Historically, farmers in Pennsylvania relied on atmospheric deposition of sulfur to satisfy the nutrient requirements of crops. However, between 1989 and 2009, sulfur deposition in the Eastern United States decreased by 50% (Burns et al., 2011). In the decade since then, sulfur deposition has continued to decline to the point where virtually no sulfur is added through rainfall anymore. 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 below the plow layer, where it is not detected through regular soil fertility testing, but can still remain in the subsoil within the root zone. Determining the depth distribution of sulfur in the soil profile and how this compares to soybean rooting depths could lead to more informed decisions about how frequently sulfur applications are needed. Consequently, soybean farmers in Pennsylvania must carefully consider how to manage sulfur in their soybeans in order to achieve high yields and high quality.

Sulfur is a macronutrient like N, P and K, and so must be available in the soil in sufficient amounts for healthy crop growth. Previous studies have found soybean yields to respond positively when treated with sulfur fertilizers at sulfur deficient sites (Agrawal and Mishra, 1994; Beaton and Soper, 1986; Chen et al., 2005). In recent trials in Indiana, soybean yields were reported to increase by as much as 13 bushels per acre when S was applied at rates between 15 and 25 pounds per acre as ammonium sulfate (Casteel, 2018). These results indicate that addressing S deficiencies will pay dividends in yield, and perhaps also in quality.
Sulfur is a key component in the amino acids cysteine and methionine which are two of the building blocks of proteins in soybean seeds. As a result, a deficiency in these amino acids will reduce the nutritional value of soybean protein (Sexton et al., 1997). Soybeans grown under optimal conditions fail to supply adequate amounts of methionine and cysteine for monogastric animal nutrition (Krishnan and Jez, 2018), and any reduction of these amino acids would require additional feed supplements beyond typical amounts. While increasing the S available in the soil may boost amino acid levels, another key component of building proteins and crop yield is nitrogen uptake.

As a legume, soybeans have the ability to fix N from the atmosphere to satisfy plant needs, and so supplemental N typically is not needed. However, previous studies have shown increases in soybean yield in response to N fertilization in some cases (Salvagiotti et al., 2008). Soybean seed protein concentrations generally do not increase in response to N fertilization, however increasing yield results in increased total protein production (Haq and Mallarino, 2005). In the trials previously mentioned from Indiana, sulfur was applied as ammonium sulfate (Casteel, 2018), and so it’s impossible to determine whether the soybean yield increase is due to added N, added S, or the combination of the two. Another factor to consider in choosing a source of N and S is cost. According to recent data in Pennsylvania, ammonium sulfate costs $0.76 per pound of S and gypsum costs $0.60 per pound of S, although if there is a benefit from the N present in the ammonium sulfate and the value of the N is accounted for, the cost is only $0.30 per pound of S. On the other hand, if yield increases are due strictly to the addition of N, then urea would likely be the cheapest fertilizer source at $0.46 per pound. Therefore, determining whether it is S, N, or the combination of N and S that improves soybean performance would be an economic benefit to soybean farmers in Pennsylvania.

Sulfur is known to have an uneven distribution with soil depth. This is because the plant available form of S, sulfate or SO42-, that is often applied in fertilizers like ammonium sulfate or gypsum is an anion and is subject to leaching. As a result, high concentrations of S can often be found in the subsoil layers in agricultural fields treated with S containing fertilizers (Caires et al., 2011), but it is unclear whether this S is useful to subsequent crops. Soybean peak S uptake rate occurs around R3 (Gaspar et al., 2018), at a time when the plant has developed a robust root system. Depending on the rooting depth of the soybean plant, and the depth of previously applied S in the subsoil, it is possible that the plant could utilize this deep sulfur during grain fill.
Atmospheric sulfur deposition no longer will supply adequate amounts of S for crop growth and will likely continue to decline in the future. We believe that managing S to maintain high yielding and high-quality crops is an important issue to address as deficiencies become increasingly common in Pennsylvania. We also believe that identifying whether soybean quality parameters will respond positively to S fertilization is an important question for farmers in Pennsylvania. Further complicating the issue is that S is often being applied to soybeans as ammonium sulfate, which raises questions about the effect added N has on soybean yield and quality.

