Irrigated Soybean Response to Nitrogen Applied During Early Reproductive GROWTH.

High yield soybean (Glycine max L.) has a high rate of N uptake during grain fill which may exceed N available from biological N fixation and mineralization of soil organic N. Other research findings show a low probability of response to N applied at early pod formation when soybean grain yield is below 4 Mg ha^-1 but less research has been conducted for higher yield soybean and with inconsistent results. The effect of applying N and S to the soil at early pod was determined for eastern and south central Nebraska. Fifty six irrigated trials were conducted and 44 had mean yield above 4 Mg ha^-1. Mean yield increases with 25 kg ha^-1 N applied were small or not significant for soybean with above 4 Mg ha^-1 yield and were 0.17 Mg ha^-1 in south central, 0.10 Mg ha^-1 in northeast, and not significant in southeast Nebraska. There was no added yield with applying 54 compared with 25 kg ha^-1 N or with the addition of 5 kg ha^-1 S. Variations in soil properties and in leaf N and S concentrations were not related to yield or to the response to applied N. Depending on the grain to fertilizer price ratio, application of N at early pod formation may be profitable, especially if the cost of application is low such as through fertigation.