Historically, farmers in Pennsylvania relied on atmospheric deposition of sulfur to satisfy the nutrient requirements of crops. However, between 2002 and 2018, sulfur deposition in the Eastern United States decreased by 81% (USEPA, 2018). As a result, sulfur deficiency is a growing concern for farmers in Pennsylvania. Soybean yield is related to sulfur uptake by the plant, suggesting that a deficiency in sulfur could result in reduced yield. Additionally, sulfur and nitrogen are critical to protein formation in soybean seeds (Gaspar et al., 2018). Because of the wide use of soybeans as animal feed in Pennsylvania, protein and amino acid content are key considerations for growers. Sulfur is also susceptible to leaching and determining its distribution in the soil profile could lead to more informed decisions about how frequently applications are needed. Consequently, soybean farmers in Pennsylvania must carefully consider managing the sulfur and nitrogen they supply to their soybeans in order to achieve high yields and high quality.

In our 2020 research funded by the Pennsylvania Soybean Board we found that prior to soybean planting, S was still present in the subsoil from fertilizers applied the prior year in the 2019 corn growing season, especially in the depths of 16-24 and 24-32 inches where S content ranged from 15 to 44 ppm. We collected whole plant samples when soybeans were at the V2 stage and trifoliate samples when plants were at R1 to measure S content in plant tissue. We found that plant tissue S content responded positively to S applied in 2020, and negatively to N applied alone, at the V2 stage compared to the control. At the R1 stage we found that plant tissue S content responded positively regardless of whether S had been applied in 2019 or in 2020 compared to no S application in either year.

We also observed soybean plant rooting depth at monthly intervals during the growing season and found that roots had extended to 28” when plants were at the R1 stage, which is within the depth at which we observed large amounts of S in the pre-season soil samples. The rooting depth data combined with the similarity in S content between 2019 and 2020 S treatments when plants were at the R1 stage leads us to believe the plants were utilizing the S stored in the soil profile from fertilizer applications made the previous year. An important implication of this finding is that the most economical way to apply S for soybean producers who grow corn-soy rotations would be to apply ammonium sulfate in the corn phase of the rotation at rates that would satisfy the S needs of both corn and soybean. In this way, the corn takes full advantage of the N in the ammonium sulfate, leaving behind surplus S for the soybean crop in the next year at a cost of only $0.30/lb S, half the cost of the next most economical option, gypsum.

At harvest, we measured grain yield and also collected soybean samples to analyze for crude protein, S content, cysteine and methionine. We found that there were no significant differences in yield or crude protein between the treatments. Average grain yield ranged from 18-28 bushels/acre and crude protein ranged from 36.5-37.3%. The growing season in 2020 in Centre County was extremely droughty, which is the main factor that caused low yields in the experiment. There was a significant treatment effect for S content in the grain with treatments which received S generally having higher S content. We analyzed cysteine and methionine content data using contrasts, and we found that the group of treatments which received S in either 2019 or 2020 had greater cysteine content than those which did not receive S and there appeared to be the same trend for methionine.

Repeating this experiment in 2021 will allow us to gain confidence that the results we saw in 2020 are consistent, and conduct the study under normal weather conditions. While we saw a positive effect of S fertilizers on plant tissue S content, grain S content, and sulfur-containing amino acid content, these results are limited to only one growing season and one field. If we see similar trends in 2021, it will allow us to make firm recommendations about S fertility to Pennsylvania soybean farmers. Additionally, central Pennsylvania suffered from an intense drought during the growing season in 2020 which negatively impacted yield. Under normal growing conditions, crop S demands will be higher, which will likely have an impact on plant tissue S content as well as the grain quality parameters we will measure. A normal growing season may also allow us to make conclusions about what effect S fertility has on soybean yield.

1. Validate findings from 2020 indicating increased cysteine, methionine, and sulfur concentrations in grain in response to sulfur fertilization.

2. Determine whether soybean yield, crude protein, or sulfur-containing amino acid levels are responsive to sulfur fertilization in either the present or previous year.

3. Identify whether there is an interactive effect between nitrogen and sulfur fertilization on soybean yield, crude protein, or sulfur-containing amino acid levels.

