LAWRENCE — A burgeoning billion-dollar industry woos farmers and gardeners with promises of achieving better, more environmentally friendly harvests through symbiotic fungi that bond with plant roots.

These fungal bonds can help plants thrive and can lock carbon that came from the atmosphere into the soil. But evidence has been piling up that shows buyers ought to eye with some skepticism the products that promise to produce them.

Now, University of Kansas scientists have combed through 250 commercial product trials detailed in peer-reviewed journals. Most of those trials checked to see if the promised fungi materialized on plant roots and helped the plants grow. And 88% of the time, the answer was no.

Problems that have cropped up in peer-reviewed studies at KU and elsewhere include:

• Some commercial products contain a pathogen that harms plants.
• Some contain undisclosed chemical fertilizer.
• Some don’t contain any spores for the beneficial fungi they’re meant to produce.
• Some contain spores that aren’t viable.

“These fungi can do awesome things,” lead author Liz Koziol said. “But not when they’re dead.”

Koziol is an assistant research professor at the Kansas Biological Survey and Center for Ecological Research, where she works with the world’s largest collection of the kind of symbiotic fungi that so many growers want in their soil. These are called arbuscular mycorrhizal fungi.

The paper in New Phytologist concluded with a plea for improving the industry. It said the U.S. “fully lacks regulations” on the quality of these products and on importing or exporting them. And it said these products could pose a risk of introducing invasive species.

“There is a pressing need for mandatory global regulation on product quality control,” the authors wrote.

Though it would cost money to enforce rules and independently evaluate products, researchers said they see significant potential for savings compared to how much money farmers and gardeners may be wasting.

Symbiotic fungi give plants vital nutrients. They also help the ground absorb water better, which improves resilience against both drought and heavy rain. They help plants cope with attacks from insects. And they protect against erosion, which is significant because U.S. farmland is losing soil faster than new soil forms.

All these benefits pique the interest of farmers and gardeners, but how can they browse the dizzying array of fungal inoculants for sale — and pick something that works?

“We need to have more transparency,” said Kirsten Hofmockel, a soil ecologist not involved in the KU research. She’s a senior scientist at the Pacific Northwest National Laboratory and recently served as president of the international Soil Ecology Society.

“There’s not a lot that the consumer can do at this moment without that transparency,” she added.

The microbiome ‘moment’

Eco-friendly soil products are a booming market for a reason.

“In a lot of ways, the microbiome is coming of age,” Hofmockel said, referring to microscopic life in the ground. “Soil health is having its moment.”

Scientific understanding of what makes soil productive has expanded greatly in recent decades. It’s amply clear that microbes play key roles. This has growers eager to explore beneficial bacteria and fungi. Many are seeking an alternative to chemical fertilizers.

“There’s a lot of legitimate concern about synthetic fertilizers,” Hofmockel said. “A lot of frustration about the cost and the environmental effects.”

The U.S.’ heavy reliance on these fertilizers since World War II boosted yields but exacerbated greenhouse emissions, polluted groundwater and surface water and fed a dead zone in the Gulf of Mexico.

Today, farmers and gardeners worldwide buy billions of dollars in microbe products each year, according to business analysts. They have linked this fast-growing market to the rising interest in organic and eco-friendly methods. Symbiotic fungi alone account for about $1 billion annually.

KU scientists published two studies this fall that focus on arbuscular mycorrhizal fungi (also called endomycorrhizal fungi on product labels).

One is their analysis of trials conducted by other scientists internationally. The other one, published in Applied Soil Ecology, laid out the results of a product study at KU.

For her research, Koziol bought 16 products on Amazon that were top sellers at the time of purchase. She also picked up two products stocked by one of the country’s biggest retailers of home improvement supplies. And she chose two from sellers that are building a niche market by targeting their soil health products to cannabis growers.

The study involved scouring the products for fungal spores, as well as testing the products in pots containing soil and seedlings for vegetables and grains.

Some of the packages didn’t contain spores. Some did but still failed to produce mycorrhizae.

On the whole, the study found commercial products bonded with roots at much lower rates than when academic scientists used fungi that they grow in-house.

Koziol said a product could underperform for a variety of reasons. Some companies may inadvertently expose their products to extreme temperatures during transport or warehouse storage, for example. Some may mix in other ingredients in ways that can harm the fungi.

Sellers generally don’t disclose their methods in detail, she said, but she wants the problems to get identified and resolved so consumers can tap into the potential of beneficial fungi.

One step that would help, she said, is for more companies to test whether their products are still good after they reach consumers.

“I don’t think that’s necessarily being done,” she said, “because as the end user of these products, they weren’t viable.”

In her papers, Koziol disclosed a potential conflict of interest. She runs a private business selling a mycorrhizal inoculant. The other authors on KU’s studies have not reported any potential conflicts of interest.

Undisclosed product ingredients

Some of the commercial products tested at KU boosted plant growth without producing mycorrhizae. This suggests that other ingredients in those products did the heavy lifting.

Sometimes labels disclose those ingredients, sometimes they don’t.

In 2022, scientists at Oklahoma State University tested six commercial products and found two contained undisclosed chemical fertilizer.

Soledad Benitez Ponce, an Ohio State University plant pathologist not involved in the studies at KU or Oklahoma State, said there are three reasons why undisclosed synthetic fertilizer could cause problems:

• If that’s the real reason a product works, is it worth the price tag? “Maybe you’re paying more than what you need for a phosphorus fertilizer,” said Benitez Ponce, whose lab works with beneficial fungi and bacteria.
• Certified organic farmers might mistakenly think that the product complies with the strict rules that govern their work. That “could compromise the certification of the whole operation,” she said, “and that is a costly process that takes a lot of time.”
• Synthetic fertilizer interrupts the mutual benefits between arbuscular mycorrhizal fungi and plants, so including it in an inoculant “defeats the whole purpose of the product.”

