Mycorrhizal Inoculants: Buy Only for a Testable Gap

Useful biology is not the same as a useful bag

Arbuscular mycorrhizal fungi form relationships with many crops and can help roots acquire resources, especially phosphorus. That well-established biology is the starting point for inoculant marketing, not proof that every commercial product adds value. A product must contain viable, appropriate organisms, reach living roots, establish under the farm’s conditions, and improve an outcome enough to repay its cost.

Field soil may already contain compatible fungi. In that case, adding more propagules can have little effect. In contrast, pasteurized or soilless media, severely disturbed soil, or a propagation system that interrupts colonization may present a clearer gap. Crop identity matters too: not every plant forms the same association, and a broad multi-species label is not evidence of fit.

Commercial performance is uneven

University of California researchers testing ten commercial inoculants in nursery media observed colonization ranging from none to substantial levels and recommended preliminary testing in the actual growing mix. A Washington State University onion study likewise found product-dependent results: one treatment produced consistent colonization under the study conditions while others did not show significant effects. These controlled results caution against treating products as interchangeable.

Recent peer-reviewed assessments have raised broader quality concerns, including limited viability and contamination in sampled commercial inoculants. Those findings do not show that no product works. They show why a label claim, species list, or propagule number needs independent verification and why results from one batch, medium, or crop should not be generalized to the field.

Screen the use case and the batch

Write down the hypothesized gap. Examples include a sterile propagation medium, a disturbed site with little topsoil, or a crop grown in a low-phosphorus system where colonization has been measured as weak. High phosphorus availability can reduce the plant’s incentive to maintain the association, so adding inoculant while applying abundant phosphorus may be a poor test.

Ask for named organisms, viable propagules measured by a stated method, batch test date, expiration, storage conditions, and independent crop data. Check whether other fertilizer or biostimulant ingredients could explain a growth response. For organic production, verify the entire formulation with the certifier. Do not collect and move wild or field soil as inoculum without considering pathogens, weeds, and local restrictions.

• Match organism, host crop, and production medium.
• Request batch-specific viability evidence.
• Review fertility, especially phosphorus, before the trial.
• Separate fungal effects from fertilizers included in the product.

Test colonization and farm value

Use repeated treated and untreated trays, beds, or rows with the same seed lot, medium, fertility, and irrigation. Measure both a farm outcome—saleable transplants, establishment, or yield—and, where practical, root colonization through a qualified lab or extension collaborator. Bigger plants without greater colonization may be responding to another ingredient rather than the fungi named on the label.

Run the comparison in the setting where the product would be used, not only in an ideal potting test. Keep the untreated control even if the crop appears stressed, because the question is whether the inoculant improves the system relative to normal practice. Scale only after a repeatable response. Protecting existing soil habitat may offer better value than repeatedly purchasing organisms already present.