Claims checked › Claim
Mycorrhizal inoculants drought-proof your crops
“Inoculating your soil or seed with mycorrhizal fungi makes crops drought-tolerant and protects yield in dry years.”
The mechanism is real and the evidence is from pots. The two European experiments built to test the claim, in Germany and Poland, found no yield benefit under drought. No European field trial of a bought inoculant in a drought year exists. The claim travels on product labels.
Where it comes from
Greenhouse studies in which colonised plants keep better water status and recover faster after a dry spell, and product pages that list "improved drought-stress tolerance" among the benefits.
Who benefits
Producers, distributors, an institute that develops the products, and the biostimulant industry council. In a drying Europe, water is a better story than phosphorus.
What the evidence says
The mechanism is real. Hyphae extend the root system, and pot-grown colonised plants show better water status and faster recovery. Nobody disputes the physiology.
The two European tests found no yield benefit. At Bielefeld, Germany (2022), spring wheat in pots under continuous and pulsed drought with a commercial single-species and a three-species product showed a "negative mycorrhizal growth dependency"; "fungal presence could not alleviate the effects of drought" (open access). In Poland (2019), fodder maize recovered faster from a two-week drought at silking, but the alleviation "was not sufficient to have a significant positive effect on long-term productivity"; forage yield and quality differences "were negligible" (open access).
No European field trial exists. The Greek rainfed barley trial (2015–16, 266–316 mm) was dry and reported +9%, but did not separate a drought effect. The field data with a drought design are non-European: semi-arid Jordan (2004), and Morocco (2023), where the large gains under half irrigation came from a native fifteen-species consortium plus compost, not a product (open access).
And the bag may be empty. Of 25 commercial products tested in Europe and Australia, 84% produced no colonisation in sterile soil. A fungus that is not there protects nothing. See the fungi on the label are in the bag.
What to say instead
"In pots, mycorrhizal plants handle drought better. In the two European experiments that tested yield, they did not, and no farm trial has shown it." The mycorrhizal inoculation page grades yield, drought and product quality separately.