Does the corn crop need more nitrogen?

URBANA, Ill. – The 2016 corn crop is off to a very good start in Illinois; fields in most areas have excellent stands, good uniformity, and very good leaf color, according to University of Illinois crop scientist Emerson Nafziger.

“The dark green color is in contrast with what we saw with the high rainfall in June of 2015,” Nafziger says. “Conditions of low sunlight and saturated soils mean pale green leaves.”

This year, starting in mid-May, warm temperatures, bright sunlight, and soils with adequate-but not excessive-moisture have allowed the corn to grow rapidly and take up plentiful soil nitrogen. Nafziger reports that in plots at Urbana’s South Farm, corn without fertilizer nitrogen is slightly smaller than corn fertilized with 200 lb of nitrogen, but both show dark green leaves.

Nitrogen management

“With warm temperatures and the crop just entering its rapid growth and nitrogen uptake phase, it seems highly likely that the crop growing in soils with the normal (nitrogen rate calculator) amount of fertilizer nitrogen should be able to take up all the nitrogen it needs. If soils dry out, that might reduce the amount of nitrogen taken up, but at this point there is little danger of developing nitrogen deficiency,” Nafziger says.

In 2015, well-fertilized corn had a total of 45 lb of nitrogen per acre in the crop when it was about 30 inches tall. When it tasseled about four weeks later, it had nearly 160 lb of nitrogen in the crop. Soil nitrogen over this period fell from 240 lb to 93 lb per acre, and total (plant plus soil) nitrogen fell from 285 to 242 lb per acre. Nafziger clarifies that this was under wet conditions, with some 8 inches of rainfall during the same period. Even with the drop in soil nitrogen to a relatively low level by the time of pollination, the crop in this treatment yielded 235 bushels per acre.

“At the estimated 1 lb of nitrogen taken up for each bushel of yield, the 2015 crop would have taken up about a third of its nitrogen after tasseling,” Nafziger states. “Given the low amount of soil nitrogen at tasseling, this additional nitrogen had to have come from mineralization and, possibly, from nitrogen deeper in the soil profile as the crop drew up water during dry weather late in the season. In any case, it’s clear that low soil nitrogen at tasseling did not result in low yields due to nitrogen deficiency.”

Nafziger cautions that it is premature to draw a strong parallel between 2015 results and what we might expect this year, but a number of signs point to a lower chance for nitrogen loss, and little danger of deficiency this year.

Despite the dark green color of most Illinois corn fields and moderate rainfall amounts this year, Nafziger says he has been hearing about producers and retailers gearing up to apply more nitrogen, some in fields that have had a full amount of nitrogen applied and where soils have not been saturated this spring.

“In fields that have already received their full complement of nitrogen, with most or all of the nitrogen applied this spring, there is no justification for adding more nitrogen,” Nafziger warns. “This does not appear to be one of those years when ‘just in case’ justifies adding more nitrogen fertilizer. It’s highly unlikely that a corn crop that is deep green at knee- to waist-high will experience nitrogen deficiency due to lack of soil nitrogen.

“While fear that the crop will run out of nitrogen has been a common theme, nitrogen deficiency symptoms that develop in the late vegetative or reproductive stages usually result from the crop running short of water. What is called ‘firing’ and looks like a shortage of nitrogen is really loss of lower leaf area as the plant dries out. As lower leaves start to shut down, they move nitrogen out to younger parts of the plant, including the ear, to keep the plant going as long as possible. Adding more nitrogen neither prevents nor cures this,” Nafziger explains.

If some or all of the nitrogen was applied at modest rates last fall or in early spring in an area that has gotten wet several times since, and if soil nitrogen sampling shows less than 15 ppm of nitrate-N in the top foot, then adding more nitrogen might be indicated. But even if soil nitrogen numbers don’t look very high, a deep green and rapidly growing crop is getting enough nitrogen, and as its root grow it will continue to get access to what’s in the soil. So adding more nitrogen may not benefit the crop.

“The crop is always a better indicator of soil nitrogen sufficiency at a given growth stage than are soil nitrogen tests,” Nafziger says. “As long as leaves keep their good, green color through pollination, we needn’t be too concerned that the crop will run out of nitrogen this season.”