There has been a lot of talk and research lately surrounding nitrogen applications in soybeans.

Questions such as, "Are soybean yields limited by available nitrogen?" and "Is there a yield response in soybeans from late-season nitrogen applications?" have come up in research in the past couple of decades.

It might come as no surprise that the answer, in most cases, has been "no." Still, as retired University of Nebraska-Lincoln soil scientist Charles Shapiro put it in 2017, when it comes to nitrogen applications on soybeans, "The hope never dies."

So, when soil scientists updated UNL's NebGuide "Fertilizer Recommendations for Soybeans" in 2018, nitrogen was one of the areas they focused on and revised.

Soybeans require about 4 pounds of nitrogen per bushel of grain, and uptake during the reproductive stage often is more than 4 pounds of N per day. Indeed, it's often said that while soybeans get most of their required nitrogen either through fixation from the air, on average, about 45% comes from the soil.

However, as Charles Wortmann, UNL professor and Extension soil and nutrient management specialist notes, routine nitrogen applications still aren't recommended on soybeans.

"Before, in our previous recommendations, we had hinted there may be value in some fertigation of nitrogen at R3, largely based on earlier work in Kansas," Wortmann says. "In Kansas, they haven't been able to repeat that result. Here, over 50 trials in Nebraska haven't been able to repeat it."

The updates also consider results from similar research across 16 states, showing that although nitrogen applications are most effective at 40 pounds per acre at planting again through fertigation with an average response of 2.5 bushels per acre, the profitability of the yield response is too low to justify the application.

Wortmann notes symbiotic fixation through bacteria on soybean root nodules accounts for 55% to 75% of the nitrogen required by the plant, however, "In high-yield situations where growers are set up for fertigation, they may want to do some on-farm trials, by dividing their pivot into 'pie slices' with and without fertigation to see if applications of 30 to 50 pounds per acre at R3 bring a profitable response," Wortmann says.

UNL also has updated recommendations for managing iron deficiency in soybeans — particularly when dealing with higher concentrations of residual soil nitrate, when there's a stronger likelihood for iron deficiency. That's because excess residual nitrate can decrease iron availability to the soybean plant. This is especially a problem if the previous crop was damaged by hail.

"It could be a situation where the previous corn crop didn't do well, was hit by hailstorm and had less nitrogen uptake than normal, or if it was a seed corn or popcorn field where more nitrogen was liberally applied with much residual nitrate-nitrogen remaining,” Wortmann says. “Those may be situations where the nitrate level could be high enough to increase the severity of iron deficiency or chlorosis. One way to address that may be with a cover crop with enough growth to get sufficient nitrogen uptake."

Often, when iron chlorosis occurs — as is the case in calcareous soils along the Platte River in western Nebraska, and in parts of northeast Nebraska — it's not uniform across the field. It usually isn't economical to treat iron deficiency across an entire field, however, and the updates include recommendations for site-specific management of iron deficiency.

"If one half of the field is historically badly affected compared to the other, it might be worth selecting a different variety, applying chelated iron at planting time, or planting at a higher density in badly affected areas," Wortmann says. "Otherwise, I wouldn't recommend making that investment."

Micronutrient recommendations

Earlier in 2018, Nebraska also updated the NebGuide "Micronutrient Management in Nebraska," something that's garnered interest in recent years with increased availability of micronutrient products and higher crop removal with higher yields.

Notably, nickel now is recognized as an essential crop nutrient. However, perhaps the biggest takeaway is, based on research in Nebraska, that soil tests and foliar tissue tests aren't a reliable predictor of crop micronutrient requirements.

"There isn't much response to micronutrient applications, but our recommendations do challenge the value of foliar testing to an extent and even soil testing," Wortmann says. "With the exception of soil testing for zinc availability, decisions have to be made based on crop response. You may have to properly conduct on-farm trials, or at least follow the results of Nebraska On-farm Research Network."