Soil microorganisms are an integral part of all ecosystems worldwide, but they often go unnoticed. These tiny pillars of the soil environment perform a variety of incredibly important ecosystem functions, such as carbon sequestration and nutrient cycling. In addition, they also help to build more resilient soils, remove soil contaminants, and can help to regulate some disease and pest populations.

Since we cannot see, touch, or feel these microscopic inhabitants of the soil profile, who number in the millions in just a few grams of topsoil, they have historically been underappreciated. In recent years, scientists around the globe have started to provide a better understanding of the incredible wealth of diversity below ground and we are starting to piece together the puzzle of complex and diverse soil microbial life that is fundamentally important to life on this planet. 

Humans' effect on soil health

As researchers have discovered more and more about biological activity in soils, we have started to learn that many human activities at the soil surface greatly reduce their populations, threatening to limit the many valuable functions they perform. When we change soils from their natural state, either for agricultural production and other large-scale types of land use or simply to build a home and lawn as our cities and urban spaces spill out into rural areas, we are changing the soil ecosystem and oftentimes to the detriment of microbial populations. 

Since natural soil formation takes many thousands of years, it's not easy to restore lost or altered soil properties that were fundamental to supporting soil fauna. In addition, soils with reduced function drastically impact plant life, further building the case for the need to conserve both the physical aspects of soil as well as the biological.

What are mycorrhizae? 

One valuable type of soil fauna, a group of plant-beneficial fungi known as mycorrhizae, function as an incredibly important, sometimes absolutely essential, part of the soil ecosystem. These valuable fungi form a symbiotic (mutually beneficial) relationship with plants by infecting roots to derive energy from plants that they cannot create on their own. In return, the plant root’s ability to absorb water and nutrients is enhanced by the fungi. In this relationship, both organisms benefit and thrive from the shared resources each can offer.

In some cases, as with many of our native orchids, plants cannot live without their mycorrhizal partners. In other cases, such as trees that we plant in urban environments, mycorrhizae may not be entirely essential to their existence, but we know that much is lost without the mutualistic relationship associated with mycorrhizae. 

Adding beneficial biota to the soil

In recent years, a number of products have become available to inoculate soil with mycorrhizae and other beneficial biota, increasing our ability to reestablish lost populations in urban soils. There are a variety of ways to apply these products, from mixing them into backfill at the time of planting or via liquid application either by drenching the soil from the surface down or by directly injecting with the use of a soil probe.    

Vertical mulching

Perhaps the most beneficial way to apply these products is by way of a practice called vertical mulching. In this application method, a soil auger in a handheld drill is used to quickly drill holes down into the soil profile under the canopy of a shade tree. The holes are evenly spaced a few feet apart to cover as much of the tree’s root zone as possible. Then the drilled holes are backfilled with amended soil that includes a mycorrhizae inoculant. This practice has the dual benefit of improving soil structure, which enhances air and water movement in soil, while adding in soil mycorrhizae and other beneficial soil biota in a single, yet labor-intensive, application.

How beneficial is it?

Currently, there is some debate among arborists as to the lasting benefit of inoculation with beneficial microorganisms in urban soils. Since microbial populations were lost during soil disturbance activities that established the urban setting, does it make sense to think we can add them back into an inhospitable soil environment and expect them to persist?  More research is needed to fully understand the effectiveness of soil inoculation in urban environments, but I think it makes a lot of sense to add beneficial biota when other practices are first applied to improve the soil environment. 

Soil preservation in action

One project on the University of Illinois campus has already sought to improve soil conditions with a variety of practices. The Red Oak Rain Garden is a 10,000-square foot space near Allen Hall on campus. In recent years, an interdisciplinary team of folks from across campus has worked to renovate the rain garden with a wide array of improvements to both soil and plant life. This effort has included advisers from various academic departments - Civil and Environmental Engineering, Crop Sciences, Landscape Architecture as well as Illinois Extension staff, university alums, and numerous volunteers from the Master Gardeners, Master Naturalist, and student groups.

The garden’s namesake is a 48” diameter northern red oak (Quercus rubra) that is around 200 years old. As part of a comprehensive plan to ensure the health of this stately oak, I was on campus last week to apply a mycorrhizae soil inoculant via soil injection to the tree’s root system.

I encourage anyone interested in rain gardens or native plants to visit Red Oak Rain Garden and see mycorrhizae in action as plant life in the garden thrives over the coming seasons. The entire garden space is open to the public and easily accessible.  More information can be found at go.illinois.edu/RORG