ITG soil injection

University of Illinois Extension horticulture educator Ryan Pankau inoculates soil with beneficial fungi using a soil-injection probe at the Red Oak Rain Garden on the UI campus while Master Naturalist volunteers pull weeds in the background.

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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 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, 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.

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 land use or simply to build a home and lawn as our urban spaces spill out into rural areas, we are changing the soil ecosystem, oftentimes to the detriment of microbial populations.

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

Currently, there is some debate among arborists as to the lasting benefit of inoculation 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 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 physical soil environment.

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. 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), Illinois Extension staff, alums and numerous volunteers from student groups Master Gardeners and Master Naturalists.

The garden’s namesake is a 48-inch-diameter northern red oak (Quercus rubra) that is estimated to be 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 the rain garden and see mycorrhizae in action as plant life thrives over the coming seasons. The entire space is open to public and easily accessible. More information is at go.

Ryan Pankau is a horticulture educator with UI Extension, serving Champaign, Ford, Iroquois and Vermilion counties. This column also appears on his ‘Garden Scoop’ blog at

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