Sulfur Fertilizer - It Depends
In a time of record high fertilizer and commodity prices, should you apply sulfur (S) fertilizer? Well, an economist would say, "It depends." As an agronomist, I would have to agree with them. Some critical factors for applying S fertilizer are S soil test level, soil type, yield goal for the crop, amount of nitrogen fertilizer being applied and the amount of S obtained from rain, irrigation and other sources.
Sulfur recommendations in Oklahoma are based on supplying a ratio of 20:1 nitrogen (N) to sulfur. A soil test will determine the amount of S available in the soil. Assuming the soil samples received at the Noble Foundation represent the soils in our service area, we can make some general observations from the results. Of the 1,436 soil samples tested for S, the median test result for S is 16 pounds per acre. This means, keeping the 20:1 ratio, that 50 percent of the time there is enough S to match 320 pounds per acre of actual N. But let's look further. If you apply 100 pounds per acre actual N, you need five pounds per acre S. Only 0.2 percent of the soils we tested had less than that amount of sulfur. Furthermore, we can expect about five pounds per acre per year of S from incidental additions such as rainfall. Table 1 illustrates the percent of our soil sample results that would require additional S at a given N rate.
So, when are producers in the Noble Foundation service area likely to need S fertilizer? Quite frankly, it is rare. One possible scenario is on very sandy soil with very low organic matter. Another scenario is when applying over 200 pounds per acre of actual N or growing very high yielding legumes.
What if you don't have a soil test? A forage test can be used. Sulfur dry matter tissue concentration of 0.2 percent or higher is acceptable. Of the 2,890 forage samples we have received at the Noble Foundation and tested for S, 39 percent have a dry matter tissue concentration of less than 0.2 percent which means 61 percent of these forages would not be expected to respond to additional S.
So again, the answer to whether or not to apply S fertilizer is "it depends." If you are on sandy soil, have low organic matter, have high yield goals or are applying high rates of fertilizer N, you may need additional S. Take the time to get a soil or forage test including S, and then visit with a soil and crops consultant.