|Abstract:||For optimal productivity, soybean [Glycine max (L.) Merr.] may require fertilizer nitrogen (N) to supplement biological N fixation. The objective of this study was to identify the best fertilizer source and the best plant growth stage for N applications to increase soybean yield using N fertilization. Trials were established at three locations in Illinois during three consecutive years for a total of nine site-years. In 2014, five different N sources were supplied and in 2015 and 2016, seven different N sources were supplied at one of four different soybean growth stages (preplant, V3, R1, and R3) at an application rate of 112 kg N ha-1 (100 lb N ac-1). The seven different N sources evaluated were: ammonium nitrate (AN, 34-0-0), ammonium sulfate (21-0-0-24S), ESN (environmentally-smart nitrogen, 44-0-0), urea + Limus (urea treated with the urease inhibitor Limus, 46-0-0), liquid urea-ammonium nitrate (UAN, 28-0-0), urea (46-0-0), and a mixture of ammonium nitrate, potassium nitrate, and ammonium sulfate (30-0-7-2S), along with an unfertilized control. When averaged across all locations, significant increases in yield occurred when N was supplied during the early growth stages (preplant and V3) in 2014 and 2016. While in 2015, significant increases in yield were more apparent from N applications during the reproductive growth stages (R1 and R3). Fertilizing with AN or ESN at preplant produced the most consistent yield increases over the nine site-years. However, when examining the individual locations, variation in N source and the application time that gave the greatest yield increase was evident, suggesting that yield increases are dependent upon a given location, N source, and/or application time.