Mycorrhizae - A Soil Amendment?

In various gardening, lawn, landscape and commercial agriculture communities, mycorrhizae have been a topic of discussion for quite some time.  There are many questions surrounding the relationship between mycorrhizae and plant nutrient uptake.  Before addressing these questions, an understanding of what it is and how it operates will be beneficial.

Mycorrhizae are a fungus that grows and reproduces in the soil from spores that colonize the roots of plants, forming a mycelium network around and within the root cells.  It has been referred to as, the “hidden half” of plant life, and often overlooked aspect of the ecosystem.  These relationships between fungi and plant roots play a fundamental role in nutrient uptake, plant health, and ecosystem stability.  Who wouldn’t want more present in their soils? 

There are different types of mycorrhizae.  Rather than getting into the scientific names of each, they can be distinguished by two types.  The first type is ectomycorrhizae.  These form a dense sheath around the roots and grow between the cells of the root’s outer layer.  The second type is endomycorrhizae.  These fungi penetrate the root cells, forming highly branched structures called arbuscules.

“Instead of competing with other soil heterotrophs for decaying organic matter, the mycorrhizal fungi obtain sugars directly from the plant’s root cells.  This represents an energy cost to the plant, which may lose as much as 5 to 30% of its total photosynthate production to its mycorrhizal fungal symbiont.  In return, plants receive some extremely valuable benefits from the fungi.  The fungal hyphae grow out into the soil some 5 to 15 cm from the infected root, reaching farther and into smaller pores than could the plant’s own root hairs.  This extension of the plant root system increases its efficiency, providing perhaps 10 times as much absorptive surface as the root system of an uninfected plant.” (Brady and Weil et al. 2016)

Mycorrhizae assists in several critical roles in the health and growth of plants.  In addition to nutrient exchange they can aid in water absorption, acting as a sponge and increasing the root surface area.  Disease resistance by competition with other fungi for space and nutrients, producing antibiotics and altering the root epidermis.  They can even regulate the amount of heavy metals or salts that are taken into the plant. 

There is no doubt that mycorrhizae have a key role in the overall health, productivity and relationship between soil and plants.  This is why many want to increase the amount present in the soil.  There are already several strands of this native, beneficial fungi in most soils.  These native species will have a much better relationship with the vegetation planted in these soils.  There are many commercial mycorrhizae packages that can be purchased and added to medium, but it has been found ineffective and unnecessary.  “In controlled studies, when mycorrhizae products were added to the soil, follow up results found no trace of the actual mycorrhizal species that was introduced.” (Kunz et al. 2022) It is best to let the native species colonize the plants and not waste resources on commercial mycorrhizae applications. 

Brady, N. C., & Weil, R. R. (2016).  The Nature and Properties of Soils (13^th ed.). Pearson.

Colorado State University Extension. (2022, July 8). Mycorrhizae: Worth the Investment?

https://arapahoe.extension.colostate.edu/2022/07/08/mycorrhizae-worth-the-investment/