Above photo: Dr. Bob Nielsen, Purdue Agronomist and Corn Specialist
In recent years, Purdue experts have studied the role of sulfur in growing crops, and whether adding sulfur to crops may be justified. Dr. Bob Nielsen, Purdue Extension corn specialist, recently offered insights as to what we know and what we don’t know about sulfur on growing crops.
For background and context, sulfur is an essential secondary macronutrient, not needed in the quantities that nitrogen, phosphorus or potassium are needed for crops. We began to notice sulfur deficiencies in crops as yields have increased, and as EPA mandates have cleaned up sulfur dioxide emissions from the air (“acid rain,” if you will). The result has been a precipitous drop in atmospheric deposition of sulfur over the last 30+ years.
“Not only have we cleaned up the power plant emissions, which is good, the unintended bad consequence for our corn and soybean crops is that we are getting less atmospheric deposition of sulfur, and so we’re not replacing what the crop is taking off as much as we were back 20, 30, 40 years ago.”
Sulfur is a component of chlorophyll, which is essential for photosynthesis. “When sulfur deficiency occurs, one of the symptoms we see is a yellowing of the crop canopy, because the chlorophyll is decreasing in response to inadequate levels of sulfur in the plant,” said Nielsen.
Specifically, Nielsen described sulfur deficiency as being more prevalent in the upper leaves of the plant, appearing as interveinal yellow-green leaf striping. He warned that other nutrient deficiencies may cause similar symptoms, so pre-side-dress tissue analysis is usually needed to confirm the symptoms.
“Corn removes about 0.05 to 0.10 lbs. of sulfur (S) per bushel of grain, and so a 200-bushel corn crop removes somewhere between 10 and 20 lbs. of S,” said Nielsen. We get some sulfur from mineralization of organic sulfur in the soil. “Depending on organic matter levels of the soil, somewhere between 2 and 6 pounds of sulfur usually become available to the crop every growing season.” Nielsen said we get the rest of the sulfur we need from what little atmospheric deposition we still receive, from fertilizers and soil amendments, and from incidental sulfur in other fertilizers.
Nielsen said that sandy and low organic matter soils are often correlated with S deficiency. Other contributors include excessively cold soils, excessively wet or dry soils, no-till, and continuous corn fields.
Nielsen reported the results of various trials conducted throughout the state, with most receiving sulfur via 2 side-dress treatments using ammonium thiosulfate (ATS). “One point I want to make is that while we’ve had locations with fairly dramatic yield responses to sulfur in corn, only 15 of the 40 trials we’ve conducted so far have been responsive to sulfur,” he said. Of the four trials conducted at Northeast Purdue Agricultural Center (NEPAC) in Whitley County, zero showed response to added sulfur.
Nielsen said that spacial variability within the field, based on soil types and other factors, can produce variable responses to added sulfur. Sulfur deficient areas can also result in delayed crop maturity.
Nielsen said they need more on-farm trials to help better identify and characterize areas in the state that are responsive to sulfur fertilization, and to better identify the predictive tools that identify the likelihood that supplemental sulfur fertilization will be beneficial. Farmers who would like to cooperate in these trials are urged to contact Nielsen and his research colleagues at Purdue University.
Find Nielsen’s voice-over video at https://extension.entm.purdue.edu/newsletters/pestandcrop/. You can also find the video on YouTube.