Approximately 95 percent of the total sulfur in soils is found in organic matter. As soil organic matter breaks down, the S in the organic form mineralizes to sulfate-sulfur, the only form that plant roots can absorb. While we have an understanding of how sulfur reacts with crops, there still is a lot we don’t know about the forms of sulfur in soil over the growing season.
A recent study funded by AFREC compared elemental and sulfate-sulfur, applied in the fall and the spring in poorly drained soils to test the need for sulfur in soils with high organic matter. With this study, we found that there are instances when sulfur should be applied to soils with high organic matter, but that the amount of sulfur required is small.
Current sulfur guidelines for Minnesota suggest the potential for sulfur response is greatest when soil organic matter concentration in the top six inches is less than four percent. However, we have noticed fields with sulfur deficiencies in Western Minnesota that do fit this relationship and required further research.
The amount of plant available sulfur can be relatively low early in the growing season. Current hybrids with increased vigor can show striping that looks like sulfur deficiency early in the season. We’ve consistently seen symptoms typical of sulfur deficiency in Western Minnesota in a number of trials. If striping goes away by the V5 growth state corn yield has seldom been impacted.
Elemental sulfur is a stable form of sulfur in the soil that is not subject to leaching. In this form, it must oxidize to sulfate before corn uptake. This oxidation process is very slow when soil temperature is cool. Recent AFREC-funded research has found no differences between elemental versus sulfate forms of sulfur when applied in fall or spring for corn. Sulfate forms of sulfur are still a better bet to ensure there is plant available sulfur in the soil.
Compared to sulfur deficiencies in soils with low organic matter, early season deficiencies of S found on high organic matter soils in western Minnesota tend disappear over time where the upper corn leaves eventually appear dark green.
It is not uncommon to find striping on lower leaves in western Minnesota with severe striping early in the growing season. When this effect was severe enough to reduce yield, an application of five pounds of S at or before planting has been shown to be sufficient to ensure maximum corn yield.
Since sulfur is immobile in the plant, bottom leaves may show signs of striping while the upper leaves look green and normal. When this occurs, plant tissue analysis may not prove to help determine if sulfur was deficient in the field. Remember this when trying to diagnose if sulfur is deficient in your field.