Just like turf, trees and shrubs are healthiest when the soil in which they are growing has sufficient nutrients and conditions to promote root development. With cooler temperatures and seasonal rainfall, roots of woody plants tend to grow at a higher rate in the fall than in the summer. This new fine root development allows for an increase in water and nutrient uptake. Nutrients taken up in the fall months are stored and ready to relocate to the leaves next spring. For this reason, fall is an excellent time to fertilize woody plants.
With concerns about the environmental impact of excess fertilizers and the high costs of some nutrients, professional arborists are promoting the use of “prescription fertilization.” This is the process of establishing goals for fertilization, collecting samples for soil and/or foliar nutrient analysis, and applying only deficient nutrients during the fertilizer application.
Goals are based on what the client expects to achieve with the fertilizer. These include promoting growth, overcoming nutrient deficiencies, maintaining health, replacing nutrients lost to leaf raking and removal, and/or increasing disease resistance. Identifying goals provides direction on the type and amount of fertilization.
Soil & Foliar Samples
Soil samples are the most common type of analysis arborists use to determine tree and shrub nutrient needs. Soil samples are collected from the area beneath the tree or shrub crown. The depth of sampling is based on the area where fine roots are in their highest concentration—in most areas, this is the upper 6” of soil. (Tree roots found growing deeper in the soil tend to be for stability.)
In cases where a nutrient deficiency cannot be determined from visual assessment of symptoms, a foliar (leaf) sample can be analyzed to determine which nutrients are lacking or in excess. Soil analysis and foliar analysis may have different results for the same plant, based on factors limiting availability of nutrient uptake from soil, such as pH. Soil and foliar analysis can be done by state or private diagnostic laboratories and most states have analysis available through a cooperative extension. Contact the lab before taking samples since there are forms and guidelines for collecting samples.
After soil and foliar samples are collected, they are analyzed for nutrient content. The results will direct nutrient selection and fertilizer application rates. In addition, soil samples are analyzed for soil pH. We find that nearly half of the soil samples analyzed are outside the optimal range of soil pH for the plant species tested. When pH is not optimal, the availability of some elements is limited, which can affect plant health. For example, in soils with high pH, red maple (Acer rubrum) often shows symptoms of manganese deficiency, while pin oak (Quercus palustris) will show symptoms of iron deficiency. In both cases, the nutrient may be present in the soil, but it has limited availability to the plant due to the high pH. It is very important to diagnose the conditions with analysis rather than assuming. And pests may result in similar symptoms at times.
There are numerous ways to apply fertilizer to trees. The simplest method is the surface application of soluble nutrients. This can be effective for mobile nutrients such as nitrogen, especially for sites where there is no organic matter, such as mulch or turf, on the surface of the soil. Organic mulch will slow the movement of nutrients to the root system and will break down more quickly with additional nitrogen. This can affect the appearance of the bed and may require more frequent reapplication of mulch. On turf, additional nitrogen can affect turf uniformity and quality.
If plants are growing on slopes where runoff is likely, or if you are applying less mobile nutrients like phosphorus, soil injection of fertilizer is the preferred application method. This is done with a high volume, high pressure water pump and a specialized soil injector. Most soil injectors are constructed to apply the fertilizer solution at a depth of 6” to 8” below the soil surface. Our research has found that when a solution is applied at this depth, the majority of fertilizer will be delivered to the area with maximum fine root concentration.
With nutrient deficient trees and shrubs, the results of soil-applied prescription fertilization can be dramatic. We often see greening of the foliage in a matter of weeks during the growing season. With fall fertilization, the results are not seen until spring, but we often see darker green, more vigorous growth at that time.
Most nutrient deficiencies can be corrected with prescription fertilization. However, some micronutrients, such as iron and manganese, are more effectively treated using a different approach. In these cases, it may be appropriate to inject the nutrient directly into the xylem of a tree to provide immediate benefit. This type of treatment can very quickly change the appearance of the treated trees during the growing season. However, if these treatments are delayed until fall, a higher rate of the micronutrient can be applied. Higher rates applied in the summer may result in foliar damage, but there is typically no damage when applications are made in the fall and the treatment can last up to three years.
Correcting nutrient deficiencies of tree and shrubs is an important component of maintaining plant health. However, there are other soil conditions that can affect plant health. These include soil compaction, lack of soil organic matter, and poor drainage. One excellent treatment for these conditions is the process called Root Invigoration™. It is used to reduce soil compaction, increase soil organic matter and in some cases, improve drainage under established trees without damaging the root system.
This process starts with an evaluation of the site in regard to several factors— such as tree species, current health status, soil moisture, presence of fine roots, fill soil, turf, and understory plantings—to determine if the treatment is appropriate and likely to be successful. Next, the turf in the treatment area is removed and the area is irrigated to bring the soil moisture level to field capacity. Then the area is tilled using high pressure (compressed) air. The air breaks up the compacted soil but does not significantly damage tree roots.
Once the soil is tilled, different types of materials can be applied to the soil surface to correct any problems detected in the soil analysis. These materials include composted organic matter, biochar, fertilizer, pH treatments, and biologicals such as mycorrhizae. Materials are then incorporated into the soil using high pressure air. To complete the treatment, organic mulch is applied to the soil surface and the treatment area is again irrigated to make sure the roots do not dry out.
Tree roots can respond rapidly to the improved soil conditions and that response is reflected in foliar color and twig growth the next spring. If the soil is not re-compacted, the results can even be seen for many years. The addition of mulch on top of the root-invigorated area is very important to help prevent recompaction. Mulch will also aid with: retention of soil moisture; reduction of soil temperature during summer months; and insulation and warmth during winter months. Additionally, the breakdown of mulch over time contributes essential organic matter and nutrients to the soil.
So just like turf, there are many challenges for growing high quality trees and shrubs. Lack of nutrients and other soil conditions are just a few of those challenges that can be managed with proper knowledge and practices.
Dr. Smiley and Dr. Brantley are scientists in the Bartlett Tree Research Laboratory in Charlotte, NC. Dr. Smiley is a Senior Arboricultural Researcher with Bartlett and an adjunct professor at Clemson University. Active in the arboriculture industry, he has co-authored many of the ISA’s Best Management Practices and the CTLA’s Guide for Plant Appraisal 10th edition. His research has lead to improved methods of: increasing sidewalk longevity near trees; protecting trees from lightning damage; improving tree root growth; and reducing tree risk.
Dr. Brantley is the Northeast Technical Support Specialist at Bartlett, and provides assistance to arborists with field diagnostics, plant health care, and tree risk assessment. She taught forestry and related topics at Penn State Mont Alto for more than 20 years before starting with Bartlett in 2019. Her research interests include bacterial leaf scorch, beech leaf disease, and wood decay.
Bartlett Tree Experts was founded in 1907 by Francis A. Bartlett and is a leading scientific tree and shrub care company with over 100 offices worldwide. The Bartlett Tree Research Laboratories is the research wing of Bartlett Tree Experts. The Lab houses a state-of-the-art plant diagnostic clinic and provides vital technical support to Bartlett arborists and field staff. For more information visit Bartlett.com.
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