Posted on Jun 13, 2016 by Emerson Nafziger

Does the Corn Crop Need More Nitrogen?

Except for some areas of southeastern Illinois, the 2016 corn crop went in well, and on June 12 was rated at 75% good or excellent. Warm temperatures have speeded up growth, and although below-normal rainfall, especially in western Illinois, is starting to cause some concern, the 2016 corn crop is off to a very good start.

The corn crop this year has excellent stands and there are few drowned-out areas, though there is some unevenness depending on when the crop was planted and how much rain it received after planting. The most noteworthy feature, though, is the dark green color of the crop, especially the crop that was planted in mid-April. This is among the greenest corn crops I have seen in Illinois.

Not only is the crop green where N fertilizer has been applied, it is also green where no N fertilizer was applied. In a June 9 photo taken in one of our N trials, the zero-N treatment shows slightly less growth than the treatment with 200 lb. N applied on April 18 as NH3, but leaf color is about the same without N as with a full N rate (Figure 1). We don’t expect this to last as N uptake kicks into high gear, but the crop has taken up a fair amount of N that didn’t come from fertilizer.

Figure 1. Photo taken on June 9 of V7 corn in a research trial near Urbana, Illinois. The crop followed soybean, and was planted on April 18.

Figure 1. Photo taken on June 9 of V7 corn in a research trial near Urbana, Illinois. The crop followed soybean, and was planted on April 18.

Soil N changes

Soils in the plots shown in Figure 1 have been sampled several times this spring to monitor changes in N. In samples taken on June 3, plots without fertilizer had 49 lb. per acre of plant-available N (PAN, nitrate-N plus ammonium-N in the top 2 feet of soil) and those with 200 lb. of N applied as early spring NH3 had 222 lb. of PAN. Unfertilized plots had about 40 lb. less soil N on June 3 than they had two weeks earlier, but most soil N numbers remained relatively constant during May. Fall-applied N has been present mostly as nitrate this spring, and, and spring-applied NH3 has moved steadily towards nitrate, going from 25% nitrate (75% of the N recovered as ammonium) after application on April 18 to 71% nitrate on June 3. Unless soils get wet soon and stay wet for some time, nitrate will stay in the soil and remain available for uptake.

We saw somewhat inconsistent changes in soil N at some of the other research centers where we’re tracking N this spring. At DeKalb, where 4 inches of rain fell on May 11, soil N following 200 lb. N applied as NH3 in April fell from 375 lb. on May 7 to 176 lb. on June 3, while the unfertilized check rose from 80 to 90 lb. of soil N per acre. At Monmouth, the unfertilized checks had 167 lb. of soil N on June 7, and plots with 200 lb. N as fall-applied NH3 had 288 lb. of N in the soil while those with 200 lb. N as spring-applied NH3 had 260 lb. of soil N.

Fields that Dan Schaefer of IFCA is sampling under the N-Watch program are showing little spring loss of soil N, and some increases. Nine sites in Sangamon County that received 225 lb. of N last fall and winter had an average of 363 lb. soil N in the top 2 ft. on June 2. Samples taken on the same date showed that eight fields in Champaign County that received about 190 lb. of fall-applied N had an average of 279 lb. soil N. Both sets of fields showed some 80 lb. of N more in early June than they had when sampled in late winter (Feb. and March). In contrast, a set of eight fields in Vermilion County that had about 160 lb. N applied in the fall had only 130 lb. of soil N on May 24, slightly less than in March. Soil N is quite consistent among fields in each group, and it’s not clear why there are such differences among the groups.

Can soil N amounts really increase by as much as 80 or 100 lb. in May without any addition of fertilizer N? If we saw this only in a few fields we might think it was sampling error. But we’re finding that soil N often increases as soils warm, and such increases tend to be greater in soils with higher organic matter. So we think this happens as nitrification – the release of N from soil organic matter by microbial action – kicks in as soils warm in the spring. Nitrogen is in the ammonium (NH4+) form when released by mineralization, but then nitrifies (is converted to nitrate NO3) quickly. Both mineralization and nitrification are microbial processes, and rates of both processes are high in warm, moist, aerated soils.

While there are some fields that seem to have less soil N than we might have expected, soil N levels are in general showing amounts at least as high as those we saw at this time in 2015. The largest difference between the two years is rainfall: May rainfall and temperatures were similar both years, but the heavy rain that fell in June, 2015 has not returned in 2016, and is not in the forecast. In fact, rainfall during the first two weeks of June has been below normal over most of the state this year. We think this will be favorable for the crop’s N supply, and expect to see a less pronounced drop in soil N as the 2016 crop moves towards pollination.