The Oklahoma State researchers tried the six products on prairie grasses and wildflowers and found “no evidence of benefit.” Some of the products caused these plants to grow less while boosting growth of invasive plant species instead.

What options do buyers have?

Benitez Ponce said researchers have been concerned for years about the quality of soil microbe products, how to regulate these and how to help consumers pick among them.

She recommended growers contact nearby university extension agents to see if they’ve tested any products locally. Fungi are living organisms, she said, so they may perform differently according to region, and local results are particularly relevant.

Also, she said, scrutinize labels. Look for as many details as possible, such as the expiration date and specifics about application and storage. A thorough label could be useful.

Still, product labels aren’t always accurate.

The problems go beyond containing undisclosed fertilizer or unviable spores. One study last year found the fungal species in some products didn’t match those listed on the package. In a 2007 study, scientists got results with several products only when they applied 5 or 10 times as much as the manufacturers suggested.

Hofmockel, at the Pacific Northwest National Laboratory, said it might be possible to make rapid spore-staining tests widely available so consumers could at least check the inoculants they purchase to see if they contain viable spores.

She also said farmers and gardeners can improve the conditions for fungi that already live in their soil. Reducing tilling and keeping roots in the ground year-round are two tried and true methods that would help, she said.

These approaches have the advantage of promoting native species, Hofmockel said. The effects of introducing fungi from elsewhere remain uncertain.

What do businesses say?

The Kansas News Service reached out to 11 of the manufacturers whose products were tested by KU.

Two of them replied. One is Groundwork BioAg, which says it is the world’s biggest producer of mycorrhizal inoculants.

The KU paper said one of the company’s products, Dynomyco, produced mycorrhizae in 1 out of 5 pots. It also noted that a pathogen called Olpidium, which can attack plants and host viruses that do the same, repeatedly cropped up on plants grown with Dynomyco.

“We take such studies very seriously,” Dan Grotsky, cofounder of Groundwork BioAg and general manager of Dynomyco, wrote in an email while questioning KU’s methods. “Groundwork BioAg tests its products regularly, using several types of standard tests, and has never received results like these, even on old or shipped and returned products, which we test as well.”

Those tests also include checking Dynomyco for pathogens including Olpidium, he said, and none have turned up.

Grotsky suggested that because KU’s study found Olpidium in products from five companies, the contamination could have happened at KU’s lab. He said his company “would welcome a transparent discussion with full access to the dataset and methods.”

KU is not alone in finding a pattern of Olpidium contamination in commercial products. Its scientists combed through 67 other peer-reviewed trials that tested commercial products for Olpidium. Eleven of those trials found the pathogen.

Koziol said KU’s methods for working with mycorrhizal fungi are used widely in this field of research.

“As curator of the world’s largest AM (arbuscular mycorrhizal) fungi collection, I train others in these techniques, including commercial inoculant producers,” Koziol said. “The fungal assessments we used were appropriate and published in highly regarded, peer-reviewed journals.”

To Benitez Ponce, the plant pathologist at Ohio State, the discovery of Olpidium in multiple products raises the question of whether something in the supply chain could explain it, such as some manufacturers unknowingly receiving an infected ingredient from a single supplier.

“The challenge is that we don’t know where the Olpidium is coming from,” she said.

The other company that replied to the Kansas News Service was MicraCulture, a small company run by Sarah Pellkofer in Seattle. Pellkofer wrote her doctoral dissertation at the University of Zurich on soil biodiversity, and she said she was pleased to see KU’s study.

Her product, Plant Probiotics, was free of the Olpidium pathogen in KU’s trial.

The product didn’t produce mycorrhizae in the trial. But that didn’t alarm Pellkofer because these fungi are just one small part of her microbe mix.

“Our recipe does not only consist of (these fungi), but a suite of microbes” including bacteria, she said in an email. This is to “boost the soil ecosystem as a whole,” because plants benefit from soil biodiversity.

The KU results suggested other microbes in Plant Probiotics helped plants grow, even though the fungal spores didn’t.

Pellkofer said her product instructions recommend that people reapply the mix multiple times during a plant’s growing cycle to increase the chance that the spores will grow and bond with the roots.

She also noted that KU scientists found exactly as many fungal spores in her product as printed on the label, which made it the outlier among the products studied. Pellkofer said this reflects that her company has “gone out of our way” to ensure accuracy.

“I welcome regulation in our field,” she said. “I’ve seen the market flooded with products that have lots of claims that maybe do not go through the scientific testing to back them, and as we see in this study, often do not contain the microbes claimed.”

A snapshot of the KU trial results

KU scientists tested fungi that they grow in-house. They also tested soil from a nearby organic farm that contains fungi. When added to plants, these two sources of fungi produced mycorrhizae 72% of the time.

Tested commercial products produced mycorrhizae 12% of the time.

Products that didn’t produce mycorrhizae when KU scientists tried them out:

• Great White Premium Mycorrhizae
• King of Mycorrhizae
• Plant Probiotics
• Root Naturally Endo Mycorrhizae
• Promix Organics
• Wildroot Organic
• New Life Agriculture Microbial Solutions

These produced mycorrhizae in 1 out of 5 pots:

• Big Foot
• Dynomyco
• Green Eden
• Happy Frog
• Mikrobs
• Myco Bliss
• Xtreme Gardening

Root Magic produced mycorrhizae in 2 out of 5 pots.

The pathogen Olpidium was a problem when KU scientists used these products:

• Wildroot Organic
• Bigfoot
• Xtreme Gardening Mykos
• Dynomyco
• Root Magic
• Kansas News Service ksnewsservice.org.