Soybean Response to Seed Inoculation with Bradyrhizobium japonicum and/or Nitrogen Fertilization

Seed inoculation with symbiotic bacteria is a commonly employed practice in soybean cultivation. As a result, nodulation proceeds properly and plants self-supply atmospheric nitrogen, requiring either minimal or no additional nitrogen fertilization. The aim of the study was to investigate the response of soybeans to the application of the recommended or double dose of commercial inoculants (HiStick ® Soy or TURBOSOY ® ) and/or mineral nitrogen fertilization compared to the untreated control. It was demonstrated that a double dose of the tested preparations had the most favorable effect on nodulation. However, the impact of weather conditions modified their effectiveness during the study years, which was especially visible in 2022. Sowing seeds without inoculation (control) resulted in the formation of sparse root nodules and consequently the lowest leaf area index (LAI) and soil plant analysis development (SPAD) measurements. In addition, the values of SPAD and LAI indices varied across the years of the study, indicating that weather conditions modified nitrogen uptake by plants. Overall, seed inoculation and/or nitrogen fertilization positively influenced the chemical composition of seeds compared to the control. The only decrease observed was in the oil content, while the double dose of HiStick ® Soy preparation reduced the polyphenol content. The double dose of the tested inoculants had the most favorable impact on yield components and seed yield. However, applying inoculation at the recommended dose or in combination with nitrogen fertilization yielded similar or slightly worse results, depending on the year. Therefore, soybean seed inoculation should be recommended, although the effectiveness of the procedure will depend on various factors, including the type of inoculant, dosage, nitrogen fertilization, and weather conditions.