Mycorrhizae (2024)

Should growers inoculate soil with mycorrhizal fungi when planting liners - Yes? or No?

Mycorrhizal fungal filaments in the soil act as extensions of a root system-- and are more effective in nutrient and water absorption than the roots themselves! More than 90 percent of plant species in natural areas form a symbiotic relationship with their beneficial mycorrhizal fungi. Aside from enhancing plant efficiency in absorbing water and nutrients, mycorrhizal interaction with root systems reduces fertilization and irrigation requirements.

“Mycor” – “rhiza” literally means “fungus” – “root” and defines the mutually beneficial relationship between the plant and root fungus.

These specialized fungi colonize plant roots and extend far into the soil. Mycorrhizal fungal filaments in the soil are truly extensions of root systems and are more effective in nutrient and water absorption than the roots themselves. More than 90 percent of plant species in natural areas form a symbiotic relationship with the beneficial mycorrhizal fungi.

There are major classes of mycorrhizae and many types within the classes. One type grows into the root forming a root extension while another grows around the root system. Both provide benefit but the actual extension of roots provides the greatest benefit.

Commercial Mycorrhizal Inoculant Products

There are numerous mycorrhizae products available for both landscape construction and all manner of nursery production. A thorough read of the available literature suggests that this industry produces good, better, and best products with some possibly shipping mostly dead product. Much research aimed at inoculation of planting soils has produced limited positive results in some cases and none in others. Some nursery field growers move soil from adjacent wooded areas to their growing fields to inoculate the soil.

Naturally Occurring Mycorrhizae

Composted manures can contain mycorrhizal populations. We amend our soils with 120 tons of compost per acre before each planting rotation but have never sought to prove the existence of viable mycorrhizae populations.

Researchers have long known that significant plant benefits exist when mycorrhizae are present in the soil. However, at least some researchers will state that the more they know the more research is required to truly understand this phenomenal development of nature.

My research indicates that we should be cautious primarily for two reasons; first the production and delivery of live mycorrhizae products is challenging, and second, research shows that consistent, repeatable soil inoculation is even more challenging. Further, efficacy may not be obvious.

Benefits of Mycorrhizae

  • Enhanced plant efficiency in absorbing water and nutrients from the soil.
  • Reduced fertility and irrigation requirements.
  • Increased drought resistance.
  • Increased pathogen resistance/protection.
  • Enhanced plant health and vigor, and minimizing stress.
  • Enhanced seedling growth.
  • Enhanced rooting of cuttings.
  • Enhanced transplant establishment.
  • Improved phytoremediation of petroleum and heavy metal contaminated sites.

What we need to know before getting started with mycorrhizae

  • Not all plants are mycorrhizal; lists are available on the internet.
  • Most plants will grow and survive without mycorrhizae.
  • If one inoculates plants with mycorrhizae, don’t assume benefit; several other soil conditions may result in robust growth conditions.
  • From the previous fact, proof of benefit does not always translate to all production or landscape situations.
  • The expense of inoculation may exceed the benefits and one should evaluate the use of mycorrhizae just as one would evaluate any other production input.
  • Application of mycorrhizae does not guarantee inoculation which can usually be verified only by microscopic or DNA evaluation.
  • If mycorrhizal inoculation is the goal, one must prove successful inoculation occurs before continuing.
  • Once plants become mycorrhizal there is no guarantee they will remain so.
  • Benefits incurred in a production system does not guarantee transfer to the landscape.
  • Changes in environmental and cultural factors can alter mycorrhizal status in the plant.
  • Mycorrhizae success in early plant development may not continue as a plant matures.

Does this all sound a bit confusing and contradictory? The benefits and advantages cited by Davies would encourage me to jump right in. Yet the facts listed by Scagel and Lindeman cause me to proceed with extreme caution. Most of us are not in a position to prove that we have successfully inoculated the plants. Unlike soil amendments with fertilizer or compost which usually produce near-term visual proof of efficacy, it appears that we may have difficulty assuring success with mycorrhizal inoculation.

It is not my intention to discourage the use of mycorrhizae. For me, based on the facts, I would be very cautious before spending money for product and labor to achieve a potentially elusive, verifiable benefit. I would be pleased to hear from anyone who has proven successful inoculation.

Mycorrhizae Resources

  1. Dr. Fred Davies, Texas A&M University
  2. Carolyn Scagel and Robert Linderman, USDA-ARS Horticultural Crops Laboratory, Corvalis, Oregon

Mycorrhizae (2024)
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