Update:
Nitrogen
The addition of N fertilizer at the rate of 50 lb per acre resulted in about a 5% lower established plant density compared with the control. Although the N was broadcasted and worked in before planting and not applied near the seed, the negative effect on plant stand was significant. As expected the plants with application of N were more vigorous early in the season. The vigor score with 50 lb N was statistically higher compared with the 0 (control) lb N application. At the end of the season the plants with application of 25 or 50 lb N per acre were taller than the control. Both rates of N resulted in a significant seed yield increase compared with the control There were no significant difference in protein or oil content.
Sulfur
There were no significant differences in plant density, vigor, height, yield or oil content among the three sulfur levels across all other factors. The only significant difference was in protein content. Average seed protein concentration for the treatment of 0, 10, and 20 kg S per acre was 33.73, 33.65, and 33.57%, respectively. These differences when expressed as rounded numbers 33.7, 33.7 and 33.6, respectively, appear not to be meaningful. However, the result of this study disagree with those by Bellaloui et al. (2011) who reported that seed protein consistently increased in field trials when fertilized with S or N and S.
N x S
Understanding the relationship between N and S and the influence of the two nutrients on yield was a main objective of this study.The control treatment was the lowest yielding of all the treatments but not significanlty different from 0N+10S, 0N+20S, and 25N+20S. All 25 and 50 lb N treatments with or without S yielded more that the control. No significant statement can be made about the effect of S. It is suggested to continue research on S rates and possibly use other sources of S to determine if there might be a benefit of a low amount of fertilizer S.
Nodulation study
In addition to the yield trial, observational plots were included in the experiment for the purpose of destructive sampling (removing the plants) and data collection. Plants from observational plots were used to determine the effect of N and S application on plant nodulation. Soybean nodulation was assessed by uprooting plants from the observational plots, counting the number of nodules and rating the nodules for size. Plants were removed from 2 feet of the center two rows at approximately R1 and R4.
Results
Environment played a key role in the magnitude of plant response to N x S treatments. For example, average nodule number per plant was highly variable between environments at the V4 and R4 growth stage regardless of treatment.
In 2016, the average nodule number per plant increased 5% and 73% in the Ransom County and Steele County environment, respectively, comparing the count at V4 and R4. Possible explanations for differential environmental response include factors such soil type and previous crop, weather conditions and level of soil N at the start of the experiment. In addition, the soil in the 2016 Ransom County environment was extremely dry and compacted at the R4 growth stage, which caused root damage during sample collection.
Average nodule number per plant was significantly different at the V4 stage between varieties (24 and 30 nodules). However, by the R4 stage variety had no significant impact on average nodule number per plant (37 and 40 nodules). This indicates that the two varieties evened out in average nodule number per plant during the growing season. A significant difference in root mass between varieties was also observed at the V4 growth stage, but evened out by the R4 growth stage.
Nitrogen influenced nodulation at the V4 and R4 stage. As the N rate increased at both growth stages, average nodule number per plant decreased. Nodule number per root for 0 lb N per acre was significantly higher than when N was applied. Results agree with those by Streeter (1988) as well as Gibson and Harper (1985) and Laysell and Moloney (1994) who indicated a strong inhibition of nodulation and N fixation activity under high nitrate conditions.
There were no significant differences in nodule number for the two S rates. There were 27 nodules for the control and 100 lb S per acre rate at the V4 and 38 nodules at the R4 stage for both rates.
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The results of this study indicate that N application significantly increased vigor, height and yield, but reduced plant stand and nodulation. Compared to the control (0 N) treatments of 25 or 50 lb N acre increased yield by 1.5 and 2.6 bu per acre respectively. The 50 lb N per acre application level yielded about 5% more compared with the control. The average yield, 49 bu per acre of the combined trial results for 2015 and 2016, was substantially higher than the North Dakota state soybean yield average of 37 bu per acre for the same period. If farmers look at economics, they need to consider their own production level and do the calculations based on a potential yield increase of 5% using their traditional obtained soybean yields. For instance, if the North Dakota State average 37 bu per acre would be used, the anticipated yield increase would be approximately 1.9 bu per acre with the application of 50 lb N per acre.
The potential benefit of N application and net profit depends on fertilizer cost, application cost, commodity price, and yield level.