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Soil management and safeguarding tree health
Soil management and safeguarding tree health
BY KYLE MCLOUGHLIN
A healthy, mature tree survives and flourishes best when its soil remains undisturbed. Mature trees are a vital component to quality landscapes, so it makes sense to take measures to avoid and mitigate the potentially harmful impacts on trees through construction, plant bed installation, and hard landscaping. (Hard landscaping will almost always disturb soil, potentially adversely affecting the health and longevity of a tree.) While it is true that trees can withstand non-optimum conditions and situations, if the project goals include healthy, good looking trees, it is critical to minimize soil disturbance. This means good soil structure, good soil chemistry, and good soil protection. Protecting the soil ensures trees live long and stay healthy, with less risk of tree death or failure, and prevents unnecessary tree removal costs.
Soil health: The soil food web
If we want to avoid damaging the soil, it helps to understand the fundamentals of how soil works and how to remediate it. Healthy soil is porous and allows for oxygen and water to move within the soil, making it available to trees and plants. Compacted soil has smaller pores for oxygen and water. A general rule of thumb for healthy soil suggests it is composed of 50 per cent pore space (holding 25 per cent water and 25 per cent air) and 50 per cent solids — composed mostly of mineral particles with about two to five per cent organic matter. This ideal mix of healthy soil provides easy penetration for an ever-expanding root system as the tree grows. This deep root penetration helps to anchor the tree — physically supporting the tree and keeping it upright. Pore spaces contain air, allowing tree roots to absorb the oxygen required to maintain respiration — the process by which all living organisms convert food into the energy that fuels living and growth. The water retained within air spaces is equally vital. Maintaining these soil services is a vital key to tree longevity.
Fertility is based on a healthy ecosystem. The role of maintaining a healthy, bio-active soil ecosystem cannot be overstated. It cycles nutrients and is meant to prevent runoff. The soil food web exists in a balance that (in natural forests) is vibrantly alive — ultimately supporting healthy, vigorous trees. These eco-processes satisfy all the requirements for a healthy soil, ensuring ease of root penetration, oxygen, water and nutrients are continuously available to trees.
Urban trees exist in a much different environment where such a vibrant soil ecosystem is not present and suppressors to health are legion. Trees are beset by limited soil for root growth, pollution and the effects of human use and construction. Where the ideal forest environment is not possible, maximizing good conditions and minimizing the bad can markedly improve the chances for urban trees to survive in a healthy state for longer than would otherwise be possible. Any measures that are taken to reduce damage and encourage health will make a difference.
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How to kill soil Compaction
Compaction will limit root penetration and thereby limit root growth, preventing the uptake of water and nutrients. In addition, as compaction reduces pore spaces it lowers the amount of oxygen and water available to the tree long term. On top of this, compaction also reduces drainage which can cause oversaturated, oxygen-poor soil — conditions that encourage disease and root deterioration. Deadly fungal diseases like Armillaria root rot are a common result of compaction and moisture damage. Preventing soil compaction that may occur during hard landscaping will significantly help maintain tree health and lifespan. Adding chemicals does not help compacted soil. Organic matter is the primary source of the nutrients that feed trees.
Fill soil
Adding fill, even where it is quality top soil, can also be damaging when it buries a tree’s stem (trunk) and deprives the roots of oxygen and increases moisture retention, depending of course on the depth of soil added. The loss of oxygen combined with the increase in moisture can actually lead to root or stem decay. Mulch can do this in much the same way. Eventually, fill will lead to root death, trunk decay, and decline.
Mislaid mulch
Some measures promoted as remedial are not helpful, and in fact, may be harmful. A small layer of mulch (one to two inches) can help retain moisture, keep exposed soil cool and help prevent string trimmer damage on tree trunks. Too much mulch can result in excess moisture retention around the tree base.
Bark likes to stay dry. Too much moisture can encourage rotting diseases. As an aside, the practice known as “tree volcanoes” whereby mulch or soil is mounded around the trunk of newly planted trees, is actually destructive in many ways — chiefly causing stem bark rot. This practice also encourages unnatural upward root growth into the mound, rather than extending naturally outward from the tree.
