Soil Amendments for Improved Plant Health

As a company delves into PHC, exploring the potential to be tree physicians rather than just tree morticians (general tree care), the array of possibilities can be overwhelming. The initial tendency is to focus on ways to control key insects or diseases that may show visible signs or symptoms on your customers’ properties. However, abiotic (non-living) causes of plant decline account for between 30-40% of the problems an arborist may encounter, and treatments for pests will not solve these problems. In fact, it can be argued that these abiotic causes of plant decline actually make plants more susceptible to pest attack and more at risk of serious damage when attacked.

Abiotic issues in a landscape can cover a wide range of issues, from improper planting depth to girdling roots, excessive mulch to construction damage, improper pruning to drought stress – and diagnosing the cause of these plant declines can be challenging. The remedies for non-pest-related issues can be equally daunting.

As new arborists begin to explore supplements and inputs they might add to the soil to improve conditions, the natural tendency is to fertilize with N, P and K, usually choosing a blend that provides some logical ratio of these nutrients. But is any fertilizer blend the answer? The answer is often no, though there are conditions, especially in newly planted trees, that might call for a slow-release fertilizer blend to help with plant establishment. It would surprise many people to learn that most soils have adequate levels of these basic nutrients already present. So we must wonder, why then are trees not growing optimally? Also, it is a well-accepted fact that plant growth is limited by that which is most limited, so figuring out what is limiting your trees is the goal.

The answer is complex, but a simple answer is that most plants are placed in artificial environments, dug from elsewhere and placed on a customer’s property and, frequently, the wrong tree is planted in the wrong place, preventing it from ever reaching its optimal potential. For instance, most people plant dogwoods in the center of a front yard to enjoy its flowers, where it will experience all-day sun, heat and water stress. Yet dogwoods are considered understory trees, and they are found in nature in a semi-shaded forest setting. It is really no surprise that many dogwoods die from borer attacks before they reach 3 inches in diameter. No amount of fertilizer can fix this situation.

There are many such examples, so it is incumbent on those planting trees to understand the limitations inherent in the genetics of the plant and to explain this to your customer, either as a limiting factor for existing trees or when considering a new plant installation.

Another critical factor which is often overlooked, especially when introducing desirable but non-native species to our landscapes, is the roll of soil pH in nutrient availability. Many plant species depend on a synergistic relationship with soil-dwelling fungi called mycorrhizae. These fungi assist plant roots in absorbing nutrients and water. However, a little-known fact is that many mycorrhizae do not have a wide range of acceptable pH, so a tree may die not because it is unhappy, but rather because its fungal partner can’t thrive.

Early in my career, I was presented with a unique example of the degree to which soil mycorrhizae can dictate plant survival. In a field in Alabama, two distinct types of clay overlapped in the field’s center. The gray clay was alkaline and the other, a red clay, was acidic. Along the fence rows, as is often the case, small stands of trees had grown up. On one side, the alkaline side, there was an entirely alkaline-loving community of plants, such as an Osage orange, while on the other side of the field, acid-loving plants thrived, as exemplified by southern pines. This was my first indication that what takes place in the soil is more important to plant health and survival than what we apply as treatments for visible pests, and, frequently, even our planting skills can’t fix sub-optimal soil conditions.

Rather than guessing at soil conditions, there are several tools a new PHC practitioner might employ to help them. The first is a simple soil probe. It can identify compaction, depth of soil, condition of the soil and even anaerobic conditions caused by sitting ground water, (which forces oxygen out) causing roots to die. Secondly, soil tests, specifically those that include cation exchange capacity (CEC), pH and micronutrient levels and their relationships, can clue you in to the overall nutritional health of the soil. In micronutrient comparisons, it is often the balance of nutrients, rather than the lack of any one of them, which causes plant-growth challenges. If those results seem acceptable for plant growth and health, yet you do not see evidence of optimal health, flowering, leaf size or internodal expansion, it is time to consider special supplements, which go beyond fertilizer.

An important concept with considerable recent popularity is the notion of feeding soil microbes to assure healthy soil. Healthy soil can then assure healthier trees and shrubs. So, what can be done and how does one feed soil microbes? First, let’s define naturally occurring soil microbes. They consist of decomposer organisms that break down organic matter into useable plant nutrients. Some of the larger decomposers include visible arthropods and insects, such as worms and beetles, but the true workhorses go largely unnoticed, including microscopic soil-dwelling fungi, bacteria, protozoa, viruses, algae and actinomycetes.

These decomposer organisms do not work in isolation, but rather in a complex interactive web. Many of the artificially reconstructed soil profiles found on landscapes have mixed horizons (A, B, C) with low nutrient-holding ability, and often the soils that are brought in won’t have adequate levels of these microbes, nor the nutritional support for them to develop a healthy presence in the soil. Decomposers also contribute to improved soil structure, which reduces soil compaction and encourages root development. Bacteria, clumping with organic matter and parent soil materials, create good soil structure, resulting in friable soil that in turn permits air movement and soil moisture flow, while facilitating nutrient availability to plant roots. There are additives which also speed this process, such as hygroscopic humectants.

