A wet and warm winter followed by a wet spring (at least in the western half of Iowa) has many producers concerned about the amount of nitrogen left for the corn crop, but what about soybeans? This topic at first glance would be dismissed by most agronomists and growers alike since soybean fix their own nitrogen. However, on a per acre basis soybeans may actually need more nitrogen than corn. So that brings up the question: Do soybeans need supplemental applications of nitrogen?

First, consider some of the facts about soybeans and nitrogen consumption. Soybeans need about five pounds of nitrogen per bushel of grain produced and about 75 percent of that is removed during harvest. To raise a 65-bushel soybean crop, we will need about 325 lbs. of nitrogen per acre. Where does all this nitrogen come from? It mainly comes from three different sources. The first is a symbiotic relationship with Bradyrhizobium bacteria which infects the soybean root and forms nodules. The soybean plant provides sugars to the bacteria and in turn the bacteria fix nitrogen from the atmosphere and provide it to the soybean plant for growth. It is estimated that 50 to 60 percent of the soybean nitrogen needs come from this. The second source of nitrogen is atmospheric deposition which is a very small part. The rest of the nitrogen comes from the soil, either through mineralization or from what is left from the previous crop.

With only 50 to 60 percent of the soybean’s overall nitrogen needs coming from biological nitrogen fixation, one could argue a point of concern for soybeans. Research from Iowa State has shown that nitrogen applications at planting do not increase yield and can reduce the formation of nodules, thereby limiting nitrogen production later in the growing season. Recent research by Dr. Emerson Nafziger at the University of Illinois saw very little response to in-season applications of urea and ESN usually applied around July to soybean. Dr. Nafziger also reported that there was no relationship between yield level and response to fertilizer N. Conversely, research from Kansas State reported a yield response to soybeans from mid-season application of urea to high yield soybean under irrigation and nitrogen stressed soybeans in low yield environments. Research from the University of Nebraska also reported a better chance of a positive yield increase when applying nitrogen in a high yield soybean environment.  

Overall, nitrogen application to soybeans has not become a widely recommended practice due to the inconsistent yield results, limited number of positive trials, and low chance of an economic return. There are instances where nitrogen application to soybeans has produced positive results, but it seems to be mainly contained to very high yielding environments and low yielding environments. One should implement strip trials first if this becomes something of interest.

Originally published in Spokesman. Tim Berkland serves the FS System as GROWMARK’s strategic agronomy marketing manager. He can be reached at [email protected].