Rethinking Variable Rate Seeding: Why Can Increasing Soybean Population Decrease Yield in Nutrient-Limited Soils?

Variable Rate Seeding (VRS) in soybean (Glycine max [L.] Merr.) cultivation is a critical strategy for managing soil spatial variability, which often constrains yield. However, conventional practices that increase plant density in low-yield zones to compensate for poor fertility may unintentionally intensify intraspecific competition for already limited resources. This study addresses the need for a mechanistically sound basis for VRS prescriptions that moves beyond this counterproductive assumption. Field experiments were conducted in the Cerrado region of Brazil to evaluate different soybean population densities within management zones (MZs) delineated according to Liebig’s Law of the Minimum. This approach identified soil potassium (K) as the most growth-limiting nutrient and was used to define MZs representing distinct yield potentials. Three seeding densities were tested, Low (200,000 seeds ha−1), Medium (240,000 seeds ha−1), and High (280,000 seeds ha−1), with particular emphasis on comparing the medium and high populations in potassium-limited zones. Results revealed that, contrary to the conventional strategy, increasing the soybean population from medium to high in low-fertility (low-K) MZs significantly decreased grain yield. This yield reduction was attributed to intensified intraspecific competition, which promoted excessive vegetative growth (increased plant height) at the expense of root development and photosynthetic efficiency. Notably, maintaining a moderate population (240,000 seeds ha−1) in these low-fertility zones produced yields statistically equivalent to those in higher-fertility areas. These findings demonstrate that applying Liebig’s Law of the Minimum to identify the most limiting factor provides a robust, mechanistically sound foundation for developing VRS prescriptions. For nutrient-limited zones, the optimal and most resilient strategy is not to increase sowing density but to maintain a moderate population that balances yield potential with resource availability. Future research should investigate the phenotypic plasticity of different cultivars under this VRS strategy and assess its economic viability at a commercial scale.