Phosphorus and potassium uptake, partitioning, and removal across a wide range of soybean seed yield levels

Maintenance of adequate soil phosphorus (P) and potassium (K) levels is critical for profitable soybean [Glycine max (L.) Merr.] production. To accomplish this, precise knowledge of soybean P and K uptake, utilization, and removal is critical, yet a comprehensive study characterizing these requirements across wide-ranging seed yield environments is nonexistent for modern soybean production systems. Using six site-years and eight soybean varieties, plants were sampled at six growth stages , partitioned into their respective plant parts, and analyzed. Distinctly different uptake patterns and rates were found between P and K, where soybean accumulated greater relative amounts of K by R1 and 91 to 100% of its season-long K total by R5.5, compared with only 68 to 77% of its season-long P total. Removal of P (0.0054 kg P kg⁻¹ grain) and K (0.016 kg K kg⁻¹ grain) with the seed was consistent across environments and varieties and displayed strong relations with yield (R² = 0.89–0.92). For each kilogram increase in yield, total P and K uptake increased by 0.0054 kg and 0.017 to 0.030 kg, respectively. The difference between total uptake and removal for each nutrient resulted in average nutrient harvest indices of 81 and 49% for P and K, respectively. However, significant variation in total uptake and nutrient harvest indices existed due to the environment, not variety, and was more pronounced for K, resulting in significant variability in the amount of K removed in stover. These results can be incorporated into future fertility recommendations to improve P and K management for profitable and environmentally sound soybean production.