Modern Organic Amendments: A Practical Approach to Urban Soil Health

In my December 2022 article in TCI Magazine, “The Plight of Urban Soils,” a majority of the content focused on the issues with urban soils and how they are notably different from natural forest and prairie soils. There is mention of modern organic-matter (OM) products and how organic-matter inputs can help improve urban soils. (Photo 1) Understanding organic matter and how to use it effectively is a challenging topic. There is no doubt that lack of understanding plays into the underutilization and underrepresentation of OM products in the tree care industry. In this article, we’ll take a deeper dive into OM as a whole and aim to clarify the use of OM in our modern world.

How does organic matter work?

Attempting to understand the science of organic matter can be difficult without first defining what exactly “organic matter” is, or even, which of the many definitions we want to use. For the purpose of this article, we are speaking more specifically about OM that directly affects our soils. Cornell University defines soil organic matter as, “… the fraction of the soil that consists of plant or animal tissue in various stages of breakdown (decomposition).”

It’s crucial to understand that OM is not a single product in a single state, but an ever-changing pool of compounds, nutrients, microbes and other materials. Ahead, we’ll talk about specific products and why you might choose one over another, depending on what stage of decomposition or form the OM is in.

First, let’s touch on the basic path that organic materials take during decomposition. (Figure 1) This path may begin when an animal dies or a fruit falls from a tree, or when you throw out that old sandwich from the back of the fridge. Regardless of how the OM gets there, when it is put on natural ground, decomposition will begin to occur.

This process is not done internally, but only through the microbes that live in our soils and are floating in the air. These microbes begin the process of decomposition, and throughout the process, different species and types of microbes continue to modify and break down organic materials.

These microbes use enzymes to break chemical bonds and modify compounds. They change and break down organic compounds into inorganic minerals, lipids, amino acids, organic acids and much more. This microbial food not only increases the microbial population but diversifies it, too. The release of nutrients and minerals from OM helps feed plants and improves nutrient cycling of the soil. This process also makes a nice home for earthworms, ants and all sorts of other little critters that work together to improve soil aggregation, aeration and water infiltration.

A visual representation of this process is easily seen in compost piles. What starts as a pile of recognizable leaves, food scraps and mulch, etc., slowly turns into a pile of black, loose, soil-like material we refer to as “compost.” Compost is OM that is still in the active phase of decomposition vs. stable. Generally speaking, once decomposed organic materials begin to look more like soil than raw materials, they are safe to incorporate into the soil. This ensures they are further down the path of decomposition and will not rob too much nitrogen from the soil as they continue to decompose.

This process continues and, over time, the OM is broken down into a black/brown group of organic substances called humus, which is the stable phase of decomposition. Humus is the single most important piece of the OM puzzle and is responsible for most of the soil-fertility actions. Humic substances can be loosely classified into three categories:

  • Humic acids.
  • Fulvic acids.
  • Humins.

The nuanced makeup of these substances can change depending on where and how the substances came to fruition and can vary greatly. And that’s only the surface of this deep organic chemistry!

These substances are quite powerful and are mainly responsible for improving and having a great impact on water-holding capacity, cation-exchange capacity, mineral chelation, soil aggregation and root/plant growth stimulation.

A few examples to demonstrate some, but not all, abilities of humates include:

  • Fulvic acids – small molecules that can bind to nutrients, making them available to the tree. Fulvic acids also can be absorbed into the roots themselves, which can stimulate root growth while bringing nutrients along with them.
  • Humic acids – can bind to clay particles, increasing their aggregation and thereby increasing pore space. They also participate in cation-exchange capacity (CEC) and can hold and make available more than 50 different mineral elements.
  • Humins – large molecules that break down very slowly, increase pore space and hold onto water longer than soil particles. They also can participate readily in the CEC of a soil.

Whereas active organic matter, meaning the quickly changing decomposition of raw organic materials (mulch, compost, etc.), is the fuel for microbes and bulk nutrition in the soil, humus is the fuel lines and gas station. Humus is stable, is longer lasting and facilitates the movement and absorption of nutrients and compounds through the soil and into the plant.

As you can see, the decomposition of OM is by no means a simple process, but a group of complex processes that benefit and restore the soil at every step along the way.

