Soybean grows best on fertile soils. For decades, the University of Kentucky has conducted field studies to establish the relationship between soil nutrient supplies and soybean yield. Adequate soil fertility must be present so that yields are not limited. Soybean removes a large amount of nutrients in the seed relative to other annual crops. For each bushel of soybean produced, 3 lb N, 0.7 lb P2O5, and 1.1 lb K2O will be removed in the seed, but considerably more is required to supply the leaves, stems and pods that are not removed during harvest (Table 1).
Ritchey, Edwin L.; Lee, Chad; Knott, Carrie Ann; and Grove, John H., "Soybean Nutrient Management in Kentucky" (2014). Agriculture and Natural Resources Publications. Paper 136.