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What is the most important nutrient for a soybean crop? Depending on who you ask, potassium, nitrogen or manganese might be the answers you receive, but it is really a trick question. There are 17 nutrients required by plants to carry out the essential biological functions that ultimately lead to a harvestable crop. While they are required in different amounts, your soybean crop craves them all. 

Essential nutrients include those that we think about frequently, like nitrogen, as well as micronutrients, including nickel, that are required in such miniscule amounts that we rarely consider supplying them as a fertilizer. Perhaps the better question to consider is, what is one of the most commonly overlooked nutrients that can make the difference between an average or an outstanding soybean yield? Although sulfur is increasingly common in corn fertility programs, it is just as important for a high yielding soybean crop.

What is the role of sulfur?
Plants, animals and humans would not be the same without proteins, one of the basic building blocks of life. In addition to being the machinery that enables biology, protein is also one of the main reasons why we grow soybeans. Nitrogen and sulfur are two key ingredients required to make proteins. Soybean plants do a great job of acquiring at least some of their nitrogen from symbiotic fixation with Rhizobia bacteria, but sulfur must be acquired from organic matter or from applied fertilizer.

How much sulfur does a soybean crop require?
Understanding how much sulfur is required to produce a bushel of soybeans ultimately helps design a fertilizer program to target the needs of a high yielding crop. In 2012 and 2013 Dr. Fred Below and his graduate students at the University of Illinois measured the uptake and partitioning of sulfur in soybean fields located near DeKalb and Champaign (Bender et al., 2015). Their study found that a 60 bu/A crop required approximately 17 pounds of sulfur in the plant at harvest, of which 10 pounds (59%) was removed with the grain.

In other words, for every bushel produced, approximately 0.28 pounds of sulfur were taken up by the plant. If you are striving for 100-bushel soybeans, nearly 30 pounds of sulfur needs to be present and available for plant uptake.

How much sulfur does the soil provide?
Sulfur is found in organic matter, and like nitrogen must mineralize (convert from organic to sulfate form) to become available for plant uptake. It is difficult to predict sulfur mineralization each year, but the University of Minnesota (Kaiser and Vetsch, 2020) suggests that three to five pounds of sulfur are released per year for each percentage point of soil organic matter. As a result, a representative central Illinois soil with approximately 3.5% organic matter might provide somewhere between 10.5 and 17.5 pounds of sulfur each season. Seventeen pounds of sulfur is adequate for the previously mentioned 60 bushel example, but is not likely to be adequate for higher yield goals. What can we do to supply the remaining sulfur?

Sulfur fertilizer sources and crop availability
There are several dry and liquid sulfur fertilizer options to consider, but the timing of when sulfur becomes available to the crop should also be considered. On the dry side, elemental sulfur (90% sulfur) is commonly available and cost effective but takes time to oxidize and become available in the sulfate form. Estimates of availability during the first season following application vary, but I take a conservative approach and suggest that no more than 25% of the applied sulfur is available during the first growing season. Ammonium sulfate (24% sulfur) is another great option to consider because the sulfur is immediately available to the crop as sulfate. The downside is that sulfate is readily lost through leaching, so spring broadcast applications are best if using ammonium sulfate. There is no need to worry about the nitrogen contained within ammonium sulfate. At the application rates likely used, based on sulfur need, the amount of nitrogen applied is not likely to negatively impact nodulation. The supplemental nitrogen might actually be a benefit, especially in high residue situations where early season soil nitrogen immobilization occurs. There are fewer options to consider on the liquid side, but ammonium thiosulfate (ATS) is widely available and can be included as a spring broadcast application.

Do not let sulfur be the limiting factor in your sulfur fertility program
Illinois soybean growers regularly apply phosphorus and potassium to their fields, while soybeans fix a portion of their nitrogen. High yield growers are increasingly turning their attention to micronutrient applications. Sulfur, as a secondary macronutrient, should not be overlooked as part of the recipe for success. It could make the difference for your yields in 2020.

References:
Bender RR, Haegele JW, Below FE. Modern soybean varieties’ nutrient uptake patterns. Better Crops with Plant Food 2015;99:7-10.
Kaiser DE, Vetsch JA. Sulfur for Minnesota soils. 2020. URL: https://extension.umn.edu/micro-and-secondary-macronutrients/sulfur-minnesota-soils. Last accessed: 03 April 2020.