Prior to 1990, significant quantities of sulfur were deposited on crop fields through rainfall. In Fond du Lac County, this amounted to about 15 pounds per acre per year. Amendments to the Clean Air Act in 1990 called for, among other things, a large reduction of sulfur pollutants through the use of industrial sulfur “scrubbers” and the burning of low sulfur coal in power plants. 

          Current local precipitation analyses now indicate about 7 pounds of sulfur are deposited per acre per year, a reduction of over 50 percent in the past 20 years.                 The ramifications for crop fertility programs are not fully known, but recent University of Wisconsin studies and an increase in sulfur deficiency observations point to the fact that producers need to know where their fields stand from a sulfur sufficiency standpoint.

          Over the past two years, samples of alfalfa plants were taken across the state to analyze plant tissue for sulfur concentration. In total, 73 samples were collected from 21 different counties. Plant tissue sulfur concentrations were considered low (less than 0.25 percent sulfur) in 63 percent of these samples. Interestingly, most of the fields sampled appeared normal in appearance (i.e. didn’t show visual signs of nutrient deficiency).

          A similar plant analysis survey had been conducted in 2000 and 2001. In that study, only 38 percent of the samples were in the low sulfur range.

          Deficiency of sulfur in alfalfa is characterized by light green and stunted plants with thin stems and delayed maturity. In corn, a general chlorotic (yellowing) takes place, similar to nitrogen deficiency. Often plant leaf veins remain green. 

          Sulfur, like nitrogen, is a mobile nutrient in the soil. This makes a standard soil test for sulfur somewhat less precise compared to those for phosphorus and potassium. Sources of sulfur not directly measured by the soil test may contribute significantly to crop requirements. In addition to precipitation, other plant sources of sulfur include organic matter, the subsoil, and manure.

          Given sulfur’s mobility and its multiple sources, actual sulfur availability can vary from field to field and even within areas of fields. Currently, plant tissue analysis is the best method to confirm sufficiency.

          Two crops that require relatively large amounts of sulfur are alfalfa and corn harvested as silage. A 4 ton per acre alfalfa crop removes about 23 pounds per acre of sulfur. Corn silage can remove 25 pounds or more of sulfur per acre. The increase of corn silage acres over the past twenty years and the increased yields obtained from both crops may also be contributing to less plant available sulfur.

          As we move forward through this decade, sulfur fertility will require more attention. If significant amounts of manure are applied to a field on a routine basis, sulfur likely won’t be an issue. However, in situations where little or no manure is applied or fields are low in organic matter, it’s likely sulfur will be deficient unless commercial sources are applied. The days of relying on precipitation as a major sulfur nutrient source are over. The Clean Air Act is working.

          The bottom line to this story is simple:  Don’t assume plant sulfur status is sufficient and don’t assume it’s deficient. Invest a few greenbacks in plant tissue analysis to know for sure.