Soil Aeration (De-Compaction)
Root Collar Excavation
Natural native soils are the result of thousands of years of biological, chemical and physical weathering (erosion of bedrock). Geologically, soils are dominated by the parent material within a given region. The ideal soil for the growth of trees and plants is composed of 50% solids, 5% to 10% of which is organic matter, and 50% pore space for oxygen and water infiltration and percolation.
Urban soils however, typically contain 10% to 20% pore space and very little organic matter. It’s no wonder we have so many problems with the trees in our urban environments. It has been said grow soil and the trees will take care of themselves. Indeed there is merit to that statement.
Another factor influencing the health of a soil, is the abundance or lack of the various microbial populations (i.e. mycorrhizae, soil arthopods, nematodes, worms and various protozoa)
Over time soils develop layers called horizons due to rainfall, heating and cooling, chemical reactions and biological activities. These horizons collectively make up the soil profile.
In natural forest settings, the soil profile is usually not disturbed to the point of serious compaction. Trees and soils are so ecologically interdependent, that it is hard to imagine separating them from one another. Truly the soil is the foundation on which a tree lives and grows. If we can grow healthy soil the trees will take care of themselves.
In the built urban environment, soil compaction has indeed become one of the biggest problems, often caused by activities such as land clearing, construction, grading and tilling coupled with vehicular and foot traffic and the presence of engineered soils to support roads and buildings etc.
All these factors combined serve to compress and compact soil particles into tighter and tighter arrangements thereby squeezing out available oxygen. Pushed to the extreme an overly anerobic situation is created, where growing conditions range from unfavorable to antagonistic or marginal at best.
Although certain trees have a habit of developing root systems close to the soil surface, many times this is caused by the trees reaction to growing in compacted, less then favorable soil.
Under anerobic conditions different types of micro-organisms primarily bacteria take over. Certain root decaying organisms thrive at low soil oxygen levels. Tree roots are capable of unconstrained growth at about 15% soil atmosphere. When oxygen levels drop below 5%, root growth stops. When oxygen levels continue to drop below 2%, tree roots inevitably decline and die.
Warmer temperatures can also disrupt oxygen use by tree roots, and other soil micro- organisms. As soil and air temperatures increase, so does oxygen demand. For every 18 degree increase in temperature (Fahrenheit) oxygen demand doubles. Increasing temperatures cause tree roots to respire faster. Respiration is a process by which trees convert stored carbohydrates into energy by using oxygen. This is yet another important reason for keeping soil in and around the roots of trees un-compacted, especially here in Florida where summer time temperatures can climb well into the 90’s with heat index readings over the 100 degree mark. Other factors that can contribute to low soil oxygen levels are overly saturated conditions like those experienced during flooding and other competing plants.
In natural forest soils, the rhizosphere (a zone of intense biological activity near elongating roots), serves to keep trees healthy and functioning properly. In many urban soils this rhizosphere is non - existant. Because the microbial populations in the rhizosphere and the trees themselves depend on well oxygenated soil to survive and proliferate, urban soils often do not posses the inherent qualities for optimal growth and development.
Certain trees will be effected more or less than others. A Live Oak tree (Quercus Virginiana) for example will often tolerate compacted conditions better than a Florida Maple tree (Acer saccharum) this tree is also commonly called a sugar maple.
The down side is that the tree must sacrifice this valuable space for accessing or storing energy reserves. If the tree cannot produce new woody tissue faster than the decay organism can spread, the tree looses and eventually declines and dies.
As mentioned girdling roots are another potential problem. This is basically a condition where certain roots have become entangled or are circling and have begun to choke off the vascular tissue of adjacent roots or the tree trunk itself. Certain species of trees have a tendency towards this condition.
(Magnolia Spp). Is a good example. Improper cultivation techniques are a very common cause for this malady. This primarily happens in the nursery, where trees are allowed to outgrow their containers and no effort is taken to correct the problem. The tree is simply placed in a larger container and the situation gets steadily worse. If no corrective measures are taken by the time the tree is planted in the landscape, it is predisposed to stunted, poor growth and an inevitably shorter life span.
Left unchecked the condition continues to worsen and the tree eventually declines and dies. We have also encountered many cases where additional soil was piled up around the base of an already established tree, for the purposes of creating a raised flower bed. This condition can also be conducive to the development of girdling roots.
Some symptoms of this problem are: Canopy die back , Small chlorotic leaves or needles and Flat, sunken or swollen areas around the base of the tree. If root decay or girdling roots can be addressed early enough, there is a reasonably good chance of survival and eventual rejuvenation, as the tree progresses to a more optimal state of health following root surgery. If not treated however a tree will continue to decline and removal will likely be the only recourse. This can take many years for some species.
