- Sulfur (S) deficiency is becoming more common in areas of fields with lower organic matter.
- Sulfur deficiency symptoms often disappear as soils warm and S mineralization increases.
- Corn response to sulfur will be less likely in high organic matter soils.
Applying nitrogen (N) in the fall or spring before planting is a good option for working towards a sufficient supply of N for plant growth, but much of this applied N can be lost before corn needs it most. On the other hand, applying excess N is an inefficient use of time and money and has negative environmental effects. Monitoring soil N availability and adding supplemental in-season N, when needed, can help minimize lost yield potential and prevent unintended consequences from over-application. Soil sampling-based recommendations and N application decision support tools are 2 options for predicting and addressing in-season nitrogen needs.
Sulfur (S) is another soil nutrient that can be in short supply as atmospheric S deposition has decreased. S is naturally present in organic matter, and like organic N, it must be mineralized before becoming available to plants. Ammonium thiosulfate (ATS) is a form of S that is easily incorporated into either starter or sidedress N fertilizers.
2020 Assessment of In-Season Fertilizer Application
The Golden Harvest® Agronomy in Action Research team conducted trials at 8 locations in 2020 to test response to sidedress N applications and any possible advantages of including S in the form of ATS. 2 IA locations were lost to storm damage, so results focus on the Bridgewater, SD, Seward, NE, Clay Center, KS, Sac City, IA, Oregon, IL, and Clinton, IL, trial sites.
Each trial consisted of 5 treatments testing N rates applied at V5-V6, S application (20 lbs./A), and N recommendations:
- No in-season N application
- 50 lbs. N
- 50 lbs. N + 20 lbs. S (ATS and urea ammonium nitrate [UAN])
- 75 lbs. N
- 75 lbs. N + 20 lbs. S (ATS and UAN)
All sidedress applications were in addition to nitrogen applied to the entire trial prior to planting (Table 1).
2020 Nitrogen Response
Hybrid response to 50 or 75 lbs./A of nitrogen was inconsistent across the 6 trial locations harvested in 2020. Extreme variability within many locations left much of the data unusable. Hybrids receiving an additional 75 lbs. of nitrogen at Clinton significantly increased yield by 13-16 bu/A (Graph 1). Smaller increases of 7-8 bu/A were seen with the 50 lbs. N rate. Numerical increase of 6.5-10.1 and 16.5-19.1 bu/A were also seen with sidedress applications of 50 and 75 lbs./A N, respectively, at Clay Center (Graph 2). No other locations consistently responded to in-season N.
2020 Sulfur Response
Bridgewater was the only site with any type of response to adding sulfur when sidedressing N. Although responses were not statistically significant at Bridgewater, similar increases of 7.8 and 12.6 bu/A were observed when S was applied with either the 50 or 75 lbs. N rate, respectively (Graph 3). The additional N did not appear to have any effect on yield at this location.
Approximately 95% of the total sulfur in soils comes from organic matter. Due to this, responses to S are observed less frequently in soils with organic matter levels greater than 4%. There are still instances where small responses can be observed in higher organic matter soils. Sulfur must mineralize from its elemental form into sulfate before plants are able to take it up from the soil. The process of mineralization is known to slow in cooler soil temperatures and thus, signs of deficiency, such as yellow striping, may be seen with young plants but not visible with later growth as soils warm and S mineralization rates increase.
Due to relatively high soil organic matter levels at the 2020 test sites, it is likely there were sufficient S levels already available to the plant. S will also be less readily available in soils with low or high pH levels, and like nitrate, once mineralized to the sulfate form, S can easily leach. Although there is no established definition of the optimum soil test sulfur level, none of the trial sites tested below 7 ppm.
Other studies across the Midwest on lower organic matter soils known to be S-deficient have found sidedressing additional S may benefit corn yield. With roughly 34 lbs. of S removed in a 200-bushel corn crop and the reduced amounts of S being delivered from the atmosphere, more response to sidedressed S is expected over time.
Scouting and documenting field areas that appear yellow, and confirming it is due to sulfur deficiency through tissue sampling, may help identify fields more likely to see a response to sidedressed S. ATS is readily available and may be easily applied with sidedressed nitrogen. Targeting lower organic matter soils and applying test strips within fields may be another good method to identify responsive fields to ensure a good return on the sidedress investment.
For more insight into N and S applications in corn, contact your Golden Harvest Seed Advisor.
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