Proper efficacy testing should be mandatory for registration of biostimulants

Biostimulants are products that stimulate plant growth independently of the product's nutrient content. Efficacy testing is crucial given that many commercial products on the market have been shown to be ineffective. In the European Union, biostimulants fall under the Fertilizer Products Regulation, which stipulates that “the plant biostimulant shall have the effects that are claimed on the label for the plants specified therein.” We suggest that this definition of product efficacy is inadequate, and should be expanded to include both the mode of action and the quality of the product. The main types of biostimulants are non-microbial and microbial biostimulants, which we describe briefly. We then evaluate the European Committee for Standardisation (CEN) EN 17700-1 Standard that provides guidance on efficacy testing. This CEN Standard permits claims without field trials, lacks mandatory controls (e.g., sterile-carrier comparisons), and allows extrapolation from a limited set of specific crops to a large and diverse group of crops. Consequently, products can reach the market that are often ineffective, as demonstrated by numerous studies showing poor colonisation by microbial biostimulants, small and variable yield responses, and occasional phytotoxicity. We recommend the CEN standard be strengthened, incorporating lessons from the European and Mediterranean Plant Protection Organisation, FAO, and recent efficacy-testing proposals, and reconsidering CEN's role in standard-setting to ensure reliable, evidence-based products for farmers.