Plant Biostimulant Act This bill excludes plant biostimulants (i.e., a substance, micro-organism, or mixture thereof that supports a plant's natural processes independently of the biostimulant's nutrient content) from regulation under the Federal Insecticide, Fungicide, and Rodenticide Act. The bill also requires the Department of Agriculture to study the types of plant biostimulants and practices of plant biostimulant use that best achieve certain results, such as increasing organic matter content.