Sulfur application questions: How much, when and where?

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For the last several years, many farmers in Iowa have learned how to recognize symptoms of sulfur deficiency by finding yellow striping on younger corn leaves. Discussions at coffee shops, field days and research conferences also suggest that sulfur applications may be needed to avoid yield losses for both corn and soybeans.

Recent sulfur problems in Iowa are not surprising for many agronomists because deposits from the air have decreased substantially during the last two decades. The use of other fertilizer sources and materials containing sulfur have substantially dropped. Higher corn yields and more stressful crop conditions have also likely fueled the demand in this nutrient.

The following are common questions farmers and agronomists ask:

• How often do crops in Iowa show visual symptoms of S deficiency?
• When and where are sulfur applications needed?
• What tools can be used to diagnose sulfur deficiency?
• What is the likelihood of positive or above break-even yield responses to first and second-year sulfur applications?

According to a 2009 Iowa State University study, six out of 11 on-farm experiments shows statistically significant corn yield responses to sulfur in central and northeast Iowa. (Click Here to view the whole article).

Results of a 2011 statewide nutrient benchmarking survey done by On-Farm Network participants suggest that at least 20 to 30 percent of corn and soybean fields might have some sulfur problems based on in-season tissue testing. (click here for more information about corn sulfur; Click here for more information about soybean sulfur).

A multi-year On-Farm Network study with SuperCal SO4 (dehydrate gypsum) suggests that about one-fourth of the 25 trials conducted in northeast Iowa had statistically significant corn yield responses. These were mostly observed in fields having sandy, high sloping areas with low soil organic matter content. Farmers who participated in these trials also reported severe early season corn sulfur deficiencies within some fields. (Click here for the Field Summary, Click here to view the Field Summary).

Studies also revealed that significant responses to sulfur in soybean fields were very uncommon across Iowa as well as the residual (second-year) effect of sulfur applications in both crops. Also, no consistent soybean yield responses to sulfur were found in studies conducted by University Minnesota researchers. (Click here to view the presentation)

During the 2014 On-Farm Network Conference, farmers and agronomists were advised to reduce the potential risk of yield loss from sulfur stress. One option is to scout corn fields for early symptoms. If the early season sulfur deficiency does not disappear rapidly, farmers should consider conducting a tissue test within at least two field areas: One that is clearly stressed and the other that does not show stress symptoms.

It is important for farmers to ask their agronomist or crop consultant to make interpretations for both sulfur and nitrogen tissue tests for the two sampling areas. Extremely high nitrogen tissue content and low sulfur tissue content are often good indicators of corn sulfur stress. (Click here to view the presentation)

If sulfur problems are detected, fertilizer sources such as ammonium thiosulfate (AMS), elemental sulfur, MicroEssential SZ (MEZS), SuperCal SO4 and others can be used as pre-plant or starter applications in a range of 20 to 30 pounds of sulfur per acre. Summaries of trials with SuperCal SO4 (dehydrate gypsum) can be found at the On-Farm Network on-line database of individual replicated strip trials (Click here to view the database).

Sulfur transformation in soils is very similar to nitrogen transformation (Click here to view the update). As a result, predicting sulfur availability and loss during a growing season is very difficult. However, using the On-Farm Network on-line resources should give farmers and agronomists more confidence in making better decisions to identify when and where to use sulfur fertilizer in crop production in Iowa.

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