Is mycorrhizal inoculant necessary in our garden soil?

I love going to Garden Writers meetings, I get to see other garden writers, editors and photographers, listen to the latest and greatest in the garden writer’s world. I am, however, ambivalent about receiving goodie bags filled with garden product. The organizers of the GWA meeting emphasized that in exchange for getting these “gifts,” we are obligated to write about them. Some of these products are great, but others…not so much.

Make Tree & Shrub mycorrhizal inoculant

Mycorrhizal inoculants

I focus my attention on soil-related product (of course!). One of the products being pushed aggressively this year is mycorrhizal inoculant. I received a tub of Myke Tree & Shrub growth supplement as well as a book explaining how mycorrhiza work and their use in horticulture.

Myke Tree & Shrub and Mycorrhizal book

I’m not surprised. Although mycorrhizae studies have verified the beneficial affects, the use of mycorrhizal inoculant in the landscape has not fared so well.

 

Beneficial effects of mycorrhiza/plant association

Nutrient and water exchange between plants and fungi

Increases soil aggregates (a good thing, it improves soil structure)

Enhances organic matter decomposition

Acidifies root zone (which facilitates uptake of nutrients)

 

Use of mycorrhizal inoculant in the urban landscape

After reviewing the latest scientific research on the use mycorrhizal inoculant in urban landscapes Dr. Linda Chalker-Scott concluded that

 

“…applying commercial mycorrhizal amendments is generally ineffective and unnecessary, give the widespread presence of indigenous inoculum.”

 

In other words, your soil already has mycorrhizae. Why waste your money and time adding something that is already there?

 

Dr Chalker-Scott also noted that generally “plant species inoculated with commercial products and installed into the landscape are equal in performance to uninoculated controls (which quickly became colonized with native fungi).”

 

It all fairness, adding mycorrhizae inoculant to sterilized potting soil is effective, as long as the containers are not overwatered and overfertilized.

 

The argument has come up some soils don’t have mycorrhizae, and the addition of mycorrhizae could help. But if the soil is impaired to the point that indigenous mycorrhizae won’t survive, mycorrhizal amendments alone won’t help.

 

So how do we keep the mycorrhizae in our soil happy?

Stop using soluble phosphate fertilizer (it stops mycorrhizal development on plant roots)

Avoid overwatering and overfertilization (increases the chances of roots being infected by these beneficial fungi)

Reduce pesticide use

Stop tilling

Increase the diversity of trees, shrubs, ground covers and perennials in our gardens

Add organic matter (it simulates the growth of native mycorrhizal populations)

References

Chalker-Scott, Linda. Mycorrhizae. What the heck are they anyway?

Soil Health website. Mycorrhizal associations

For the visual learners

Society for General Microbiology. The New Green Revolution: Arbuscular mycorrhizas


Written by Cristina da Silva
Thursday, April 23, 2015 in Book & Product Reviews

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