Biofertilizers are a large part of the global agricultural economy. Recently, there has been an increase in the number of companies producing fungal inoculants. Whether these inoculants are useful is not clear; they are difficult to monitor in the field. The unintended consequences of inoculants in natural systems is not known, but if invasive, they may pose a threat to soil and plant biodiversity and ecosystem functioning. A plain language summary is available for this article.