Secondary and Micronutrient Uptake, Partitioning, and Removal across a Wide Range of Soybean Seed Yield Levels

Reduced atmospheric S deposition, in conjunction with higher grain sale prices and steadily increasing yields of soybean [Glycine max (L.) Merr.], has many growers considering an increase in secondary and micronutrient applications. Limited information exists quantifying requirements of S, Mg, Ca, Zn, Mn, Cu, Fe, and B across a wide yield range for modern soybean production systems. Using six site-years and eight varieties, plants were sampled at six growth stages and partitioned into their respective plant parts and analyzed. Nutrients were acquired heavily (48-73%) from R1 through R5.5 with peak uptake rates near R3. Yet, uptake after R5.5 represented a greater portion of total S uptake as yield increased from the low (24.9%) to high (32.2%) yield level (3608 vs. 5483 kg ha(-1)). This coincided with seed S accumulation, which relied more heavily on continued uptake after R5.5 (58%) vs. vegetative S remobilization (42%). Across all environments (site x year) and varieties, total S uptake (0.004 kg S kg grain(-1)) and removal (0.003 kg S kg grain(-1)) showed moderate (R-2 = 0.58) and strong (R-2 = 0.76) relations with yield, respectively. These relations for each micronutrient were much weaker (R-2 = 0.13-0.66), due largely to the main effects of environment and variety along with their respective interactions with yield. Furthermore, micronutrient concentrations in leaf tissue varied considerably (CV = 28-46%) during recommend testing stages. Thus, previously reported inconsistent yield responses to foliar application of these micronutrients may primarily be due to the large variability in leaf tissue concentrations and nutrient requirements.