Updated November 6, 2019:
In 2015, 2016, 2017, and 2018 the Delaware Soybean Board funded trials to evaluate nitrogen and sulfur fertigation on irrigated full season and double crop soybeans. In 2015 and 2016, nitrogen (N) + sulfur (S) was applied at R1, R3, R5, R3 + R5, and R1 + R3 + R5 at 30lbs N + 15 lbs S per acre at each timing. Soybeans received a total of 30 lbs N + 15 lbs S per acre in the R1, R3, and R5 treatments, whereas the R3 + R5 treatment received a total of 60 lbs N + 30 lbs S per acre and the R1 + R3 + R5 treatment received a total of 90 lbs N + 45 lbs S per acre. In 2017, the number of treatments were reduced due to severe deer damage early in the season. Treatments included N + S applications at R1 (30 lbs N + 15 lbs S), R1 + R3 (60 lbs N + 30 lbs S total), and R1 + R3 + R5 (90 lbs N + 45 lbs S total). In 2018, N + S and UAN (without S) were applied at multiple timings and rates. The application timings were selected based on previous Delaware research results. Applications were made at R1 and at R1 + R3. Total N and S applied ranged from 7.5 lb N/A + 3.5 lb S/A to 90 lb N/A + 45 lb S/A. A high fertility treatment was also included for comparison in an attempt to eliminate fertility as a yield limiting factor. The high fertility treatment consisted of a broadcast granular application of a complete fertilizer and a foliar application of Molybdenum at R1.

In 2015 and 2016, the largest yield response was from the R1 treatment where soybeans yielded 8 bu/A greater than the no fertigation treatment. Yield was not improved and was slightly less than the R1 treatment when single applications were delayed to R3 and R5 and when multiple applications (R3 + R5 and R1 + R3 + R5) were made. The most noticeable increase in tissue N concentration over the no fertigation treatment was when N was applied at R1; however, all treatments had similar tissue N concentration in the final tissue analysis after the R5 treatment was applied.

In 2017, the R1 treatment yielded similar to the no fertigation treatment. However, there was a significant increase in yield (8 bu/A) when N + S was applied at R1 followed by another application at R3 for a total of 60 lbs N + 30 lbs S. This was the first year that evaluated the R1 + R3 application. Yield was not improved by applications at R1 + R3 + R5 (90 lbs N + 45 lbs S) over the R1 + R3 application. There was a noticeable increase in tissue N after all application timings compared to the control. With S tissue levels, the most noticeable increase occurred after the R3 application was made compared to the control. In the final tissue analysis, N and S levels remained highest in the multiple application treatments.

In 2018, all treatments yielded similar to the no fertigation treatment. The treatments that included only N (UAN) were the lowest yielding. Although there were no yield differences across treatments, tissue analysis revealed increases in N and S due to the N and S applications. With the high fertility treatment, no significant increase in tissue levels were observed with P and Ca, however, K and Mg tissue levels were higher than nontreated plots 1 week after application, but not at other tissue sampling timings. All micronutrient (B, Mn, Zn, and Mo) tissue levels in the high fertilility treatment were higher than nontreated treatments at all tissue sampling timings after application. It is very interesting that the increase in K, Mg, and micronutrient tissue levels did not improve yield. There were no significant differences in plant height compared to the nontreated control.

Over all years, a significant yield increase with N + S applications compared to the control have been observed in 3 out of 4 years. Data indicates that early reproductive stage application timings provide the highest yield increase; however, applications before R1 during the vegetative stage have not been evaluated. In addition, it is necessary to again evaluate N applied without S. It is interesting to note that in all 4 years, average yield of all treatments combined was around 75 bu/A. Although we did not see a yield response to the high fertility treatment in 2018, there was an influence on tissue nutrient levels observed. It would be interesting to gather more data on tissue levels between high fertility and University recommended fertility and their corresponding yields.

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In research conducted at the University of Delaware Warrington Irrigation Research Farm in 2015, 2016, 2017, and 2018 trends in soybean response to nitrogen (N) and sulfur (S) fertigation are developing. Over all years, a significant yield increase with N + S applications compared to the control have been observed in 3 out of 4 years. Data indicates that early reproductive stage application timings provide the highest yield increaseIn 2015 and 2016, there was an 8 bu/A increase in yield when N + S (30 lbs/A + 15 lbs/A) was applied at the R1 growth stage compared to the no fertigation treatment. In 2017, the R1 application yielded similarly to the no fertigation treatment, however there was a 7 bu/A increase in yield over the no fertigation treatment when N + S was applied at R1 and again at R3 (60 lbs/A N + 30 lbs/A S total). In 2018, there was not a significant yield response with any rate or timing of N + S. Although we did not see a yield response to the high fertility treatment in 2018, there was an influence on tissue nutrient levels observed.