Role of Biostimulants in Sustainable Soybean (Glycine max L.) Production: A Systematic Review

This systematic review critically evaluates and synthesizes current evidence on the efficacy of biostimulants in enhancing soybean seed yield and quality. A comprehensive literature search was conducted following the PRISMA approach using the Web of Science (WoS) database, focusing on peer-reviewed studies from 2014 to 2025 reporting on the effects of biostimulants applied alone or in combination with other agro-inputs on soybean performance. Over 500 publications were retrieved from the database, of which 72 were included in this review. Extracted data were used to calculate changes in yield (kg ha−1), percentage yield increase (%), oil content (%), and protein concentration (%). Our synthesis demonstrated that the sole application of biostimulants, including seaweed extracts, humic acids, amino acids, and beneficial microbes (Bradyrhizobium, PGPR, AMF), consistently enhanced soybean yield by 4% to 65%, while their interaction with other agro-inputs was shown to be capable of increasing yield by more than 150% under abiotic stress conditions, indicating strong synergistic effects. These improvements are mediated through various physiological mechanisms such as enhanced nutrient uptake, improved root growth, increased photosynthetic efficiency, and elevated stress tolerance. Furthermore, biostimulant application positively affects seed quality, increasing oil and protein content by 0.4–5.5% and 0.5–7.3%, respectively, by optimizing source–sink relationships and metabolic pathways. Overall, the greatest benefits are frequently observed through synergistic combinations of biostimulants with one another or with reduced rates of mineral fertilizers, highlighting a promising pathway toward sustainable crop intensification in soybean systems.