4. Validate the depth distribution of sulfur in the soil profile following a year of sulfur fertilization in corn production observed in 2020 and that soybean plants can access and utilize this sulfur.

In 2020 we established plots at the Russell E Larson Agricultural Research Center in the corn phase of a corn-soy rotation. We applied S at a rate of 40 lbs per acre using gypsum, ammonium sulfate (AMS), elemental sulfur and poultry litter and maintained four unfertilized check plots. During the 2020 corn growing season, we took monthly soil samples to monitor S availability in the top 8” of soil. After corn harvest, we took soil cores to a depth of 32”, separated in 8” segments, to determine whether the S had moved downward through the soil profile in each of the treatments. We will use this same set of plots when the crop rotates to soybeans in 2021.

Prior to soybean planting in the spring of 2021, we will collect soil cores to 32” again to determine whether the S that was in the soil profile in the fall of 2020 has leached out of the soil profile or whether it remains adsorbed in the clay rich subsoil. Like we saw in our 2020 experiment (Figure 1), we expect that there will be S remaining in the soil profile for the treatments which received S, but this S may be too deep in the soil profile to be initially utilized by seedling plants with shallow root systems. After soybean planting, we will apply gypsum, ammonium sulfate and urea to the plots which did not receive any S in 2020 and maintain one untreated control plot (Figure 4). The fertilizers will be surface applied at a rate of 40 lbs S per acre and should be readily available in the topsoil for the seedling soybean plants and continue to supply S throughout the growing season. The urea will be applied at a rate to match the N supplied in the AMS treatment. By applying AMS, gypsum and urea alone, we should be able to make some conclusions about whether yield increases are due to the added S or added N if we observe any yield increases in these treatments.

Following the establishment of the plots, we will collect soil samples to a depth of 8” at monthly intervals after fertilization until physiological maturity of the soybean crop to monitor S availability. We will also take soil cores to 32” in order to monitor the depth to which the soybean plants have rooted. These cores will be collected at monthly intervals after soybean planting. Because of the destructive nature of the root coring from equipment traffic in the plots, root cores will be taken from additional plots on the edge of the experiment, not directly from the fertilizer response plots. The rooting depth data will be compared to the depth distribution of S in preseason soil core data to estimate when the plants can access S deep in the soil profile.

Plant tissue samples will be collected at the V2 and R1 stages of development. These samples will be analyzed for S content at the Penn State Agricultural Analytical Services Lab. Current Penn State recommendations are that soybean plant tissue be in the range of 0.21-0.41 % S on a dry weight basis to be considered normal. Using these results, we will be able to determine if plants are accessing sufficient S and if there is any increase in S uptake by the soybean plants in response to the various S treatments compared to the no S control. Like we saw in our 2020 experiment, we expect treatments which received S in 2020 but not 2021 will be similar to the control at the early sampling date, but after developing a root system which can reach the S in the subsoil, will have greater S content than the control at the later sampling date.

We will measure soybean yield from each of the treatments at harvest using a small plot combine. At the time of harvest we will also collect soybean grain samples from each of the treatments. We will submit these grain samples to a laboratory for analysis of total S content and crude protein, to quantify what effect the different treatments had on grain S uptake and grain crude protein. We will also have the samples analyzed for the amino acids cysteine and methionine to quantify any effect the treatments had on sulfur containing amino acid content.

Updated August 26, 2021:
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Updated March 30, 2022:
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In 2020 and 2021, we conducted an experiment to evaluate the response of soybean yield and grain quality to sulfur and nitrogen timing of application in a corn-soy rotation. Funding for this experiment came from the Pennsylvania Soybean Board. At corn planting in 2020, we applied sulfur at a rate of 40 lbs/acre using ammonium sulfate, elemental sulfur, gypsum and poultry litter as the sulfur source. We used the same plot area in 2021 rotated to soybeans, in order to observe the effects of the previous year’s sulfur application. At soybean planting in 2021, we applied sulfur as ammonium sulfate or gypsum, and nitrogen as urea, to the 3 of the 4 2020 no sulfur plots and maintained 1 no S check in each replication.