Nitrogen management

With warm temperatures and the crop just entering its most rapid growth and N uptake phase, it seems highly likely that, unless soils start to run out of water in the next two weeks, the crop growing in soils with the normal (N rate calculator) amount of fertilizer N will be able to take up most of its N over the next few weeks with little danger of developing N deficiency.

In a trial at Urbana in 2015 with 200 lb. of N applied in April, the crop was at stage V9 and had 45 lb. of N in the plants on June 12. By tasseling time on July 13, it had 159 lb. N per acre in the plants. Soil N between these two dates fell from 240 lb. to 93 lb. per acre, and total (plant plus soil) N fell from 285 to 242 lb. per acre. Rainfall totaled more than 8 inches between these two dates. Even with the drop in soil N to a relatively low level (about 6 ppm nitrate-N and 5 ppm ammonium-N) by pollination, the crop in this treatment yielded 235 bushels per acre.

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

It’s premature to draw a strong parallel between the 2015 results and what we might expect this year, but with drier weather this year, soil N levels similar to those we saw in 2015, the crop darker green, and a root system that is likely to be somewhat deeper this year, all signs point to the likelihood of less chance for N loss and deficiency than we saw in 2015. In 2015, yields in most of our trials were high or very high, indicating that N loss and deficiency were not yield-limiting; exceptions were in fields where root damage form standing water was severe, and crops could not fully recover. While this looked like N deficiency, adding more N to such damaged crops often didn’t help very much in 2015.

Despite the dark green color of most Illinois corn fields in mid-June and soil N numbers that show no shortage, we are continuing to hear about producers and retailers gearing up to apply more N, including in some fields that have had a full amount of N applied and where soils have not been saturated this spring. In fields that have already received their full complement of N, with most or all of the N applied this spring, there is no clear justification for adding more N.

This does not appear to be one of those years when “just in case” justifies adding more N fertilizer. It’s highly unlikely that a corn crop that is deep green at knee- to waist-high will experience N deficiency due to lack of soil N. When N deficiency symptoms do develop in late vegetative or reproductive stages, this usually results from the crop’s running short of water to keep photosynthesis going at full speed. What is called “firing” and looks like a shortage of N is really loss of lower leaf area as the plant dries out. As lower leaves start to shut down they move N out to younger parts of the plant (including the ear) to keep the plant going as long as possible. Adding more N neither prevents nor cures this.

If some or all of the N was applied at modest rates last fall or in early spring in an area that has gotten wet several times since, and if soil N sampling shows levels of less than 15 or so ppm of nitrate-N in the top foot (2-ft. samples will capture N that has moved down but aren’t always practical) then adding more N might be indicated. We can’t accurately estimate the chances that applying more N will pay its cost, but if the crop is deep green and growing rapidly despite what seem to be low soil N numbers, that’s a hint that chances of getting a return may not be very high. The crop is always a better indicator of soil N sufficiency at a given growth stage than are soil N tests.

For those heading out to apply more N, remember that applied N has to get to the roots in order to do any good. If we get average rainfall over the next few weeks, that won’t be a problem. But if it stays dry, N is likely to stay close to where it lands in or on the soil. Roots pull water from the surface soil first, and there will need to be enough rain to bring soil moisture levels up to activate roots and to move surface-applied N into the soil before root uptake can resume. Placing N close to the rows in tall corn was only slightly higher-yielding in our trials last year than applying the same amount at normal sidedress time. The soil is a good reservoir for N, and so N applied a month or more before the crop takes it up is usually available. Even in the wet June of 2015 it was neither necessary nor cost-effective to spoon-feed N to the crop. All signs point to even less benefit to that approach in 2016.

Some people are using slowed-release forms of N for applications made at or after the normal sidedress time. When N is applied when crop uptake is close to its maximum, which starts at about the V7 stage, the main risk is that N won’t be released in time for the plant roots to take it up. Any slowing of the release of N increases that risk. Uptake of N remains at a high rate for only about three weeks, and it’s unlikely that N, especially when applied as urea or ammonium, will convert to nitrate and move out of the rooting zone in the few weeks before N uptake starts to slow. That’s especially the case now, with dry soils more common than wet soils, and extended wet periods not in the forecast.

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