Soybean Yield and Biomass Response to Nitrogen Additions.

Biological N fixation is the most significant benefit that soybean (Glycine max (L.) Merr.) has over other grain crops. 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. 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 2014 and 2015 research was established at the Delta Research and Extension Center 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). Results from 2014 suggested no biomass differences were observed among treatments for research conducted on very-fine sandy loam soils or clay soil sites. Soybean grain yield was significantly influenced utilizing alpha = .1  by only the main effect N source at both soil sites, very-fine sandy loam (p = 0.0783) and Tunica Clay (p = 0.0507).  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).