Co-inoculation with Azospirillum and Bradyrhizobium modifies yield components and phenological timing in soybean (Glycine max L.)

Soybean productivity in tropical environments is often constrained by nutrient limitation. This study evaluates how microbial inoculation modifies soybean yield formation under field conditions in the Philippines, with emphasis on phenology, root development, and yield components. Treatments included no inoculation (control), inoculation with Azospirillum spp., Bradyrhizobium spp., their combined application, and mineral nitrogen (N) treatment. Grain yield differed significantly among treatments, with co-inoculation producing the highest yield (2.52 t·ha−1) compared with the control (1.57 t·ha−1) and mineral N fertilization (2.23 t·ha−1). Yield increases under co-inoculation were associated with earlier flowering (36 d) and greater root length (13.01 cm), rather than increased vegetative biomass. Reproductive development was strongly altered, with pod number increased to 316 pods·plant−1, while seeds per pod declined from 1.64 to 0.62. Despite reduced seed number, individual seed size increased markedly, with hundred-seed weight rising from 8.03 to 16.04 g. Correlation analysis showed that grain yield was positively associated with seed weight per plant (r = 0.94), hundred-seed weight (r = 0.91), pod number (r = 0.88), and root length (r = 0.87–0.98), while seed number traits were negatively related to seed size. Overall, microbial co-inoculation enhanced soybean productivity in tropical low-input systems by shaping yield components and phenological development, offering a viable alternative to mineral N fertilization.