Biostimulant or biological? The complexity of defining, categorizing, and regulating microbial inoculants

Agronomic use of specialty products known as biostimulants to improve crop productivity is growing. Traditionally, biostimulants are defined as any substance or microorganism applied to plants to enhance nutrient use efficiency, tolerance to abiotic stress, and crop yield and/or quality. However, grouping non-living products with living microbial inoculants poses regulatory guideline challenges. Moreover, peer-reviewed literature and regulatory legislation utilize “biostimulants” while industry and farmers have recently adopted the term “biologicals,” confusing discussions related to product regulation and policy. To better understand the challenges associated with this input sector, we have focused on four critical aspects: (1) the current regulatory status, (2) terminology disparity of biostimulants and biologicals, (3) key attributes that distinguish microorganisms from non-living biostimulants, and (4) mechanism of action differences between plant growth-promoting microorganisms and plant growth regulators. Therefore, we propose that living beneficial microorganisms and non-living biostimulants be separated with distinct regulatory requirements.