Is sulfur a mobile nutrient? Carryover, soybean needs, and S fertilizer product differences

By: Daniel Kaiser, Extension nutrient management specialist

One thing I hear a lot from people is that sulfate, the form of sulfur available to plants, does not hang around in the soil from one year to the next. In most soils, the rate at which sulfur leaches is much slower compared to nitrogen in the nitrate form (the exception being sandy soils which can quickly leach most nutrients). While it is true that sulfate has a negative charge and can be leached, the rate of leaching really depends on the soil.

What does current research say about how sulfate carries over in Minnesota soils?

Currently, I have multiple research projects funded by both the Agricultural Fertilizer Research and Education Council (AFREC) and the Minnesota Soybean Research and Promotion Council studying how sulfur carries over in our soils.

In my AFREC-funded long-term sulfur trial, we applied sulfur over a period of four years in continuous corn. In year five, we divided the plots where sulfur was applied into two areas: one where sulfur applications continued and one where we stopped applying sulfur. Annual sulfur rates were 5, 10, and 20 pounds of sulfur per acre, comparing sulfate to elemental forms of sulfur fertilizer.

The 2026 growing season will be the eighth and final year of corn grown at these locations. What has been interesting at the Rosemount and Waseca locations is that application of 10 pounds of sulfate per acre for four years resulted in sulfur being carried over for at least two years, covering the corn crop’s sulfur needs for those two years. There appeared to be some loss in yield in the first year without sulfur with only five pounds of sulfur applied, which most likely is about what the corn crop needed at minimum at both locations each of the first four years. 

This is a good indicator about the carryover of sulfate in Minnesota soils and can explain some of the lack of responses in many of my research sites around 2015. While we were applying treatments on soils that should have been prone to sulfur deficiencies, many of the growers we were working with at the time were applying sulfur and it is likely that enough sulfur was carried over for the crop the year we were conducting research. You cannot expect to find a sulfur response on what should be a deficient soil if you do not know the rate and source of sulfur previously applied.

Will sulfur application benefit my soybean crop?

Soybeans do need sulfur to grow and develop seed. The question is not whether soybeans need sulfur, it is whether you should be applying it.

I have had several trials in Minnesota over the past 18 years and have only found a direct application of sulfur to benefit soybean grain yield in two of around 30 trials. In one of those trials, back in 2009, sulfur fertilizer was not applied by the farmer in the years before the trial, so there was no carryover of sulfur in the soil. The second trial was on an irrigated sand where the irrigation water had little to no sulfate present.

Two current and one past trial looking at sulfur applied ahead of corn in a two-year corn-soybean rotation showed that application of 20 pounds of sulfur per acre, regardless of the source, was enough to benefit both the corn and soybean crop. When applied only to the corn, there was always an increase in sulfur concentration of soybean leaf tissue and soybean yield was less without sulfur starting in year four at each location. It was common to measure any response to sulfur for the first two years of the studies, which I attribute to the crops in the first rotation cleaning out any residual sulfur left from previous fertilizer applications. In six years of trials, it was common for corn to respond to sulfur fertilizer treatments in years three and five and then see differences in soybean grain yield in years four and six.

Soybean will benefit from sulfur but not sulfur applied directly to them. I have actually seen instances where too much sulfur fertilizer can result in a slight depression in yield, which I attribute to fertilizer applied directly ahead of the soybean crop increasing vegetative biomass of the crop. Too much biomass can lead to greater disease pressure and may increase plant water requirements.

If I am applying sulfate, is there a best time of year to apply it?

This question can be taken two ways. First, I really suggest applying sulfur to crops that will respond to it the most. Corn, alfalfa, and canola are three high demand crops and will take advantage of sulfur fertilizer applications the most. Second, I still get questions about application of sulfate in the fall. While there are soils where I would not suggest fall application of sulfate, if most of my data shows carryover of sulfate applied in the spring ahead of corn to the next years’ soybean crop on medium to fine textured soils, it is likely that sulfate would be available in the spring if applied the previous fall for a corn crop.

Some growers have opted for elemental sulfur for fall application but there are a few things to consider if applying elemental sulfur to ensure that enough sulfate will be oxidized to meet crop needs. In several studies looking at sulfur-bentonite products like the Tiger-Sul products, there have been mixed results depending on when incorporation of the products occurs relative to when the product is applied. Neither elemental sulfur nor the bentonite (clay) are water soluble, so immediate incorporation of the product (even with band application) can results in poor oxidation of elemental sulfur.

In my long-term trials with spring application of Tiger-90, we immediately incorporated the product into the soil and I have found poor availability of sulfur. In my long-term trials, it has taken five to six years for the same rate of sulfur applied as Tiger-90 to yield the same as an annual application of a sulfate source of sulfur. The available sulfur from Tiger-90 all appeared to be coming from what was applied the previous year.

I would suggest that anyone using a sulfur-bentonite fertilizer source not incorporate the material for at least two weeks after application, which should help the material to disperse with the tillage pass. Or, if you are incorporating the material within two weeks after application, it may be worth applying an additional source of sulfate or thiosulfate in the spring for a few years until there is enough time for the previous sulfur applications to become available. Sources of elemental sulfur such as MicroEssentials or MST have greater availability. The addition of the bentonite as the dispersal agent is the bigger issue for availability of these products.

The good news is that any source of sulfur will work given enough time, but knowing how and when different sources become available is key to ensuring that your crops will not suffer a deficiency.

Thank you to the Agricultural Fertilizer Research and Education Council (AFREC) and the Minnesota Soybean Research and Promotion Council for funding these research projects.

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