1. Determine whether soybean yield, crude protein, or sulfur-containing amino acid levels are responsive to sulfur fertilization in the present or previous year.
2. Identify whether there is an interactive effect between nitrogen and sulfur fertilization on soybean yield, crude protein, or sulfur-containing amino acid levels.
3. Identify the depth distribution of sulfur in the soil profile following a year of sulfur fertilization in corn production and whether soybean plants can access and utilize this sulfur.

In 2019 we established plots at the Russell E Larson Agricultural Research Center to demonstrate the effectiveness of different sulfur containing fertilizers in corn. We applied S at a rate of 40 lbs per acre using gypsum, ammonium sulfate (AMS), elemental sulfur and poultry litter and maintained two unfertilized check plots. During the 2019 corn growing season, we took monthly soil samples to monitor S availability in the top 8” of soil and soil acidity in the top 2”. Sulfur was extracted from soil samples using a Mehlich III extraction method and extractants were analyzed using an ICP at the Penn State Agricultural Analytical Services Lab. 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. While S concentrations were similar in the top 8” of soil in the treatment plots and check plots, there was greater S deep in the soil profile in the treatment plots. We will use this same set of plots with soybeans in 2020.

Prior to soybean planting in the spring of 2020, we will collect soil cores to 32” again to determine whether the S that was in the soil profile in the fall of 2019 has leached out of the soil profile or whether it remains adsorbed in the clay rich subsoil. Our hypothesis is that there will be S remaining in the soil profile of the gypsum, AMS, poultry litter and elemental S treatments that could be accessed by the soybeans, 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 2019 and maintain one untreated control plot. The fertilizers will be surface applied at a rate of 40 lbs S per acre and should be readily available in the top soil 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 any S deep in the soil profile.

Plant tissue samples will be collected at the V2/V3 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 0.16-0.41 % 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. Our hypothesis is that treatments which received S in 2019 but not 2020 will be S deficient at the early sampling date, but after developing a root system which can reach the S in the subsoil, will be sufficient 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 protein. We will also have the samples analyzed for concentrations of the amino acids cysteine and methionine to quantify any effect the treatments had on sulfur containing amino acid content.

Updated August 30, 2020:
Please see uploaded report.

View uploaded report

Updated April 4, 2021:
Please see uploaded final report.

View uploaded report

In this experiment we hypothesized that soybean yield and quality would respond positively to sulfur (S) fertilization. We tested S fertilizers applied at 40 lbs/ac S applied to corn in the year before soybean production (2019) to S fertilizers applied in the soybean production year (2020). Our results indicated that there was not an effect of S fertilization on grain yield or crude protein concentration, however, grain S concentration increased as well as the concentration of the S-containing amino acids cysteine and methionine. This suggests that in cases where S doesn’t limit yield, there are still some effects of the addition of S, especially if producers are concerned about the amino acid concentration of their grain. We also sought to describe the depth distribution of S in the soil profile after S application, and did so at three time points: after corn harvest in 2019, prior to soybean planting in 2020, and after soybean harvest in 2020. We found that even after two seasons of crop production, subsoil S content was higher in after soybean harvest in treatments which had received S in 2019 than treatments which received no S in either year. This is an important finding because it illustrates the fact that the subsoil can act as a reservoir to hold S for use by multiple cropping cycles. In addition, our soybean plant rooting depth data and plant tissue testing data indicated that once roots reached the deeper soil layers containing S, plants readily took it up and assimilated it into their tissues. Finally, when considering the potential interaction between N and S, our plant tissue, soil testing, and grain analyses all indicated that gypsum and ammonium sulfate performed equally well. There was no difference in S content between the two treatments at either plant tissue sampling date, there were no differences in yield, and both treatments resulted in increased grain S, methionine, and cysteine concentrations when compared to the control. Therefore, if a producer’s intends to add S to their soybean crop, the cheaper of these two products should be selected, since the results so far indicate similar crop performance. It also appears that if S is added during the corn year of the rotation, producers should be able to rely on excess S stored in clayey subsoils.

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. We expect soybean yield to respond most to the fertility treatments applied in 2020, especially the AMS (Figure 3). 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.