In my blog from July 26, 2021, I discussed ‘The Hidden World Beneath Your Feet,’ a look at some of the microscopic inhabitants of the soil that influence soil health and crop productivity. In this article, we will take a closer look at mycorrhizal fungi to understand what they are and why they might be important to crop productivity.

Are your crops networking? I am not suggesting that your soybean plants should attend a cocktail party or join a popular online networking app. Instead, I am referring to the presence of mycorrhizal fungi in the soil and the association or ‘network’ that plant roots form with these fungi. Chances are that you may have heard the term ‘mycorrhizal’ at some point in recent years, but do you understand what this means or how mycorrhizal fungi are different from other common fungi?

Fungi are present in almost every environment on this planet and are a diverse group. Some, like mushrooms, are easily visible with their above-ground fruiting structures, while others, like yeasts, are important to society for the roles that they play in creating bread, alcohol, and other industrial compounds. Within the soil, a special group of fungi are known as mycorrhizae: ‘myco’ refers to fungi and ‘rrhizae’ is a reference to roots. In other words, a fungus that associates with plant roots!

Types of mycorrhizal fungi

There are two classes of mycorrhizal fungi: ectomycorrhizal and endomycorrhizal. ‘Ecto-’ and ‘Endo-’ simply refer to whether the fungus invades the cell walls of a plant’s roots. Ectomycorrhizae will grow between root cells but not invade them, while endomycorrhizae invade the cell walls. Generally, ectomycorrhizal fungi are more commonly associated with hardwood trees and conifers, while endomycorrhizal fungi are preferred by herbaceous perennial plants and annual crops.

Why are mycorrhizal fungi important?

Although farmers and agronomists strive to promote healthy and extensive root systems on crops like soybean and corn, there is a limitation to how extensive a plant’s root system will be. The size and architecture of a root system is determined by the plant’s genetics and physiology, natural soil properties like depth to bedrock, or management induced soil characteristics like compaction. It has been estimated that a plant’s root system only explores approximately 1 percent of the soil volume that it could potentially access. If you consider that many essential plant nutrients are taken up by the processes of root interception or diffusion that require or favor roots being in immediate or close contact to the location of the nutrient in the soil, the size and surface area of the root system has a huge impact on nutrient uptake and crop yield. Endomycorrhizal fungi, which are also known as arbuscular mycorrhizae (arbuscular meaning tree like), have extensive branching, and through their association with the plant’s root system increase the volume of soil explored and increase plant nutrient uptake.

In exchange for retrieving nutrients that are tightly bound in the soil and delivering them to the plant, mycorrhizal fungi receive carbohydrates and other beneficial exudates from the plant’s roots. This last point is an important one because it has been suggested that mycorrhizal fungi may be the largest reservoir for carbon storage in the soil. In other words, efforts to promote soil health and carbon sequestration will likely rely on healthy populations of mycorrhizal fungi.

Tips for managing mycorrhizal fungi

These fungi do exist naturally in soil and there are also commercial sources of mycorrhizal inoculum to apply to soils. When considering commercial products, it is important to inquire about the shelf-life and viability of the fungal spores contained within the product, as well as the applicability of the species or strains of mycorrhizal fungi to the local crops and environment. Also, keep in mind that crop inputs and management impact the success and importance of mycorrhizal fungi in the soil. For example, although tillage does not eliminate mycorrhizal fungi, it does break up the hyphal structure of soil fungi. Fungi are generally slower growing compared to bacterial organisms and will need time to grow and repair damaged hyphae. As another example, mycorrhizae are commonly promoted as increasing phosphorus uptake for crops. While that is a true statement, it is important to understand that phosphorus soil test levels affect whether a plant will establish a relationship with mycorrhizal fungi for the purpose of accessing phosphorus. If soil test levels for phosphorus are high, then the plant may not have a need to expend valuable carbohydrate exudates on its mycorrhizal companions.

Closing thoughts

I began this series in July with a brief overview of ‘The Hidden World Beneath Your Feet,’ where we took a look at soil microbes like bacteria and fungi. While we can’t see or easily understand how this secret society functions on our behalf, we do know that soil microbes play important roles in cycling soil carbon and nitrogen, cause or prevent plant diseases, and ultimately influence crop productivity and quality. I will finish up this series next month with my thoughts on how biological organisms will be combined with other crop nutrient management practices in the future.