Soybeans fix their own nitrogen, but high-yielding beans need a lot of nitrogen – 4 to 5 pounds per bushel total with some 3 pounds per bushel in the harvested seed, according to Emerson Nafziger, University of Illinois Department of Crop Sciences.
“That’s why some people try applying nitrogen to boost yields,” he says. “You would think if you put on 100 to 200 pounds per acre, there would be a response.”
Not so, say leading soybean experts who have tested the idea in university trials.
In a study of high-input systems in Arkansas, Iowa, Kentucky, Louisiana, Michigan, and Minnesota, “we haven’t seen any response to nitrogen in any of our locations,” reports Seth Naeve, University of Minnesota Extension agronomist.
“We put on 100 or 150 pounds nitrogen, which wouldn’t be economical, just to see if we could get a response, but it was minimal,” Naeve says. “My advice to farmers would be not to bother with nitrogen on soybeans. Even if it does work, the amount of nitrogen you need means your chances of an economic return are near zero.”
At Purdue University’s Department of Agronomy, Shaun Casteel, has seen no consistency in soybeans’ response to N. “We overloaded the system to see if the genetic potential was limited by the nitrogen supply. Yield responses (about 10 to 12 bushels per acre) were promising, but not economical with maturity Group 3 beans in Illinois and Indiana but not with maturity Group 2 in Wisconsin and Minnesota,” he says.
“We’ve seen some seasonal greenness and greater leaf retention with the non-limiting supply of N,” Casteel says. “We’re trying to build the nitrogen reservoir (leaf retention) without inhibiting the plant’s nitrogen-fixing bacteria, but that is walking a narrow line. “I would say don’t do nitrogen on beans yet unless you just want to play around.”
Want more news from CSD? Subscribe to CSD Extra  and get the latest news right to your inbox!
Nafziger reports similar results: “In the soils in Iowa and Illinois we just don’t find a yield response. We don’t see that many plants around that look like they need nitrogen, and even if we had a deficiency develop, it’s hard to get it to the crop when it needs it.”
Nafziger’s latest work, funded in part by the Illinois Soybean Association, looked at a variety of inputs (foliar fertilizer, foliar insecticide, lactofen herbicide, seed treatment, foliar fungicide, cytokinin, and nitrogen fertilizer) that might increase soybean yields. “With nitrogen most times we didn’t boost yield at all. And it’s not a matter of getting a response only when yield potential is high. In 2013 we had some 90 bushel beans that didn’t respond.”
His recommendation for farmers echoes Casteel’s: “If they are interested, farmers could do this in strips and see for themselves, but so far we haven’t figured how to boost yields at all with nitrogen fertilizer, let alone how to make it pay out. Until we learn more, we’re just poking around in the dark.”