Updated November 8, 2019:
In 2017, the Delaware Soybean Board funded a study to evaluate nitrogen and sulfur fertigation on irrigated full season soybeans. Treatments were planned to be the same in 2017 as those conducted in 2015 and 2016, however significant deer damage early in the season resulted in lost plot area and treatment number was reduced to 4.
Nitrogen (N) + sulfur (S) was applied at R1, R1 + R3, 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 treatments, whereas the R1 + R3 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.
Yields ranged from 72 to 79 bu/A. The largest yield response was from the R1 + R3 treatment where soybeans yielded 7 bu/A greater than the no fertigation treatment. At maturity, soybean heights ranged from 38 in. to 39 in.
There were some differences and trends observed in nutrient tissue content. Tissue samples were collected before fertigation treatments were initiated at R1 and 1 week after each fertigation timing on 7/20, 8/9, and 8/22. On the 7/9 sample date before fertigation, tissue nitrogen levels ranged from 5.82 to 5.92 ppm. The largest increase in tissue N concentration was observed when N was applied at R1 and R3. N + S applied at R3 resulted in the greatest tissue N concentration on the 8/22 sample date.
Tissue sulfur levels were within the sufficient range or above throughout the season in all treatments. Sulfur tissue levels were highest on the 8/22 sample date when treatments included N + S applied at R3.
Other nutrients to note in this full season study are potassium, magnesium, and molybdenum. These nutrients were found to be below the sufficiency range on at least 1 sample date, which may have limited yield. On the 8/22 sample date, potassium was below the sufficiency range. Magnesium was below the sufficiency range on the last 2 sample dates. Tissue molybdenum levels were found to be well below the sufficiency range on the last sample date. These are interesting results that we would like to explore in 2018.
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In research conducted at the University of Delaware Warrington Irrigation Research Farm in 2015, 2016, and 2017 trends in soybean response to nitrogen (N) and sulfur (S) fertigation are developing. There was a significant yield response in all three years when N + S was applied over the no fertigation treatment. In 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 (Table 3). 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).