History of OM use

The history of organic matter is as lengthy as our soil-dwelling plants are old. Since we know that OM is the way soil nutrients are replenished and cycled, we know that nature has used OM since the dawn of time. But how long have humans knowingly used organic matter?

There is no shortage of evidence that farmers and gardeners have been improving the soil through practical application and management of OM for centuries, with written evidence of compost use going as far back as 4,000 years ago.

In the Andean culture, farmers used terraced platforms and organic material to turn a mountainside into a lush, regenerative farming system. (Photo 2) Certain Native American tribes used the techniques of cover cropping, controlled burns and organic material to grow multiple crops over the whole growing season, all the while maintaining soil fertility.

While some areas of the world never lost touch with the power and potency of OM, other areas never fully utilized OM to their advantage. This was a costly mistake among the agriculture industry, where poor tillage and farming practices amplified the decomposition and loss of OM, resulting in an unstable and eroded topsoil.

In the U.S. in the 1930s, this specific combination of factors led to a horrific situation where the topsoil simply blew away in some areas. Having lost its ability to aggregate and hold water, drought and windstorms eroded and blew the topsoil away. You could reasonably argue that, if the OM levels had been kept higher through application of organic materials, instead of being depleted, the soils in that area might have made it through the drought and windstorms without major issue.

We can learn a lot about history from both the situations just described. We can learn about how treating the land better physically reduces the loss of the natural system of OM. We can learn that you can take an area that does not do well with plants, invigorate it with OM and restore the soil’s growing capacity. This brings up an interesting question when we apply these ideas to the world of urban forestry: Are we improving our urban soils or simply treating the symptoms of poor tree health?

Modern organic-matter products

When considering OM products applicable to the tree industry, typically you will find a list of organic materials somewhere in the decomposition process. The most commonly used would be mulch, manure, compost, peat or some form of amended potting soil, etc. These are all in the bulk form, and are generally applied topically or worked into the soil using an air-excavation tool during planting or soil remediation.

Use of bulk organic materials should be encouraged whenever possible on urban soils. Large mulch rings with the proper amount of mulch and use of quality compost in large quantities are essential to increasing the percent of soil organic matter, helping improve soil health and buffer nutrient and/or pH issues you may be facing. The use of bulk products is not always feasible, as this may not be cost effective for some clients. But it should be encouraged and used whenever possible.

This brings us to the topic of what I would define as the “modern” organic-matter products. These products can often have greater complexity and, unfortunately, do not possess the large body of research to legitimize their use the same way bulk organic matter does. Examples of products in this category would be liquid or dry humates (humic acid/fulvic acid), biochar, liquid or dry microbial inoculants, bio-stimulants, etc.

A few examples include:

  • Liquid humates (humic/fulvic acids) – Most products utilizing liquid humates are using an extracted and concentrated form of humates found in Leonardite deposits in the United States and Canada. These humates are not technically the same thing as humus found in OM, but instead are a concentrated and more diverse form that is closer to coal than compost. With that in mind, these humates contain highly beneficial humic/fulvic and other organic acids that can have a positive benefit on soil and nutrient availability.
  • Biochar – The easy way to describe biochar is by calling it the “tiny rock sponge.” I call it that because biochar is a high-carbon-based, very porous, high-surface-area, charcoal-like substance that has some very cool effects on soil. Biochar is made through a process called pyrolysis, where woody organic material is burned at high heat using little oxygen. This process locks the carbon into a highly stable form with very few contaminants. Biochar is approximately 70% carbon, which means adding it to soils adds carbon. Biochar also can chelate toxins and metals from the soil and participate in CEC, and has a huge surface area that can make a nice home for water, oxygen and microbes. (Photo 3)
  • Biostimulants – This is an interesting product category, as technically all organic material and its specific products are “biostimulants.” However, this category of products typically follows a similar path, which is a mixture of carbohydrate and amino-or fatty-acid-based organic liquids, often with fermented products, such as fish and kelp, or other various organic compounds. These products tend to be more about quick delivery of highly usable nutrition and compounds to stimulate microbial activity and plant/root growth. Of course, every biostimulant product can be a little different, so always ask the manufacturer about the ingredients used and why they chose them.

Choosing one or all of these products and becoming confident in their use can be challenging. It’s not always a question of whether they do anything, but more one of what effects they have and how much is needed. The use of bulk organic material is more straightforward; use [X] pounds of quality organic material to increase [X] amount of the area’s soil organic matter by 1%.

