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SOYBEAN PHOSPHORUS AND

SOYBEAN PHOSPHORUS AND POTASSIUM 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 • Greater genetic yield potential, combined with physiological changes and improved management practices, has lead to annual increases in the average national soybean yield (0.4 bu/acre/year) and greater frequency of growers achieving yields greater than 75 bu/acre throughout the Midwest. • Phosphorus and potassium fertilization is likely the most critical annual fertility decision growers make. Fertility programs should not limit soybean yield, while still being environmentally sound. • Therefore, precise knowledge of soybean Phosphorus (P 2 O 5 ) and Potassium (K 2 O) requirements (specifically crop removal) across a wide yield range is critical for environmentally and economically sustainable fertility programs, while in-season P 2 O 5 and K 2 O utilization is beneficial for various management decisions. 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 P 2 O 5 AND K 2 0 UPTAKE Total P 2 O 5 Uptake (lbs/acre) Total K 2 O Uptake (lbs/acre) 105 90 75 60 45 30 300 250 200 150 100 P K y = 0.90x + 1.01, R 2 = 0.70, S.E. = ± 6.72 y = 2.30x + 15.62, R 2 = 0.53, S.E. = ± 23.65 40 60 80 100 Yield (bu/acre) Figure 1. Total whole plant P 2 O 5 and K 2 O uptake at growth stage R8 (full maturity) across all environments and varieties. • Across all environments and varieties, the soybean plant took up an average of 0.9 lbs of P 2 O 5 /bu/acre. Potassium uptake was more than double this at 2.3 lbs of K 2 O/bu/acre. Therefore, an 80 bu/acre soybean crop accumulated 73 and 200 lbs of P 2 O 5 and K 2 O, respectively, by R8 (Figure 1). • More variability existed in the relationships between P 2 O 5 and K 2 O uptake and yield than the relationship between nitrogen (N) uptake and yield (R 2 = 0.80). This was due to the fact that both P 2 O 5 and K 2 O uptake were greatly affected by the environment, with K 2 O experiencing the greatest variability in uptake associated with environmental differences and therefore, lowest R 2 value. • As displayed in Figure 3, 90% of total K 2 O uptake is accumulated by R5 (beginning seed fill), but seeds/m 2 and seed size are far from being determined. This lack of overlap and the fact that soybean is a luxury K 2 O consumer certainly contribute to the moderate relationship between total K 2 O uptake and seed yield (R 2 = 0.53) (Figure 1). 126

eturn to table of contents P 2 O 5 AND K 2 0 UPTAKE RATE P 2 O 5 Uptake Rate (lbs/acre/day) 1.25 1.00 0.75 0.50 0.25 0.00 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 Phosphorous, S.E. = 0.03 Potassium, S.E. = 0.12 Figure 2. P 2 O 5 and K 2 O uptake rate through the growing season for a 66 bu/acre soybean crop. Duration of peak uptake period is represented by a horizontal black line. • The potassium uptake rate accelerated quickly after V2 to a peak rate of 4 lbs K 2 O/acre/day at R2 and then sharply decelerated through R6 (Figure 2). • Phosphorus displayed a more gradual build and decline before and after the peak uptake rate (1 lb P 2 O 5 /acre/ day), which occurred at R3 (Figure 2). P 2 O 5 AND K 2 0 PARTITIONING AND UTILIZATION Cumulative % of Total P 2 O 5 Cumulative % of Total K 2 O 100 90 80 70 60 50 40 30 20 10 90 80 70 60 50 40 30 20 10 P K 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. P 2 O 5 and K 2 O uptake, partitioning, and remobilization through the growing season for a 66 bu/acre soybean crop. 5 4 3 2 1 0 60 55 50 45 40 35 30 25 20 15 10 5 160 140 120 100 80 60 40 20 0 K 2 O Uptake Rate (lbs/acre/day) Total P 2 O 5 Uptake (lbs/acre) Total K 2 O Uptake (lbs/acre) • Relative uptake prior to R1 was minimal for P 2 O 5 (13%) but greater than 25% for K 2 O. Leaves and stems were large storage organs for P 2 O 5 , while K 2 O was more equally distributed to all plant parts. • Seed potassium accumulation relied more heavily on vegetative remobilization compared to P 2 O 5 (76 vs. 52%), yet the nutrient harvest index (HI) was much higher for P 2 O 5 (81%) than K 2 O (49%) and was similar to the N HI (83%) at the same yield level (Figure 3). • The large K 2 O uptake requirement (2.3 lbs K 2 O/bu/acre) and low K 2 O HI makes stover removal a major pathway for K 2 O removal and soil K 2 O depletion if not replaced with fertilizer. P 2 O 5 AND K 2 0 REMOVAL WITH THE GRAIN P 2 O 5 Removal (lbs/acre) K 2 O Removal (lbs/acre) 80 70 60 50 40 30 135 120 105 90 75 60 P K y = 0.74x – 0.32, R 2 = 0.89, S.E. = ± 2.93 y = 1.17x + 4.20, R 2 = 0.92, S.E. = ± 3.90 40 60 80 100 Seed Yield (bu/acre) Figure 4. Total P 2 O 5 and K 2 O removal with the seed at growth stage R8 across all environments and varieties. • Total P 2 O 5 (0.74 lbs./bu/acre) and K 2 O (1.17 lbs/bu/ acre) removal with the grain was consistent across all varieties and environments. Therefore, an 80 bu/acre soybean crop removed 59 and 98 lbs of P 2 O 5 and K 2 O, respectively (Figure 4). • In comparison a 200 bu/acre corn crop would remove similar amounts of P 2 O 5 but only half the amount of K 2 O. CONCLUSIONS • Soybean K 2 O uptake varied considerably by environment and was utilized and removed with the grain in greater quantities than P 2 O 5 . • Moreover, the K 2 O HI was much less than that of P 2 O 5 , making stover harvest a large avenue for K 2 O removal. • Peak uptake rates occurred at R2 and R3 for K 2 O and P 2 O 5 , respectively, with K 2 O partitioned more equally between all plant parts compared to P 2 O 5 , which accrued mainly in leaves and stems. 127

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