In fall 2020, spring 2021, and fall 2021, we collected soil samples to 32” in order to determine the distribution of sulfur in the soil profile. At all three sampling dates, we found that there were no significant differences between the treatments at any of the sampling depths. However, the soil profile contained large quantities of sulfur below the routine soil sampling depth of 0-8”.

During the 2021 soybean season, we collected plant tissue samples when the plants were at the V2 and R1 growth stages, as well as soil samples to monitor plant rooting depth. At the V2 sampling, the soybean plants had roots extending to 14 inches, which was above the soil depth at which we observed increased sulfur. The 2021 gypsum and 2020 elemental sulfur treatments had greater sulfur tissue content than the No S check at the V2 growth stage. All other treatments had similar sulfur concentrations to the No S check, except for the urea treatment which was significantly less. At the time of sampling at the R1 stage, the plants had roots extending to 29.5 inches, well within the portion of the soil profile at which we observed increased sulfur. We found that treatments which received sulfur in either year had greater sulfur content than those which did not. This suggests that by the time the soybean plants reached the reproductive phase in their development, their root systems were accessing sulfur in similar quantities, regardless of which year the sulfur was applied.

Soybean grain was harvested in early November. We found that there were no significant effects of sulfur source or year of application on grain yield. Grain yield ranged from 62 to 70 bushels per acre. During harvest, we collected grain samples which were analyzed for sulfur, crude protein, methionine, and cysteine. There were no significant treatment effects on crude protein, sulfur, or methionine concentrations. The results indicated a trend for the group of treatments which received sulfur in either year to have greater cysteine concentration than those which did not receive sulfur. Despite the lack of a treatment effect on methionine concentration, we did find a significant treatment effect on the cysteine:methionine ratio in soybean grain. Generally, treatments which received sulfur in either year had a cysteine:methionine ratio closer to 1:1 than those which did not. This is an important consideration for animal feed, as recent research has shown that a cysteine:methionine ratio of 1:1 is optimal for broiler performance.

Our results indicated that there was not an effect of sulfur fertilization on grain yield, however, plant tissue sulfur concentration increased with sulfur fertilization, and it resulted in closer to optimal cysteine:methionine ratios. This suggests that in cases where sulfur doesn’t limit yield, there are still some effects of the addition of sulfur. We also found that even after two seasons of crop production, there was greater sulfur concentration in the subsoil than at the routine soil sampling depth of 0-8”. This illustrates the fact that the subsoil can act as a reservoir to hold sulfur for use by multiple crops. Our soybean plant tissue testing data indicated that once roots reached the deeper soil layers containing sulfur, plants readily took it up. Finally, when considering the potential interaction between nitrogen and sulfur, our plant tissue, soil testing, and grain analyses all indicated that gypsum and ammonium sulfate performed similarly. There was no difference in sulfur concentration between the two treatments at the R1 plant tissue sampling date, there were no differences in yield, both treatments resulted in increased grain cysteine concentrations when compared to the control, and both resulted in cysteine:methionine ratios close to 1:1. Therefore, if a producer intends to add sulfur to their soybean crop, the cheaper of these two products should be selected, since the results indicate similar crop performance. It also appears that if sulfur is added during the corn year of the rotation, producers should be able to rely on excess sulfur stored in clayey subsoils. These results will benefit farmers by giving them more flexibility in their sulfur fertility programs.

We expect that this research will help to identify whether one S fertilization event can meet the needs of both a corn and soybean crop, or if there is a benefit to applying S fertility to both crops in the rotation. We will also be able to identify where in the soil profile S remains after fertilization in the corn year of production, and whether S at this depth is available to soybean roots in sufficient quantity to meet S needs. We expect to also be able to determine whether the benefits seen from fertilizing soybeans with AMS are due to the added N, added S, or the combination. Additionally, we expect to use the information about the benefits of N, S, or the combination to better inform the economics of decision making regarding which fertilizer source to apply. Finally, we will measure what effect these S fertility treatments have on soybean yield, crude protein and amino acid content. These results will aid Pennsylvania farmers in managing the S supplied to their soybean crop, either from previous year’s fertilization, or fertilization in the current year in order to maximize yield and grain quality for benefit of the grain producers as well as the livestock industry.