Part 2: Microbes

Good tree root health

In discussing root health, we have determined that oxygen is perhaps the most critical issue that will affect positive root growth and vigor in trees. The aerobic nature of soil is what facilitates the physiological processes that occur in the root zone of trees. Of course life is not so simple as "just add oxygen," though. In good soil there is a complex soil food web. Let's examine it.

Something that should be considered is that tree roots do not necessarily act alone in their function of absorption of water and nutrients from soil. Growing on the root caps of the fine feeder roots are tiny mycorrhizal fungi. These fungi engage in a symbiotic relationship with the tree. In return for carbohydrates that they are able to take from the photosynthetic tree, the fungi provide microscopic extensions that greatly increase the tree roots' ability to absorb water and minerals from the soil. The presence of these fungi on tree roots can make a tree much more resistant to drought stress and disease.

In the forest soil these microbes live in abundance. The forest floor provides an ideal environment for their propagation, and they are constantly cycling through the soil food web. In contrast, urban and suburban soil can be seriously lacking in these microbes. Heavy fertilization, disturbed and compacted soils, pesticides and fungicides, and low oxygen levels combine to provide a harsh environment for their propagation.

These microbes can be cultured or "brewed" in compost teas. The brewing process heavily aerates a compost solution that contains the spores of these microbes and some nutrients to get them activated. When matured, the tea is ready for introduction into the soil for the benefit of the soil food web. The benefit of a tea is much greater than a synthetic fertilizer because of its ability to restore the soil food web instead of simply dosing the soil with nitrogen.

If the soil being treated is heavily compacted, as is often the case in urban soil, it is not enough to simply spray the solution on the surface. The absorption barrier at the level of the compacted soil will stop the migration of water, microbes, and nutrients to a good depth. The absorption barrier must be broken so that the tea can be introduced. Vertical mulching and rootwell installation are the best ways to break this barrier and deliver the tea to where it will benefit root growth.

Work with your arborist to employ strategies that can bring the characteristics of forest soil to the soil in your trees' root zones. It's amazing what can happen when you apply nature's gifts to your property.