Optimizing Crop Spatial Arrangement Improves Yield and Yield Stability of White Mustard (Sinapis alba L.) Under Dry Steppe Conditions

White mustard (Sinapis alba L.) is a valuable crop for dryland agroecosystems, yet its productivity is strongly influenced by crop spatial configuration and hydrothermal variability, particularly under dry steppe conditions. Nevertheless, systematic evidence for optimizing cultivation parameters for white mustard in the dry steppe of northern Kazakhstan remains limited, while substantial interannual yield fluctuations continue to challenge production stability. This study evaluated the effects of seeding rate and sowing method on crop growth, biomass accumulation, and yield formation on southern carbonate chernozem soils. Field experiments were conducted during 2023–2025 using a two-factor design with three seeding rates (1.5, 2.5, and 3.5 million viable seeds ha−1) and two sowing methods (25 cm row spacing and 50 cm wide-row spacing). Seed yield was significantly affected by seeding rate, with 2.5 million viable seeds ha−1 producing the highest overall mean yield. Both lower and higher seeding rates were associated with reduced productivity, particularly under moisture-limited conditions. Sowing method also influenced yield, with row sowing (25 cm) generally producing higher yields than wide-row sowing. Yield declined progressively from 2023 to 2025, reflecting substantial interannual environmental variability, while the significant interaction (p < 0.05) among sowing method, seeding rate, and year indicated that treatment responses varied across growing seasons. Spearman’s correlation analysis indicated that seed yield was more closely associated with plant height at flowering and pod formation than with early-stage biomass accumulation. Although biomass variables were strongly interrelated across growth stages, their direct association with seed yield was limited, indicating that reproductive-stage plant development was more closely associated with yield variation under moisture-limited conditions. The combination of 2.5 million viable seeds ha−1 and 25 cm row spacing produced the highest mean yield (558 kg ha−1), representing a 51.2% increase in yield and a 476.9% increase in the stability index compared with the lowest-yielding treatment (1.5 million viable seeds ha−1 with 50 cm wide-row spacing). This study provides field-based evidence of the combined effects of seeding rate and sowing method under contrasting hydrothermal conditions and identifies 2.5 million viable seeds ha−1 combined with 25 cm row spacing as the most favorable crop spatial configuration for improving white mustard productivity under the dry steppe conditions in Northern Kazakhstan.