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I. Optimum sulfur application for soybean (Glycine max) in Oklahoma

Wehmeyer , Gwendolyn Bess
Soybean (Glycine max) are a vital legume crop present in the United States with substantial nutritional value and oil content. In 2017, 36,421,707 ha of soybean were harvested in the US (USDA, 2017). As soybean markets increase and stabilize, input strategies become exceedingly beneficial to further increase yield and monetary oil content potential for producers. Various articles indicate that the addition of sulfur (S) can assist in the synthesis of amino acids and proteins, and also increase nitrogen (N) fixation, further increasing oil content for oilseeds. The objective of this study was to evaluate the effect of S application rate and timing on soybean to further increase oil content and grain protein. The application of fertilizer nitrogen (N) either before planting and/or during the growing season impacts final grain yield differently depending on the environment. Oklahoma State University initiated a long-term winter wheat trial in 2002, to evaluate a combination of preplant and topdress rate on wheat grain yield and N response. This long-term field experiment is located at Lake Carl Blackwell, Oklahoma. Treatments included combinations of all N applied preplant and added sidedress applications in February and March. Total N rates ranged from 0 to 150 kg N ha-1. Preplant N applications were made using ammonium nitrate (34-0-0) (N-P-K). Midseason February and March applications used urea ammonium nitrate (UAN) (28-0-0) as the N source. For all years, winter wheat was planted in October and harvested the following July. From 2002 to 2018, grain yield, nitrogen response, and normalized difference vegetative index (NDVI) were analyzed to decipher the optimum preplant and topdress N combinations that would maximize wheat grain yields, over a wide range of environments (years). It was clear over the ten years where yield data was collected, split N applications resulted in consistently higher yields.