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SOYBEAN NITROGEN AND

SOYBEAN NITROGEN AND SULFUR UPTAKE, PARTITIONING, AND REMOVAL by Adam Gaspar, Ph.D., Field Agronomist, and Shawn Conley, Ph.D., Associate Professor, Department of Agronomy, University of Wisconsin-Madison return to table of contents RATIONALE AND OBJECTIVES • Soybean growers are often concerned that soybean yield is restricted by limitations on biological N 2 fixation and soil nitrogen (N) mineralization. • Atmospheric sulfur (S) deposition has been reduced, but the wide spread availability of animal manure has many growers questioning the need for application of additional S. • Proper management of these two soil-mobile nutrients throughout the growing season is critical for high yielding soybeans. • The objective of this study was to characterize actual soybean N requirements and utilization across a wide seed yield range for modern soybean production systems. STUDY DESCRIPTION • Test Environments: 2 years at 3 locations with nonlimiting 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: Phosphorus and potassium, presented in fertilizer equivalents (P 2 O 5 and K 2 O) • 6,672 tissue samples analyzed that span a yield range of 40 to >100 bu/acre Catch container used to collect all fallen leaves and petioles throughout the growing season from each plot. TOTAL N AND S UPTAKE Total N Uptake (lbs/acre) Total S Uptake (lbs/acre) 500 450 400 350 300 250 200 150 25 20 15 10 5 N S y = 3.75x + 1.94, R 2 = 0.80, S.E. = ± 20.20 y = 0.21x + 1.83, R 2 = 0.58, S.E. = ± 2.06 40 60 80 100 Yield (bu/acre) Figure 1. Total whole plant N and S uptake at growth stage R8 (full maturity) across all environments and varieties. • 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 N 2 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 (R 2 = 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. 124

eturn to table of contents N AND S UPTAKE RATE N Uptake Rate (lbs/acre/day) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Nitrogen, S.E. = 0.1 Sulfur, S.E. = 0.01 0.0 0 10 20 30 40 50 60 70 80 90 100 110 Days After Emergence VE V2 V4 R1 R2 R3 R4 R5 R6 R7 R8 Growth Stage 0.30 0.25 0.20 0.15 0.10 0.05 0.00 Figure 2. N and S uptake rate through the growing season for a 66 bu/acre soybean crop. Duration of peak uptake is represented by the horizontal black line. • 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 to R5 (Figure 2). N AND S PARTITIONING AND UTILIZATION Cumulative % of Total N Cumulative % of Total S 100 90 80 70 60 50 40 30 20 10 90 80 70 60 50 40 30 20 10 N S Seeds Pods Stems Petioles Leaves Fallen Petioles & Leaves 0 0 10 20 30 40 50 60 70 80 90 100 110 Days After Emergence VE V2 V4 R1 R2 R3 R4 R5 R6 R7 R8 Growth Stage Figure 3. N and S uptake, partitioning, and remobilization through the growing season for a 66 bu/acre soybean crop. 225 200 175 150 125 100 75 50 25 14 12 10 8 6 4 2 0 S Uptake Rate (lbs/acre/day) Total N Uptake (lbs/acre) Total S Uptake (lbs/acre) • 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). N AND S REMOVAL WITH THE GRAIN N Removal (lbs/acre) S Removal (lbs/acre) 400 350 300 250 200 150 100 22 20 18 16 14 12 10 8 6 N S y = 3.30x – 10.35, R 2 = 0.89, S.E. = ± 13.02 y = 0.18x + 0.46, R 2 = 0.76, S.E. = ± 1.15 40 60 80 100 Yield (bu/acre) Figure 4. Total N and S removal with the seed at growth stage R8 (full maturity) across all environments and varieties. • 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 did not differ between varieties or environments. S removal did vary by variety, but differences were small enough to have no meaningful management implications. CONCLUSIONS • Total soybean N requirements were less than previously reported due to greater vegetative remobilization, increased uptake directly to the seed after R5, and a higher N harvest index. • Early season N and S applications are likely nonbeneficial as peak uptake occurred near R3 to R4, while approximately 50% of total N uptake happened after R5. • Growers should focus on building organic matter and maximizing N 2 fixation to provide a season long N supply rather than focusing on a single in-season N fertilizer application. 125

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