Since bulk organic material provides all functions of decomposition over time, we don’t need to wonder what exactly it’s doing. With the more concentrated products, you’re using more narrowed forms of OM, somewhere along the process of decomposition, in hopes of specifically targeting or improving soil characteristics. Simply stated, concentrated and specific forms of organic matter should work just like bulk OM to improve soil. The questions that always follow are: How much do I need to use? What exact effects are there? How fast can I see results? Can I, or should I, still use synthetic fertilizers?

Controlled testing

These questions plagued me over the last few years, so I attempted to answer them myself. In late 2021, I orchestrated a controlled experiment in the yard area of my employer’s shop. This soil mimics the soil we see in our city of Columbus, Ohio – mixed, rocky, high pH, low organic matter. From May to October 2022, we conducted the experiment to see the effects of different fertilizer products on soil. I made three separate plots, where each trial would be repeated. Each plot would have the same seven treatment types, which included:

  • Control.
  • Liquid humate (low rate).
  • Liquid humate (high rate).
  • Liquid humate (low rate), plus a biostimulant.
  • Liquid humate (high rate), plus a biostimulant.
  • 20-5-10-15S (nitrogen, phosphorus, potassium and sulfur) synthetic fertilizer.
  • Combination of the liquid humate, biostimulant and NPKS (nitrogen, phosphorus, potassium and sulfur) fertilizer.

Each plot contained seven testing circles that were 6 feet in diameter, where each product would be injected into the soil in their respective circle at their respective rates.

Soil samples were analyzed by a third-party lab. They were tested before treatment in May, again in August post-treatment and lastly in October post-treatment. We also tested the microbial biomass and fungal-to-bacterial ratio of every sample using Soil MicroBIOmeter testing kits. The third-party testing is still being calculated at this time, however, I was able to get the results of the microbial testing before this article was published.

The graphs in Graphics 1 and 2 demonstrate a significant increase in both microbial biomass and fungal-population ratio (to bacteria). When we compare the humate groups against the control groups, the increase in microbial biomass from May to October was 60% in the control groups and 153% and 210% increases in the humate and humate-and-biostimulant groups, respectively. This nets a 93% and 150% average increase in microbial biomass over the control groups, respectively.

A similar result showed in the fungal-to-bacterial (F:B) ratio tests, which means these humate products are increasing the number of fungi in the soil. This is pivotal for trees, considering they prefer a higher fungal-to-bacterial ratio than our urban soils have.

What’s even more interesting is that the NPKS groups showed no statistically significant change in microbial biomass or F:B ratio over the control groups. This should not come as a surprise, since there is plenty of research showing the often negative effect NPK fertilizers can have on our soil microbes if used improperly. One way to help counteract some of those issues is to reduce NPK fertilizer rates and pair them with a humate blend as a tank mix. In the “ALL” groups, you can see that the power of the OM products was not negated by the addition of the NPKS fertilizer when used together.

More research like this is needed, and I hope to continue this study in 2023 and beyond.

Future of urban-soil health

The urban soils desperately need our help, the same way our agricultural soils need help. The nature of urban-soil development and running for-profit businesses puts us in a bit of a pickle. While what our soils truly need is large quantities of bulk OM on an annual basis to increase the percentage of soil organic matter, that is often just not practical or advantageous for many clients.

Tree care companies should be using bulk OM as much as possible. The more modern and concentrated OM products that enable us to practically administer OM are a massive step toward correcting the issues. The fact that these products can increase synthetic-fertilizer efficiency and thereby reduce fertilizer use is profound. So is the fact that they can improve some tree health (and soil) issues on their own without the use of pesticides or synthetic fertilizers. These new products give us a front-line, foundational “fertilizer” that companies can implement to help the soil and tree before moving to a synthetic fertilizer.

This can provide the advantage of assisting the nutrient availability and soil microbes that are already present in the soil, giving a head start on soil correction and allowing time to better assess the soil and tree for other nutrient issues. We can then appropriately apply complementary treatments such as growth regulators, air excavation and synthetic fertilizers as needed.

More research is always needed to better hone specific product ingredients and rates. The future of urban-soil health will benefit and ultimately depend on the use of these newer products. With that in mind, consider a fresh mindset about how to approach urban tree and soil issues.