Soybean [Glycine max (L.) Merrill] Growth and Yield Responses to Cultivar and Nitrogen Fertilization

The aim of the study was to determine the effect of cultivar and nitrogen fertilization on the morphological and physiological traits and yield of soybean (Glycine max (L.) Merrill) grown in central-eastern Poland. In a strict, two-factor field experiment, four soybean cultivars were used: ‘Abelina’, ‘Malaga’, ‘Coraline’, and ‘Petrina’, and three nitrogen rates: 0, 30, and 60 kg N ha⁻¹. The moderate rate (N30) was applied before sowing, while the higher rate (N60) was divided into two parts, with 50% applied before sowing and 50% top-dressed at BBCH 61. The studies were conducted during two growing seasons. It was shown that both the cultivar and nitrogen fertilization significantly affected plant height, leaf area index (LAI), leaf greenness index (SPAD), and chlorophyll fluorescence indices (Fv/Fm, PI). The interaction among cultivar, fertilization, and years was significant for SPAD and Fv/Fm index, indicating a strong influence of environmental factors on plant response. Nitrogen fertilization increased plant height and chlorophyll content but reduced fluorescence indices. Among the cultivars studied, the late-season cultivar ‘Malaga’ was characterized by the highest SPAD index (502), Fv/Fm (0.800), and PI values (4.3), and achieved the highest seed yield (5.06 t ha⁻¹) and thousand-seed weight (230 g). In contrast, the medium-season cultivar ‘Abelina’ showed the lowest SPAD (454), and significantly lower Fv/Fm and PI values (0.790 and 3.51, respectively), resulting in the lowest yield (4.25 t ha⁻¹) and TSW (169.7 g). The application of a moderate rate of nitrogen (N30) improved the physiological indicators of plants and elements of yield structure without reducing the potential photochemical efficiency of PSII, while a higher rate (N60) did not result in a significant increase in yield, despite a greater number of pods and seeds per plant, which may have been due to a reduction in thousand-seed weight. The results highlight the importance of cultivar selection and moderate N fertilization of soybean grown in temperate climates and indicate the need for further research on the physiological mechanisms that determine cultivar-specific nitrogen use efficiency and yield stability under environmental stress.