Soybean Yield and Tissue Response to Nitrogen Additions.

Biological N fixation is the most significant benefit that soybean (Glycine max (L.) Merr.) has over other grain crops. However, it is often overlooked that biological N fixation often does not meet the N requirement for ultra-high yielding soybean production. Historically, addition of N fertilizer to soybean produced in Mississippi has been minimal. The agronomic value of N fertilizer addition to soybean has not yet been determined in Mississippi or other areas in the Mid-South. Therefore our objective was to evaluate soybean biomass and grain yield response to supplemental N fertilization. A secondary objective was to determine the appropriate N fertilization rate and application timing if an N response was observed. In 2013 and 2014 research was established at the Delta Research and Extension Center, near Stoneville, MS to evaluate the value of N addition to soybean. In both years experiments were conducted on clay and very-fine sandy loam soil textures.  At each siteyear the experimental design was a split-plot. The whole plot consisted of N application timing (V4 or R2).  The subplot consisted of a three (N source) x four (N rate) factorial arrangement of treatments plus an unfertilized control. Subplot levels of N rate ranged from 0 to 179 kg N ha^-1 in 45 kg N ha^-1 increments. The three N sources were urea, ammonium sulfate, and environmentally smart nitrogen (ESN). Preliminary results from 2013 suggested no tissue concentration or soybean grain yield differences were observed among treatments for research conducted on very-fine sandy loam soils. For clay soil experiments, soybean grain yield was significantly influenced by only the main effect N source (p = 0.0323).  Averaged over N application rates, soybean yields were greatest when ESN was applied (4368 kg ha^-1), and lowest from the untreated control (3965 kg ha^-1).