Part 1: Oxygen

Good tree root health

In our urban and suburban forest we are challenged to help our trees to overcome the handicaps of living in the built environment. Most of the problems that affect tree health are found underground. We know this to be true because of all of the research that has been done regarding tree health in the built environment. Sadly, very little effort is put into the root zone of trees, though. And what little effort is made in urban arboriculture is not very helpful to root health.

You may have already spoken to an arborist about improving the root health of your trees. Unfortunately, the response from plant health care professionals is likely to be fertilization. In the vast majority of cases involving declining root health, the problem has nothing to do with fertilization. The problem with urban tree roots is usually respiration.

It is not often talked about, but tree roots must respire to survive. In the tips of the roots where absorption takes place oxygen is critical to root life. If oxygen levels drop too low, then roots will die. It is this low oxygen level that is more likely to be the cause of declining root health than any nutrient deficiency.

The reasons for oxygen deficiency can be:

Poor Drainage

If the soil supporting tree roots is saturated beyond field capacity and fails to drain, then the soil will become anaerobic and suffocate the roots of trees. By contrast, good drainage draws oxygen into the soil. As the water leaves the macro pores of the soil, air is drawn in.

Soil Compaction

Heavy traffic (pedestrian or vehicular), construction, plow lines, and even the weight of constant surface irrigation can compact soils to the point that they will no longer support the oxygen levels needed for root growth. Compacted soil can be irrigated every day and still not ever support tree roots.

Competing organisms

Rises in soil temperature can drastically increase the demand by aerobic organisms within the soil for oxygen. This competition can lead to depleted levels and poor tree health.

There are corrective measures that can be employed as mitigation for all of the contributing factors to oxygen deficiency in soil. For drainage issues, trenching in drain pipe may be the answer. For soil compaction, air spading and structuring, or simply installing rootwells can achieve the necessary levels at depth. When there is too much competition from microbes, a more stable organic matter can reduce the level of soil composting.

An arborist can help to solve the problem, but beware of one who pushes the idea of "deep root fertilization" as this is not going to do anything to deliver oxygen. Of the things that are necessary for root activity, oxygen is the most important to the trees that we care for.