Growers have been asking about applying nitrogen (N) to soybeans. Soybeans have been shown to require more nitrogen than corn; roughly 5 lbs. per bushel, while corn requires only 1 lb. And 60-bushel soybeans need 300 to 350 lbs. of available N, about twice that of 200-bushel corn.

Nitrogen can be applied to soybeans preplant, top dress or foliar if there is a need, but often biological nitrogen fixation by the plant and supplies in the soil provide enough N for the average crop. Supplemental applications often show no benefit and results are so inconsistent that it is difficult to make a trusted recommendation.

A recent study¹ published in the Agronomy Journal states what most agronomists now believe; that the plant and soil can provide enough N for 55 to 60 bushels, but for high yields above 60 bushels, fixation won’t be enough to fill the last seeds. This is because nodule activity decreases exponentially toward the end of the soybean plant lifecycle, when pods are filling.

The Agronomy Journal study stated that fixation wasn’t great enough to meet the requirements of a high-yield soybean crop under no-till conditions. It reported that foliar N application at the R3 (beginning of pod) to R4 (full pod) stages may be needed to fill out the last seeds of the upper-third of the plant as fixation and soil mineralization wanes.

The study also reported that foliar N was an efficient method of improving the yield in N-limiting situations, but any increase in yield may not be enough to offset the additional cost of fertilizer, even though application rates may only range from 5 to 10 lbs. of actual N per acre.

The results showed that foliar N applications may sometimes increase soybean yield under N-limiting conditions. The challenge is knowing what the yield potential will be and when N may become a limiting factor.

So, the debate about whether to apply supplement N to soybeans continues.

¹ Moreira A, Moraes LAC, Schroth G, Becker FJ, Mandarino JMG. Soybean yield and nutritional status response to nitrogen source and rate of foliar fertilization. Agron J 2017;109(2):629-635. Available at dx.doi.org/doi:10.2134/agronj2016.04.0199. 

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.