilizer shelves, you may find a variety of different products. Most of those products are going to be N-P-K fertilizers and will have some ratio of primary nutrients (macronutrients), such as 27-9-9 or 10-10-10, combined with a short list of micronutrients and the percentage of each present in the fertilizer. There also may be some form of organic-matter mix like peat moss or compost, or a mixture of the two. The biggest questions I had when I first started building a plant-health-care (PHC) program were which product to choose and whether or not I should have multiple products.

I found out quickly that there are a lot of hard-and-fast rules for different fertilizers when it comes to turf care and, in some sense, agriculture. But when it comes to trees, I was finding that the soil and foliar tests we took weren’t always matching up with the fertilizer options.

The goal of this article is to explain the difference between organic and synthetic fertilizer options and how to best approach long-term fertilizer treatments.

What is organic matter (OM)?

Soil organic matter, as defined by Cornell University, is “… the fraction of the soil that consists of plant or animal tissue in various stages of breakdown (decomposition).” This definition is most certainly applicable to the organic-
matter content in our soils as well as the bulk organic matter (mulch or compost) we physically add to our soils, usually in the form of topdressing. We also may use organic fertilizers, which may contain myriad different ingredients but, in general, are all fully decomposed.

Fully decomposed and stable OM is essential if putting it under the surface in the active root-growing area. If unstable OM, that which is not fully decomposed, is used regularly in the subsoil area, you risk the chance of soil microbes robbing the soil of nitrogen to decompose the carbon-based OM (search “carbon-to-nitrogen ratio”). According to Cornell, fully decomposed, stable organic matter is referred to as “humus.” You may also see humus referred to as humates or humic acid. This type of organic matter is what we will mainly be discussing in this article. Never underestimate the power of top-dressed mulch and compost, but for our conversation, we are mainly speaking about subsoil treatments, usually administered with a root-feeder probe or via an air-excavation tool.

What does organic matter (humus/humates) do for our soils?

Organic matter has the unique ability to positively affect all three properties of soil: physical, biological and chemical.

You may understand that chemical soil properties mainly consist of nutrients and minerals and the interaction of those. OM has nutrients in it, and therefore, when you add OM, you are adding nutrients, much like when you fertilize with N-P-K fertilizers. However, there are some key differences, mainly that OM has a higher cation exchange capacity (CEC) than soil itself. According to Cornell, OM can have between four and 50 times higher CEC than clay soil particles. This means that, on top of adding nutrients to the soil, OM can improve or assist in the ability of the soil to exchange nutrients within the soil. This translates to an increase in nutrient availability to the tree growing in the soil by buffering negative soil characteristics, such as too low/high pH and low CEC (sandy soils).

OM also has a positive impact on biological and physical soil properties, often one impacting the other. OM provides a food supply to microbes such as bacteria and fungi. Increased biodiversity results, and microbe populations and species diversity increase. These increases improve pore space and soil aggregation, resulting in better water infiltration, water-holding capacity, improved drainage and less compaction. OM itself acts like a sponge, holding up to 90% of its weight in water.

These improved physical soil properties all have a positive impact on the ability of the tree to grow new roots, take up much-needed water (especially during drought) and form mutually beneficial relationships between microbes and tree roots.

Are OM-based fertilizers the same as N-P-K-based fertilizers?

If you attended TCI EXPO this past November and took in my presentation on Urban Tree Nutrition and Fertilization, you should be very familiar with two words: balanced and specific. I used the words balanced and specific continually to describe the difference between OM and N-P-K fertilizers, respectively.

Now, some of you may be thinking that your standard N-P-K fertilizer is most certainly a balanced approach to tree health. It has multiple primary, secondary and micronutrients, it can be applied over half the year and it’s the “normal” treatment you go to when you see a stressed tree. While an N-P-K fertilizer may not be as specific as something like a micronutrient injection, it’s also far from being a balanced approach. We already talked about how OM improves the physical and biological aspects of the soil, but how does it differ chemically?

Figure 1 (graphic) shows a lab analysis of a humate blend from American Biochar Company. This lab analysis shows the amounts of all the macronutrients and micronutrients in the OM product: 55-plus total nutrients, including a large portion of active carbon. This is what is meant by a more balanced approach. You’ll notice that it does not have an extreme amount of any one nutrient, like with an N-P-K fertilizer, which may have as much as 30% nitrogen. This balanced and broad nutrient makeup, combined with the improvement in soil properties other than chemical, is what is meant by a “balanced” approach. This would imply that N-P-K fertilizers should no longer be considered as a broad treatment, and instead should be used in a more specific manner.

Urban tree fertilization: How should we use fertilizers?

The point of this article is in no way meant to take aim at N-P-K fertilizers as the “wrong” approach, but more to clarify that we need to change our mindset on what we think of as a foundational fertilizer, such as OM versus something like an N-P-K fertilizer. N-P-K fertilizers should be used more specifically after testing or in situations that are known or previously tested for, whereas OM fertilizers should be used more broadly, or before testing or deficiencies can be discovered.

You may be wondering whyN-P-K fertilizers shouldn’t be used generally, without testing or as a broad treatment. When I talked earlier about the nutrient makeup of fertilizers, I mentioned that N-P-K fertilizers can have up to 30% (and sometimes more) nitrogen, along with higher percentages of other macronutrients as well. When we use synthetic forms of nitrogen in higher amounts, this can have a serious effect on trees and how they use their energy. Excess nitrogen or excess macronutrients without the compliment of all the other required components (sugars, secondary metabolites, micronutrients, etc.) can cause imbalances in the tree. When you combine imbalance (deficiencies) with excess nitrogen, you get a recipe for forced growth without the accompanying responses and energy to deal with that growth. This can result in greater micronutrient deficiencies, reduced pest defense, water/nutrient uptake issues, etc. This is why fertilization without testing or understanding what the tree needs can be problematic.

In fact, Clemson University specifically says, “Fertilizer should not be considered a cure for ailing plants when unadapted or unhealthy plants are chosen, carelessly planted or improperly watered. When fertilizing trees and shrubs, keep these two points in mind: (1) Fertilizer is beneficial when it is needed; but (2) Use it in the right amount, at the right time and in the right place.”

In our urban area, this could easily mean that the overwhelming majority of trees should not even be considered for N-P-K fertilizer use until further evaluation is done, with some of those trees possibly never getting an N-P-K fertilizer.

Now, I understand that we function in a practical world in the tree industry, and if we remove the N-P-K fertilizer as your general-use fertilizer, then what are you supposed to use in these situations? This is where OM-based fertilizers come in and allow you to create a foundational fertilizer that can be used in any situation, before testing, and you know you’re improving the soil and the tree. Obviously, this does not guarantee the tree will magically get better, but it can be used as the base or foundation to your PHC program. You would then build upon these initial health treatments with testing, IPM or prescribed fertilization (N-P-K fertilizers).

No matter how big or small a company you have, optimizing and improving your plant-health-care protocols should always be on your mind. Just like the introduction of knuckle-boom cranes and grapple trucks have changed the way we do tree work, modern PHC equipment and new research should change the way we do plant health care. If we are to call ourselves stewards of the urban forest, then we must care for our urban trees in a way that makes them the most resilient and optimally healthy trees, much like their forested sisters.

Zack Shier, Certified Arborist 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 and holds the TCIA Plant Health Care Technician credential.