Finally, because of its depth and cone shape, such mounding (especially when done immediately upon planting),, will deprive the tree of water where it is most needed and reduce the availability of oxygen. This practice should be discouraged.
Misused fertilizer
Another problematic trend is the overuse and misapplication of fertilizer. Beware the use of chemical fertilization to mitigate the harmful effects of root damage, soil removal, fill or compaction, without fully understanding its impact on the tree. Far from reducing compaction, the addition of nitrogen will accelerate the activity of soil organisms to consume organic matter, thus reducing this vital soil component along with all of its benefits. Nitrogen can also negatively impact already diseased trees, such as Austrian pines suffering from Diplodia tip blight.
Remember too, that any observed benefit in the tree canopy (more and greener leaves) will be temporary (and risk vulnerability to pests and disease). This effect can be likened to consuming a high sugar chocolate bar after missing a meal. The temporary satisfaction and energy rush is shortly replaced by a loss of vigour. Simply adding fertilizer to the soil does not make it fertile. Never add fertilizer without a soil test — you wouldn’t trust a doctor to give you a cancer diagnosis without doing any diagnostic tests first.
Tree roots and soil
When talking about soil in relation to trees, damaging tree roots is the most obvious outcome to avoid. Beyond damaging, or worse, severing structural roots, one can easily observe the destruction of masses of fine feeder roots when only a few inches of soil are removed from a tree’s root zone. A tree’s fine, delicate feeder roots are most usually in the top three to six inches of soil, because that is where nutrients, water
and oxygen are most immediately found. Even minor soil disturbances can injure feeder roots.
Removing a large portion of the absorbing roots on a tree can set it back significantly, as well as make it more susceptible to disease and other destructive infestations. Tree root structure is important for aesthetic as much as it is for stability. Significant root damage will become apparent through dieback in the crown of the tree.
The roots are the foundation of the whole system: both the health and structure of the tree. The roots will only be as robust as the soil. Research says the best way to determine a tree’s root spread is by measuring its trunk diameter. Studies have found the ratio of root radius to trunk diameter (at breast height) to be about 38:1 where growth is unrestricted (Day, Susan D., et al. “Contemporary Concepts of Root System Architecture of Urban Trees.” Arboriculture & Urban Forestry. 2010.) For example, a 30 cm (1 foot) diameter tree can have a root system that extends 38 feet in all directions from its trunk. While root growth is influenced by many factors, this formula helps establish the area that should ideally be protected. The idea is to avoid any root damage through soil disturbance, fill or compaction within this area.
Soil and the water table
Beware of changing the water table on your client and their trees. This happens when soil at the site is removed and replaced. Where soil must be removed and remedial landscaping is scheduled following the hard surface installation, the top soil removed should be stored and used in the subsequent soft landscaping, if required. This advice assumes the existing soil is of high quality. Even if the soil is not ideal, modifying that soil will probably be less expensive than bringing in new soil and will constitute one less change (less potential shock) to the biological system and trees. Continuity is a good thing.
Each tree species has a preferred habitat of varying tolerances for different environmental conditions. For example, a floodplain tree like a silver maple or willow is more tolerant to water and compaction than an upland tree, like a sugar maple or white oak. It is important to take these factors into consideration when selecting species for planting, where to transplant a tree, and how to care for pre-existing trees within a specific area or jobsite.
Conclusion
To ensure trees are healthy for years after a construction project, it is important to remember that tree health and soil health are inseparable. Good soil requires a good soil food web and a balanced structure of pore space and nutrients. For projects working around mature trees with a goal of healthy trees at the end of the project, protect the soil as much as possible. When planting trees, understand that planting
soil and matching the tree species to it will make a big difference in your planting success. Many tree problems (young and old) come from impacting the soil through construction. In our next article, we will explore more specific details on tree protection practices, especially in relation to their roots and the soil.
Kyle McLoughlin is a Board Certified Master Arborist, the City of Burlington’s Supervisor of Forest Planning and Health, and owner and principal arborist of Ironwood Arboricultural Solutions.