Soybean Nitrogen and Sulfur Uptake, Partitioning, and Removal

• Test Environments: 2 years at 3 locations with non-limiting fertility levels, resulting in 6 different testing environments.
• Soybean Varieties: 8 Pioneer® brand soybean varieties (RM 1.0-2.5)
• Planting Dates: Early and late May
• Plant Sampling: Collected at the V4, R1, R4, R5.5, R6.5, and R8 growth stages and partitioned into the following parts:

-Stems                 -Petioles

-Leaves                -Pods

-Seeds                  -Fallen Leaves/Petioles

• Nutrients Quantified: Nitrogen and Sulfur
• 6,672 tissue samples analyzed that span a yield range of 40 to >100 bu/acre.

• The soybean crop required 3.75 lbs of N/bu/acre and 0.21 lbs of S/bu/acre, meaning an 80 bu/acre soybean crop would require 302 lbs/acre of N and 19 lbs/acre S (Figure 1).
• Current estimates of biological N2 fixation (300 lbs of N/acre) and annual mineralization capacities (20 lbs N / 1% organic matter) have the potential to supply enough N for yields approaching 100 bu/acre.
  • Therefore, N supply is rarely the limiting factor for high yielding soybeans.
• More variability existed in the yield – S relation, but this relation was not affected by the environment or variety. In comparison the yield – N relation was much stronger (R2 = 0.80), however, total N uptake did vary by the environment (Figure 1).
  • Therefore, N and S uptake requirements are independent of the variety used, while total N uptake is field- and year-specific.

• N and S uptake rates lagged until V2 and then accelerated to peak rates (3.5 lbs N/acre/day and 0.25 lbs S/acre/day) from R2-R5 (Figure 2).

• Uptake prior to R1 was minimal. Leaves were shown to be large storage organs for N and S, while stems also held large amounts of S. After R5.5, 68 and 50% of vegetative N and S, respectively, were remobilized to the seed (Figure 3).
• This remobilization, combined with large amounts of seed N and S acquired from uptake after R5.5, resulted in nutrient harvest indexes of 83 and 68% for N and S, respectively (Figure 3).

• Total removal with the harvested grain was 3.30 lbs of N/bu/acre and 0.18 lbs of S/bu/acre, resulting in an 80 bu/acre soybean crop removing 254 and 15 lbs of N and S, respectively (Figure 4).
• N removal per bushel of grain didn’t differ between varieties or environments. S removal did vary by variety, but differences were small enough to have no meaningful management implications.