There are even mechanical methods to rapidly change soil conditions around a struggling plant without injury to sensitive roots. The Supersonic Air Knife is one such tool, a device that uses high-velocity air directed into the soil to break it up, without injury to roots. After the use of such a tool, nutritional supplements often are added to improve soil structure or enhance microbial activity. The simplest addition may be the use of composted, decaying plant matter. How then do we “feed” or provide a better environment to encourage soil microbes, which in turn improve plant growth conditions?

Two important components of any additive to the soil, which are designed to encourage microbial enhancement resulting in soil structural improvement and greater nutrient recycling, are those containing carbon and sugars. Mineral-based carbon is often locked tightly into the soil matrix, so organically (plant and animal) derived carbon is easier for microbes to use and for plants to eventually have available for growth and development of their roots and above-ground tissues.

There are quick- and slow-release carbon additions that can be made to soils to improve them, and they are found in numerous products available to arborists. Humic and fulvic acids are great slow-release carbon sources and are found in supplements. Molasses, an unusual addition, provides both quick-release carbon and sugars, which are utilized rapidly by plant-friendly microorganisms. If you can identify a soil supplement that provides sugars, humic acids, carbon and other beneficial ingredients, you don’t have to make your soil enhancement very complicated. Some products do contain many of these necessary requirements in one product.

Another key-ingredient group found in soil supplements, which vastly improve plant responsiveness, is known as plant growth hormones. Products that contain cytokines (often derivatives of seaweed) will stimulate root development to assure that a newly planted tree or even a struggling plant can build a stronger, more effective root system. They also may improve plant-stress tolerance. As you search for products designed to help plants grow, consider supplements that include seaweed extracts or cytokines.

An often-underutilized group of soil supplements is known as plant growth regulators (PGRs). New PHC companies may fear that such products can overregulate the growth of a tree or shrub, but many PGRs have very clear and detailed labels to assure proper treatment. There are even devices which take the guesswork out of these soil applications. It seems counter-intuitive to suggest a plant growth regulator when you may be thinking that the best evidence of your effectiveness on a property would be the robust growth of the plants under your care. In many ways, PGRs have far more value and benefit to the tree or shrub than their ability to regulate its growth.

PGRs serve as one of the best solutions for a tree under stress or recovering from stress, pest damage or poor growing conditions. Here are some of the benefits that warrant your exploration of these products sooner than later. PGRs will:

  • Increase chlorophyl production, resulting in darker appearing leaves.
  • Increase the control exerted by “guard cells” around leaf stomata, thus reducing water-loss stress.
  • Suppress the release of plant hormones typically identified with top growth and expansion (gibberellins), and increase the release of abscisic acid, a chemical that increases root development and stress tolerance.
  • Increase carbohydrate storage in plants and even improve the chlorotic appearance of plants growing in conditions that limit their micronutrient availability.
  • Reduce labor resulting from reduced rapid and often uncontrolled growth of a plant’s above-ground components.

As PGRs also increase cuticle layers on leaf surfaces, there is some empirical and peer-reviewed evidence of reduced disease risk to leaves and needles.

As our experience increases in plant health care, we discover that programs of plant health care can serve as a map to rejuvenate or enhance the growth of the plants placed in our care. There are several such programs.

Depending on soil tests or plant age, micronutrients or macronutrients may play a role in plant health. Adding organic matter to planting sites can create an environment that fosters microbial health. This can even be stimulated in existing plant locations, with radial trenching using an air tool and then administering products such as compost, compost teas or products containing carbon and sugars.

Some products on the market contain soil microbes, bacterial, mycorrhizal or fungal elements. However, I must remind you that adding extra microbes might be a short-term solution if the conditions discussed previously to assure plant success have not been met. “Carrying capacity” is the ability of a site to functionally support a level of any animal or microbe, and we must remember to make our landscapes more suitable for long-term microbial survival; added microbes will rather quickly revert to levels existing in those poorer soils if we do not improve conditions for microbe success.

Another aspect briefly discussed above is the ability of water-holding products containing hygroscopic humectants to enhance soil-water holding (reducing the impact of drought), increase root-available liquid water and support plant growth. Other water-impacting products, often called wetting agents, can break surface tension of root balls or drought-induced hydrophobic conditions on soil surfaces. These also may be found in products designed for water holding within the soil. Plant growth hormones and stimulants are vital tools in the arborist’s toolbox to stimulate root activity in struggling trees and in new transplants as well.

Importantly, I hope you have seen that PHC is not just about controlling the visible pests and diseases, but also is used to provide optimal growing conditions for the plants in your care. This often can be achieved by improving soil health and creating an environment that encourages microbial survival and microbial growth, which, together with available nutrients, will allow a plant the best chance to reach its genetic potential. It will help, of course, if you can choose the right tree for the right site, and for that you will need to communicate in simple terms with your customers and explain why you might discourage the planting of a given plant in a location that creates a struggle for survival right from the beginning.

Rob Gorden is director of urban forestry with Arborjet, Inc., a 20-year corporate member company based in Woburn, Massachusetts

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