Survival after root surgery is also dependent upon tree species, as certain trees respond better or worse than others. Root collar excavations are performed with the same basic tools as radial trenching or soil de-compaction that is with an air spade.
If your tree is exhibiting symptoms of dieback or decline it is of ultimate importance to get a professional evaluation, as there are many tree diseases and disorders that can mimic symptoms of other problems and can be very difficult to diagnose by the untrained person. If you have questions pertaining to the health or condition of your tree, don’t hesitate to call us. We can assist you in diagnosing the problem and in determining whether or not you’re tree is savable, and thereby suggest the proper course of action to be taken. Also we can provide a proper treatment regimen to restore a sickly tree to a more optimal state of health. After all we’re in the business of saving trees.
(This soil is of poor quality and is typical of developed areas)
(Microscope view of beneficial soil microbes in this case Azospirilum and mycorrhizae)
( View of mycorrhizal roots under magnification )
( Side by side comparison of poor soil vs good soil )
( The soil horizons or profile )
(This diagram shows a comparison of non-compacted soil aggregates and the effect of soil compaction on soil structure)
( A common anerobic soil bacterium in this case (Pseudomonas) under magnification )
(This chart shows the basics of how tree respiration works)
( Rhizosphere bacteria under magnification )
( Alive oak tree (Quercus virginiana )
(A florida maple tree (Acer saccharum).
Radial trenching is a method of soil aeration and de-compaction that involves using what is known as a high pressure air spade, to remove soil from the root zone of a tree in a radial spoke like pattern. During this process the soil is greatly loosened thereby relieving compaction. This method is normally used in conjunction with vertical mulching, which basically involves the addition of organic soil amendments, namely mulch or well draining, well composted soil in order to stimulate fine root hair growth. Sometimes fertilizer and mycorhizal inoculums may be added to enhance the effect. Several treatments may be needed or recommended as the tree responds depending upon the degree of soil compaction and soil volume in the root zone.
It is important to remember that the air spade is a much better alternative for removing soil around the roots of trees as opposed to the standard shovel or pick which is highly labor intensive, not to mention much more harmful to the root system. The air spade offers the arborist a much less invasive method of removing soil from the root zone, and in a fraction of the time. It is by far and large the best tool for the job. Soil is removed yet roots are undisturbed. It really lets us get a good look at what is going on below ground.
Prior to the invention of the air spade, many more trees had to be removed because of the overly invasive nature of exposing tree roots with earlier implements. When this work was attempted often times more damage than good was done to the tree.
(Mechanical damage like this will most certainly have a negative impact on this tree)
It is also important to note that the average air compressor purchased from your local hardware store or home center, is severely inadequate for such work. The type of air compressor needed must deliver at least 185 cfm to be effective. As always hiring a professional is best if optimal results are to be expected. Sometimes we encounter trees that are sickly and yet the soil is not compacted. In some cases a root collar excavation may be necessary. This is a procedure that is carried out to expose the root collar, whereby an inspection can be made to look for such problems as: root decay and or girdling roots. In many cases we have been able to isolate these conditions and correct them, (depending on the severity) through selective cutting.
Whenever root decay affects a tree, the tree attempts to wall off or separate itself (the living part of the tree) from the infected area. To ensure optimal health a tree must regulate it’s mass, that is it’s dynamic mass (living tissue) with it’s static mass (non- living or non- conductive tissue). If a tree has too much decay whether in the roots or the trunk, it must sacrifice the infected areas by (compartmentalizing) or walling them off, to prevent further infection.
(A root collar excavation on this tree reveals a healthy root system)
(A root collar excavation on this tree reveals large girdling roots which if left unchecked will most certainly shorten the tree's life span)
( The c.o.d.i.t. model an acronym for compartmentalization of decay in trees this theory was greatly expounded by the late Dr. Alex Shigo. )
( A minor root collar excavation reveals decay in the butress roots of this tree )
(These pictures reveal just how damaging girdling roots can be. Early detection is always best)
( These pictures of defective nursery stock show girdled root systems, the example on the far right has been cleaned for better viewing )
(This trees root system has been covered with fill dirt which can cause the proliferation of girdling roots as well as create conditions favorable for root decay organisms)
(The dieback in this tree is being caused primarily because of soil compaction)
(This tree is exhibiting symptoms of iron chlorosis which is also being caused by soil compaction in nutrient poor soil)
(The base of this tree had been covered with soil which was removed only to reveal a large girdling root that had developed over the years you can see where the root was removed the tree was saved but will inevitably have a weak spot at the base)
(This tree is being prepared for relocation to another site. By using the air spade to unearth the root system the risk of transplant shock is greatly reduced)