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Sulfur Influence on Soybean Yield and Grain Protein Level

Categories: GROWING, SOYBEANS
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  • Decreased atmospheric sulfate deposition is resulting in more frequent corn and soybean sulfur (S) deficiencies.
  • Soybean yields can significantly decline when the plant-available form of sulfate-sulfur is limited.
  • Correcting sulfur deficiencies can also improve soybean grain protein levels.

Introduction

S is 1 of the 16 essential elements and 1 of 3 secondary macronutrients for crop production. S deficiency in young plants often appears as yellowing of leaves and is more pronounced in new growth due to the nutrient’s immobility within the plant. S has recently started to become yield-limiting in many geographies, as atmospheric S deposition has decreased with improved air quality standards and as crop removal rates have increased with yields.

S mineralizes from organic matter in the soil into sulfate which makes it more subject to leaching, similarly to nitrate nitrogen (N). Deficiencies are often noticed in coarse, eroded or low organic matter soils that are less able to mineralize the plant-available sulfate form. Mineralization will often slow with cool soil conditions, sometimes making soils that otherwise test high in S, show deficiency symptoms until warming and sulfate mineralization speeds up. Due to this, soil testing procedures for S are often unreliable and typically only recommended for use on sandy soils. Plant tissue samples are often needed to differentiate from other nutrient deficiencies.

S is also a component of amino acids cysteine and methionine, and it is essential for protein synthesis in plants. Grain protein levels are dependent on both N and S availability to the plant. Soybeans are a high protein grain that are processed for oil and commonly used in animal feed. The high level of protein and energy supplied from soybean meal is an essential feed component in livestock production. Increasing the nutritional feed value of soybeans could be useful in meeting rising demand for protein in livestock production.

Figure 1. Comparing soybeans with 20 lbs./A of S (right) to no S (left) at Geneseo, IL. 2021

Increased awareness of managing S needs in corn has created interest in better understanding soybean response to S. In addition to yield improvement, many grain end-use markets are interested in exploring new ways to increase soybean protein levels. The Golden Harvest® Agronomy in Action team established trials in 2021 to determine if S fertilizer could improve grain yield and protein content in soybeans.

Materials and Methods

Trials were established at 9 locations across IL, IA, KS, NE and SD to understand if soybean yield and protein content could be influenced by applications of S. Ammonium thiosulfate (ATS) 12-0-0-26S, a form of S that is easily applied in a liquid form, was used to supply 20 lbs./A of sulfate at the time of planting as a surface dribble 3 in. to each side of the row (Figure 1). Non-S treated plots were treated with 9 lbs./A of N in the form of urea nitrate (UAN) using the same application method and timing to provide an equivalent amount of N as was applied to the ATS-treated plots. Treatments were applied to 4 soybean varieties at each location to measure any potential response interactions. All treatments were replicated 4 times within each location. Research combines were used at harvest to collect grain analysis samples, grain moisture and determine yield.

Trial Yield Response to Sulfur

Changes in soybean yield from ATS applications ranged from no response at several locations to as much as 16 bu/A depending on the location. Of the 9 locations, the Geneseo, IL, and Clay Center, KS, sites observed the largest yield increases of 16 and 8 bu/A respectively (Graph 1). Soil test S results for both responsive sites  were in the low to very low S category. Yield responses at other sites also testing in the low category (<14 ppm) were much less responsive (0-1.1 bu/A), illustrating the challenges of predicting S response based solely on soil test results. Yield response to S also varied by variety planted within each location. Golden Harvest soybean varieties GH3475X, GH3732X and GH3934X were all highly responsive to ATS applications at Clay Center, KS, averaging an additional 8 or more bu/A than non-S treatments (Graph 2). Golden Harvest soybean variety GH3546X was less responsive at Clay Center, but still yielded 3.2 bu/A better with ATS applications. Averaged across 6 locations with relative maturity (RM) ranging from 2.7-3.0, Golden Harvest soybean varieties GH2722XF, GH2788X and GH3088X yielded 2.2 to 5.5% better as result of ATS applications, whereas Golden Harvest soybean variety GH2872XF at the same locations was not responsive to S (Graph 3). Early RM varieties, ranging from 2.1-2.5 planted at Sac City, IA, did not have a statistically significant response, although Golden Harvest soybean varieties GH2102XF and GH2329X yielded 3-5% better with ATS applications (Graph 4). Golden Harvest soybean varieties GH2230X and GH2552X were not responsive at the same location. 

Graph 1. Individual trial location response to S compared to site sulfate test results 2021
   
Graph 2.
   
Graph 3.
   
Graph 4.

Sulfur Effect on Grain

Due to the involvement of S in protein synthesis, grain samples were also collected and analyzed using near-infrared (NIR) spectroscopy to understand if S applications changed grain protein, oil and S content. S applications increased grain S content at 4 of the 9 trial sites (Graph 5). Of the 4 locations, the Geneseo, IL, site was the only site with a yield increase. Even though yields increased at Clay Center, KS, grain components were unchanged. Increases in S uptake at 3 locations not observing a yield increase indicate S availability was likely limited there, just not limiting enough to influence yield. In addition to S, grain protein levels increased in all varieties planted at the Geneseo, IL, Elwood, IL, and Keystone, IA, sites (Graph 6) even though yield responses were not observed at all of these same locations. Grain oil levels consequently decreased at Elwood and Geneseo in response to large increases in protein level from ATS applications (Graph 7).

Graph 5.
   
Graph 6.
   
Graph 7.

Summary

Soybean S needs should be considered when plant-available S is a yield-limiting factor. S deficiencies at 2 locations resulted in significant yield responses from ATS applications, proving that S management is equally important in soybeans when limited. Non­responsive sites testing low in soil S levels likely overcame deficiency through adequate in-season sulfate mineralization occurring as result of higher organic matter and warming soil temperatures. Both Geneseo, IL, and Clay Center, KS, the 2 most responsive sites, had much lower soil organic matter levels than all other sites.

Grain protein levels were positively influenced by applications of S, where limited. S applications should be considered as a means of improving grain protein levels to improve feed value when S deficiency is suspected.

S deficiency has become more common in crop production today largely due to reduced atmospheric deposition as a result of the evolving environmental emission standards. As remaining soil S levels continue to deplete, S deficiencies are likely to become more common. Monitor fields closely with coarse, eroded or low organic matter soils for signs of S deficiency and build nutrient plans around those suspected of being insufficient. Observations can be confirmed with leaf tissue samples to verify deficient fields.

To learn more about our 2021 trial on soybean sulfur fertilization treatments, watch this video.

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