Foliar Application of a New Biostimulant at Key Growth Stages Improves Soybean Performance

Soybean is one of the most important crops worldwide, but its productivity is frequently challenged by abiotic stresses such as drought and heat, which impair physiological and metabolic processes. Biostimulants have emerged as sustainable tools to improve plant performance under adverse conditions. This study evaluated the effects of foliar application of a new biostimulant, “SB”, on soybean photosynthetic efficiency, antioxidant metabolism, biometric traits, and grain yield. SB was applied at different doses (0.5, 1.0, 1.5, and 2.0 L ha−1) at the V4 and R1 growth stages during two seasons (2023/2024 and 2024/2025). Foliar SB application enhanced soybean leaf chlorophyll levels, RuBisCO activity, and gas exchange parameters, resulting in higher photosynthetic rates, carboxylation efficiency, and water use efficiency. In addition, foliar SB application reduced hydrogen peroxide and malondialdehyde accumulation, indicating lower oxidative damage and improved redox balance. These physiological and metabolic improvements contributed to greater root development and plant height and significant increases in yield components. Grain yield was consistently improved by all SB application rates, but the 1.5 L ha−1 dose produced the most stable and positive effects across both seasons, with an average increase of more than 500 kg ha−1 compared to the control. Overall, foliar SB application proved to be an efficient and promising management strategy to enhance soybean resilience and productivity under variable climatic conditions.