Trade-Off between Root Efficiency and Root Size Is Associated with Yield Performance of Soybean under Different Water and Phosphorus Levels

(1) Background: Root traits play important roles in acclimating to water and phosphorus (P) shortages. However, the relative importance of root size efficiency under these conditions is unknown. (2) Methods: This study investigated role water- P-limited environments. Three soybean genotypes with contrasting sizes were grown tall cylindrical pots compare grain yield, density, nutrient-uptake efficiencies two (well-watered water-stressed) three P levels (0 (P0), 60 (P60), 120 (P120) mg kg−1 dry soil). (3) Results: Water or deficit, combined deficit significantly decreased which was associated greater uptake per unit weight (DW) stress. The genotype Zhonghuang 30 (ZH) greatest water, nitrogen, uptakes DW had highest yield at P60 P120 stress P0 well-watered conditions, but ZH lowest due its small size. (4) Conclusions: High efficiency—which correlated high density—improved P- water-limited restricted potential when not limited.