As higher soybean yields become more common due to improvements in genetics and management practices, nitrogen additions may be needed to maximize potential yields, according to DuPont Pioneer, although 2015 research does little to support the added cost of nitrogen.
A nitrogen “budget” developed from numerous research studies shows that soil and fixed nitrogen are generally sufficient to supply nitrogen needs at yields up to 60 bushels per acre. As yields increase to 80 bushels per acre and higher, a nitrogen deficit may result, suggests Pioneer researchers who have conducted recent research.
An experiment was conducted in Johnston, Iowa, in 2015 to evaluate yield response of two Pioneer brand soybean varieties to nitrogen fertilizer applied at the R2 growth stage. The researched used varieties P93M11 and P25T51. Nitrogen applied were zero, 25 pounds and 50 pounds per acre. The N was hand-applied as ammonium nitrate at R2 growth stage, which is full flowering.
The average yield of P25T51 was significantly greater (+2.4 bu/acre) than that of 93M11.
Application of 50 lbs/acre of nitrogen significantly increased average soybean yield relative to the non-treated check (+4.8 bu/acre).
No significant effect on soybean yield relative to the non-treated check was observed with the 25 lbs/acre nitrogen application.
The yield effect of nitrogen treatment did not significantly differ between soybean varieties.
It is notable that yield levels in this study were below the range where a nitrogen deficit might be expected based on previous research, but a significant yield increase with nitrogen application was still observed.