Abstract

Adequate fertilization is essential for optimizing soybean productivity and seed quality, while supplementary fertilization plays a key role in correcting nutrient deficiencies and supporting plant performance under varying environmental conditions. This study evaluated the effects of the foliar application of macro- and micronutrients enriched with free amino acids and Ascophyllum nodosum extract applied at two phenological stages—six fully developed trifoliate leaves (V6) and beginning flowering (R1)—on soybean grain yield, protein content and oil content. Experiments were conducted over two growing seasons at ARDS Turda, Romania, using a randomized complete block design with three replications to compare three fertilization treatments: basic mineral fertilization (control), mineral fertilization supplemented with foliar application at the V6 vegetative stage, and mineral fertilization supplemented with foliar application at the R1 reproductive stage. Basic fertilization was performed before sowing using granulated nitroclacium at a rate of 100 kg ha−1. Foliar fertilization included Naturamin WSP applied at a rate of 0.5 kg ha−1 and Pleniflor and Naturfruit at a rate of 2 L ha−1. Foliar fertilization at the V6 stage significantly improved grain yield in most cultivars, with increases ranging from 1% to 18%, particularly in the 000 and 00 maturity groups (MGs). Across the maturity groups, the highest average yields were recorded in MG 0 cultivars (up to 2845 kg ha−1). In contrast, foliar application at the R1 stage was more effective in increasing seed protein content, with improvements of up to 11% in early maturity cultivars. Oil content showed only minor and inconsistent responses, with maximum increases of 4% depending on genotype. Linear Discriminant Analysis (LDA) revealed that environmental conditions and genotype explained most of the observed variation (Axis 1 = 89.91%, Axis 2 = 7.79%), indicating that cultivar response to foliar fertilization was strongly influenced by genotype × environment interactions. The effectiveness of foliar fertilization depends on application timing, cultivar maturity group and environmental conditions.

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