Do Soybeans Need Nitrogen Fertilizer?

According to a University of Illinois crop sciences researcher, there has been a great deal of interest recently in the idea of using nitrogen fertilizer during the growing season to increase soybean yields.

“This is somewhat surprising given that there has been so little evidence from published and unpublished reports showing that this practice increases yields, let alone provides a return on the cost of doing this,” said Emerson Nafziger.

Soybean plants in most Illinois fields produce nodules when roots are infected by Bradyrhizobium bacteria early in the season, Nafziger said. Bacteria growing inside these nodules are fed by sugars coming from the plant. “In one of the more amazing feats in nature, these bacteria are able to break the very strong chemical bond between nitrogen atoms in atmospheric nitrogen gas (nitrogen gas makes up some 78 percent of the air but is inert in that form.) This ‘fixed’ nitrogen is available to the plant to support growth,” he said.

The soybean crop has a high requirement for nitrogen; the crop takes up nearly 5 pounds of nitrogen per bushel, and about 75 percent of that is removed in the harvested crop. Nafziger explained that it is generally estimated that in soils such as those in Illinois, nitrogen fixation provides 50 to 60 percent of the nitrogen needed by the soybean crop. A small amount of nitrogen comes from atmospheric deposition, including some fixed by lightning. The rest comes from the soil, either from that left over from fertilizing the previous corn crop or from soil organic matter mineralization carried out by soil microbes.

Nitrogen fixation takes a considerable amount of energy in the form of sugars produced by photosynthesis in the crop. “Estimates of the amount of energy this takes range widely but could be in the vicinity of 10 percent of the energy captured in photosynthesis, at least during part of the season,” Nafziger said. “Because photosynthesis also powers growth and yield, it seems logical that the crop might not be able to produce enough sugars to go around, especially at high yield levels, and that either yields will suffer or nitrogen fixation will be reduced.”

Would adding nitrogen fertilizer fix this problem and result in higher yields?

Nafiziger explained that he has looked at adding nitrogen fertilizer in a series of trials over the past several years, with some of the research funded by the Illinois Soybean Association. These studies involve application of urea, in some cases with Agrotain® (urease inhibitor) or as ESN (slow-release nitrogen) during mid-season, usually in July.

Yields ranged widely among these studies, from in the 30s to nearly 90 bushels per acre. But in only one case did adding nitrogen fertilizer significantly increase yield (by 6 bushels per acre), he said. There was no relationship between yield level and response to nitrogen fertilizer.

“These results provide no support for the idea that the higher the yield, the more response to fertilizer nitrogen. In fact, yields above 70 bushels seemed more likely to show yield decreases from adding nitrogen, though these differences were small and not statistically significant,” he said.

While these results don’t prove that adding nitrogen fertilizer can’t increase soybean yields, Nafziger said it’s clear that it shows that an increase cannot be expected.

“It is possible that soils often contain more nitrogen than we realize, especially under good mineralization conditions, which are also good growing conditions. It is also possible that we don’t really understand the photosynthetic capacity of soybeans under field conditions, and that our guessing that yield is limited by photosynthetic rates when the plant is also fixing its own nitrogen is just incorrect,” he said.

The usual signal of nitrogen deficiency in crops – light green leaves – is rarely seen in soybean plants during the period of pod setting and seed filling, unless the crop is under prolonged drought stress. Late in seed filling, leaves start to mobilize their nitrogen as chlorophyll and photosynthetic proteins break down and much of this nitrogen moves to pods and into seeds as photosynthesis winds down. Nafziger said if there was a way to get more nitrogen into the leaves early in this process, it might be possible to maintain photosynthesis longer and move more material into seeds. “But it is clear that getting this to happen consistently will be difficult, especially under an unpredictable water supply,” he said.

“Until and unless we find a way to learn to make nitrogen application to soybeans work consistently, or in most cases to work at all, this practice increases both economic and environmental risk. Under dry late-season conditions, such as those we experienced in 2013, much of the nitrogen we apply will fail to get into the plant, but will stay in the soil and become part of the mobile pool of soil nitrogen going into the fall,” Nafziger said.

The crop scientist recommends putting in strip trials in farm fields to get a better look at nitrogen on soybeans over a wide range of fields and soils. He explained that these can be done using aerial or ground application but that ground application is easier to track.

Source: University of Illinois

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