Mycorrhizal fungi are some of the most important partners a plant has. They colonize roots and are closely tied to how crops access nutrients, which is why phosphorus levels come up so often in this research. So it makes sense that products promising to add these fungi to your soil sell well, with one secondary estimate putting the market for arbuscular mycorrhizal (AM) inoculants at close to a billion dollars.

The problem is not the biology. The problem is what is inside the bag. Independent researchers on several continents have now tested commercial products side by side, and the same pattern keeps appearing: many products contain few or no viable fungi, and field benefits are inconsistent.

This post walks through the key study, Salomon et al. 2022, the reviews that followed, when inoculation is most likely to work, and how to test a product before you spend real money on it.

Key takeaways

The study at the center: Salomon et al. 2022

The most detailed independent test to date is "Global evaluation of commercial arbuscular mycorrhizal inoculants under greenhouse and field conditions", published in Applied Soil Ecology in 2022. The team bought and tested 28 commercial AM inoculants from three continents.

What they found

Test condition Result
Sterilized soil, colonization 84% of inoculants led to no mycorrhizal root colonization at all
Sterilized soil, plant biomass Biomass increased with 5 of 25 products
Non-sterilized soil, colonization No increase in colonization over uninoculated controls
Non-sterilized soil, plant biomass Biomass increased in only 1 of 25 treatments
Lab-cultured reference (Rhizophagus irregularis) 48–79% root colonization
Field trials Outcomes varied and depended on shifts in the existing fungal community

Why the design matters

Two details make these results hard to dismiss.

First, the sterilized-soil test is the easiest possible case for a product. With no native fungi competing, any living inoculum should colonize roots. When most products failed even there, the most likely explanation is that the fungi in them were not viable, or were not present in useful amounts.

Second, the reference fungus worked. A lab-cultured Rhizophagus irregularis reached 48–79% root colonization under the same conditions. That shows the plants and methods could support colonization; the commercial products were the weak link.

The authors called for standardized quality control across the industry. A related paper in iScience proposed a quality management framework for commercial AM inoculants in July 2022.

What the wider evidence says

Salomon 2022 is not a lone result. Several later papers look at the same question from different angles.

Meta-analysis: "commercial inoculants fall short"

Koziol, McKenna and Bever published a meta-analysis in New Phytologist (online November 2024) titled "Meta-analysis reveals globally sourced commercial mycorrhizal inoculants fall short". A secondary summary on the MycoStories blog reports that it analyzed 302 trials and found fewer than 12% of commercial products showed both a growth benefit and viable colonization, while lab-produced inoculants gave positive results in 63% of trials. We have not checked these figures against the paper itself, so treat them as reported rather than confirmed. The direction, stated in the paper's own title, matches Salomon 2022.

Product assessments from 2024 and 2025

The researchers are not anti-inoculant

It would be wrong to read this research as proof that mycorrhizal fungi don't matter. Salomon and colleagues followed up in 2025 with a New Phytologist paper on realising the promise of AM fungal biofertilisers through more applied research. The message is that the fungi have real potential, but the products and the field evidence need to catch up.

Why regulation hasn't solved this

In the United States, the Plant Biostimulant Act of 2025 (H.R.3783, with a Senate companion, S.1907) would amend federal pesticide law so biostimulants are not regulated as "plant regulators" and would require a USDA study on uniform definitions and labeling. As of September 2026, we could not confirm the bills had passed, so check Congress.gov for current status.

The European Union's Fertilising Products Regulation reportedly allows only a few microbial genera, mycorrhizal fungi among them, in CE-marked biostimulant products, and defines those products by their claimed function. Neither system, as described in our research, guarantees that the fungi in a given bag are alive when you open it.

When inoculation is most likely to help

Independent tests suggest inoculation works best under specific conditions, with products shown to be viable.

More likely to be worth testing:

Less likely to pay off:

A buyer's checklist for mycorrhizal products

Before you buy, ask the supplier for answers in writing:

Test it yourself

What to do next

  1. Protect the fungi you already have. Field results in Salomon 2022 depended on the existing fungal community in the soil.
  2. Save inoculants for where they make sense: sterilized media, disturbed ground and low-phosphorus conditions.
  3. Demand viability data before buying, and walk away from products that can't provide it.
  4. Run a small side-by-side trial before treating a whole field.
  5. Check organic status with your certifier.
  6. Share your results with your extension office. Farmer trial data helps everyone.

Sources