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Research Highlights

Research Highlights
Putting the S in Your Soybeans (Where S = Sulfur)

Photo: USB

By Lisa M. Balbes, Ph.D.

Sulfur plays many important roles in soybean plants, from helping them absorb nitrogen to appearing in essential amino acids. Now that we have a good understanding of how to balance nitrogen, potassium, and phosphorus in the soil for growing soybeans, research is turning to how to manage sulfur, which is needed in quantities similar to phosphorous.

Sulfur deficiencies are showing up more often in fields. The deficiencies are caused by a number of factors including:

  • Reduced atmospheric deposition from industrial emissions. Prior to the Clean Air Act and other regulations in the 1990s, 15-20 lbs. of sulfur was deposited per acre annually from air pollution. Today, it’s closer to 5 lbs/acre.
  • Higher crop yields remove more sulfur from the fields in the beans themselves. In 70 bushels of soybeans, 13 pounds of sulfur is used.
  • Increasing use of fertilizers without sulfur, as opposed to sulfur-containing manure.
  • Sulfur often leaches out of the root zone, even more so with excessive rainfall or irrigation. 

 At the same time that the amount of sulfur in the soil is decreasing, the importance of sulfur in the soybeans themselves is increasing. Sulfur is incorporated into soybean protein in the amino acids cysteine and methionine. Animals cannot synthesize either of these, so must get them from dietary sources. In fact, sulfur-containing amino acids (SAAs) are often the limiting factor when using soybean meal as animal feed.

Sulfur is full of contradictions — it is one of the most mobile and variable nutrients when in the soil (making soil testing for sulfur problematic), but it has low mobility once it is incorporated into the plants. In addition, sulfur can interact with other nutrients that are applied at the same time, such as nitrogen, phosphorus, potassium and manganese.

Approximately 95 percent of the sulfur in soil is in the form of organic matter, which is broken down (mineralized) to sulfate-sulfur (SO4-S) over many years, depending on temperature and moisture levels. It is the SO4-S that is actually absorbed by plant roots. Once in the plant, sulfur is not transported easily.

Most sulfur is transported through the plant as a hitchhiker. When plants suffer from a lack of nitrogen, they deconstruct older leaves and tissues and transport the nitrogen-containing compounds to new growth areas or for use in seed production. Since some of these compounds also contain sulfur, the sulfur is transported to seeds as well. However, if plants have sufficient nitrogen, they do not transport the nitrogen-containing compounds, so the seeds may end up deficient in sulfur, and thus deficient in sulfur-containing amino acids. So having enough nitrogen can actually make sulfur deficiencies worse! 

Recent research has started untangling some of the other dependencies of sulfur needs, but the results to-date have been not been consistent. 

  • In Maryland, foliar spray on flowering soybeans resulted in improved protein quality (double the percentage of methionine and cysteine), as well as a 5-10 percent increase in yield, when treated with gypsum (calcium sulfate) or Epsom salts (magnesium sulfate). They are also experimenting with different methods of application – dry powder or spray rig. 
  • In 2018, trials in Tennessee with different levels, methods, and timing of sulfur application showed that only sulfur applied late in the growing season resulted in higher protein levels. 
  • In Indiana, ammonium sulfate and pelletized gypsum both significantly increased yields, regardless of the growth stage at which it was applied. Their conclusion was that reference strips of soluble sulfur should be applied as close to planting as possible, and the crops tracked over several years to identify any increases in yield and/or protein quality.
  • And in a greenhouse in Kazakhstan, it was the application form that made the difference — powder and soluble sulfur applications increased growth and yield, whereas a paste did not. 

So, what do researchers do when there are many studies, but the results disagree? They look at the very big picture and conduct a meta-analysis of the results of all the studies. Two such analyses have recently been completed by USB-funded researchers.

The first group, led by Dr. Ignacio Ciampitti of Kansas State University, looked at a total of 44 unique site-years over eight states and three growing seasons. Their analysis found applying sulfur caused increases in both seed protein concentration (when applied at sowing) and SAA — sulfur amino acids cysteine and methionine — concentration (regardless of application timing), but not under drought conditions, and only in fields with intermediate levels of soil sulfur and soil organic matter before the application.  The gains were all highly variable and site specific, indicating there may be other variables at work.

Dr. Anna M. Locke and her team at the USDA in North Carolina/Professor at North Carolina State University conducted the second study. They examined the effects of irrigation, nitrogen application, plant density, row spacing, sulfur application, and tillage on soybean yield, protein and oil concentration, and protein yield using data from 76 peer-reviewed journal articles. This review found that the largest increase in seed protein resulted from sulfur application. In fact, the application of sulfur fertilizer had a positive effect on protein and oil concentrations, as well as on yield. However, rainfall was high in many of the studies, possibly washing away existing sulfur, making the added sulfur more valuable.

We are just beginning to understand the complex relationships that affect how soybeans react to added sulfur, in order to predict what is likely to increase protein concentration and/or yield, and by how much, to ensure that the benefits outweigh the costs of the intervention. Sulfur applications are just starting to show promise, and the more we understand, the more and higher quality bushels we can produce.