Climate variability and widespread synthetic agrochemical use have increased interest in biostimulants (BS) that enhance plant growth, stress tolerance, and yield by stimulating natural plant processes. A two-site field study, conducted under no-till and tilled systems, evaluated the effects of the foliar biostimulant “Source” on soybean growth and yield at three phosphorus (P) rates (0%, 50%, and 100% of soil test recommendations) because of its potential to replace phosphorus inputs. A complementary greenhouse study was conducted to evaluate the effect of the biostimulant on different soybean hybrids. Measured at various dates after planting (DAP), leaf relative water content (LRWC) and normalized difference vegetation index (NDVI) mostly showed insignificant responses to P treatments, but significant responses to BS. Grain yield increased significantly with individual BS and P applications in both tillage systems. Under no-till conditions, BS increased yield by 13.0% (3.05 vs. 2.70 Mg ha−1), and P100 increased yield by 13.5% (3.0 vs. 2.65 Mg ha−1). Under tilled conditions, BS and P100 increased yield by 19.6% (2.75 vs. 2.30 Mg ha−1) and 19.2% (2.72 vs. 2.28 Mg ha−1), respectively, compared with the control. Yield gains were primarily driven by increased pod density and grain number. Greenhouse experiments supported these trends, with BS-treated plants producing more grains per plant (187.6 vs. 171.3) and higher yield per plant (28.8 vs. 25.7 g). Results indicated that biostimulant application improved physiological performance and increased soybean yields, comparable to full-rate phosphorus, highlighting its potential as a sustainable approach under increasing environmental and input-related challenges.
Thapa et al. (Tue,) studied this question.