Background and Aims <p>The critical nitrogen (N) dilution curve, a key diagnostic tool for cereal crops, remains contentious for soybeans due to their dual N sources: soil uptake and biological fixation. Existing soybean N dilution curves conflate these sources, masking the true "critical" N dilution. This study aimed to: i) review existing knowledge on N dilution curves for soybeans; ii) establish a critical N dilution curve using non-nodulating genotypes; and iii) investigate physiological mechanisms governing soybean N demand.</p> Methods <p>A literature review compiled data on soybean N dilution curves. A field experiment was conducted using four soybean genotypes (two nodulating and two non-nodulating) under four fertilizer N rates (0, 100, 200, and 300&#xa0;kg N ha⁻¹). Shoot biomass, N concentration, and organ-specific N allocation (leaves, stems, pods) were measured across five growth stages. Statistical analyses included Bayesian hierarchical modeling for curve fitting and power models to evaluate stoichiometric relationships.</p> Results <p>Non-nodulating genotypes exhibited 38% greater N dilution than nodulating ones. Nodulating genotypes showed higher specific leaf area (17%) and specific leaf nitrogen (15%)&#xa0;compared to the non-nodulating ones. Soil-derived N followed typical dilution with biomass, while atmospheric-N increased with growth. Pod N concentration remained stable, with nodulating genotypes maintaining higher values (4% vs. 2.5%)&#xa0;relative to the non-nodulating ones.</p> Conclusion <p>A critical N dilution curve in soybeans was established using non-nodulating genotypes, revealing greater dilution than previously reported in the literature for nodulating soybeans. Nodulating plants optimized N storage across organs (leaves, stems, pods), supporting light interception and seed demand.</p>