July 7, 2011

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Soybean demand for water and nitrogen (N) is greatest after the R3 (beginning pod fill) growth stage. Soybean meets much of its N need from fixation of atmospheric N, but 50% of the N may come from the soil. Nitrogen fixation typically peaks during podset and early pod fill (R3), and ends during late pod fill. Soil application of N at R3 may enhance N uptake with no negative effect on N fixation.

Research results have not been consistent but generally indicate a low probability of yield response to N applied at R3-R4 when yields are less than 60 bu/ac; a response is more likely when yields are  more than 60 bu/ac. We conducted 56 trials supported by the Nebraska Soybean Board to determine whether N fertilization at R3 is economical where yields are more than 60 bu/ac.

Mean yield increases with 27 lb/ac N applied and more than 60 bu/ac yield were 2.5 bu/ac in south central Nebraska, 1.6 bu/ac in the northeast, and 0.3 bu/ac (not significant) in southeast Nebraska. There was no added yield from applying 54 lb/ac N compared with applying 27 lb/ac N or when adding 4.5 lb/ac sulfur (S).

Variations in soil properties and in leaf N and S concentrations were not related to yield or the response to applied N. Depending on the grain to fertilizer price ratio, fertigation of 25 lb/ac N at early pod fill may be profitable if the pivot is already equipped for fertigation.

Charles Wortmann, Extension Nutrient Management Specialist, Lincoln
Charles Shapiro, Extension Soils Scientist - Crop Nutrition, Haskell Agricultural Laboratory, Concord
Richard Ferguson, Extension Soils Specialist, Lincoln