Sulfur (S) assists crops in forming beneficial amino acids, proteins and oils. It’s also needed to produce chlorophyll.
In the past, crops were able to utilize atmospheric S. An unintended effect of anti-pollution efforts is the reduction of levels of atmospheric S. Consequently, crops now often need S supplementation.
Dr. Dan Kaiser is an associate professor in the University of Minnesota’s Department of Soil, Water & Climate. He wants farmers to understand how important S is, especially since so many place a great deal of emphasis on nitrogen.
“If we don’t have enough sulfur, the plant can’t build the right proteins, no matter how much nitrogen you apply,” he said.
“There are two main sources of sulfur,” Kaiser explained. “Sulfate forms include ammonium sulfate or calcium sulfate, which is gypsum. There’s also elemental sulfur, which comes in multiple packages. I caution growers that it takes a while for elemental sulfur to oxidize or become plant available.”
He encouraged producers to focus on the more readily available sulfate forms like ammonium, potassium sulfate or calcium sulfate, since they will provide S directly to plants without needing further conversion.
Conversely, elemental S needs to be oxidized by soil microbes before it can be utilized by crops – an often-slow process, subject to fluctuations in soil temperature. Kaiser said it might be prudent to combine a sulfate source with elemental S to optimize S availability throughout the growing season.
Dr. Dan Kaiser
Kaiser’s research highlighted several key points for farmers when considering S applications:
- Sulfate forms are preferred – Opt for sulfate-based fertilizers like ammonium sulfate, potassium sulfate or calcium sulfate as they provide readily available S to the plant.
- Elemental S limitations – While elemental S can be a cost-effective option, it needs to be oxidized by soil microbes to become plant-available, which can be a slow process, especially in cool weather.
- Consider application timing – If applying S in autumn, elemental S might be suitable as it can oxidize over winter, but for spring applications, prioritize readily available sulfate forms to ensure S is available when the crop needs it most.
- Know your crop – Target S application to crops that are more sensitive to S deficiency. Kaiser’s research found that targeting fields in a corn year delivers a higher chance of return, as some S is carried over for the soybean year.
- Soil conditions matter – Soil type and temperature can impact how quickly elemental S is converted to sulfate, so consider these factors when making your selection.
- Know your soil – “Understand what’s in your soil and what type it is,” said Kaiser. “Determine if atmospheric deposition of sulfur can help meet sulfur requirements. If you irrigate, learn how much sulfate is in the water. These affect how plants respond to available sulfur in the soil, and they’ll help you determine if you need to add the nutrient.”
Remember to regularly check the level of S in your soil profile regularly. Go deeper than six inches, because there can be accumulation of S in heavier clay at 12 or 18 inches and the roots are going to get to it. Simply testing the first six inches of soil won’t give you the whole picture.
- Know where to place your S – Kaiser warned that most starter fertilizer options can possibly cause issues with seed placement. “You can’t apply in furrow,” he said. “Any of your sulfate sources, even a potassium thiosulfate, can damage seedlings. Source and placement must factor into your application method.”
Finally, Kaiser urged farmers to be mindful when calculating the amount of S to add to their soil. He said most crops will remove about 25 lbs./acre of S, but two-thirds to three-quarters of that amount will come from the soil, so applying 25 lbs./acre is likely too much.
by Enrico Villamaino
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