Estimating nitrogen levels in late spring

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According to a University of Illinois crop scientist, the Illinois corn crop is off to a good start in many fields and in most areas just starting its rapid growth phase (V5 to V8 stage).

From now until near maturity, the crop will add an average of some 200 pounds of dry matter per acre per day and will take up to 3 to 4 pounds per day of nitrogen before pollination, after which the nitrogen uptake rate will slow, said Emerson Nafziger.

“The spring of 2014 has not been a wet one overall in Illinois, but rainfall has been very unevenly distributed with some areas having received 6 inches or more over the past month,” Nafziger said. “In wetter areas, getting sidedressed nitrogen applied has been challenging, and some who applied the full amount of nitrogen are concerned about how much might have been lost.”

Low temperatures through the winter and early April helped preserve fall-applied nitrogen and the small amount of residual nitrogen left after last year’s big crop. Nafziger reported that April and May temperatures were normal to just above normal, with maximum soil temperatures at 4 inches deep under bare ground reaching the mid-70s by mid-May and into the 80s during warm periods in late May and early June.

Soil temperatures in the 70s and 80s increase activity of soil microbes, both responsible for mineralization (release of plant-available nitrogen from soil organic matter) and those that convert ammonium to nitrate. Results from six Illinois fields sampled for nitrogen in May confirm that nitrogen loss was not excessive by mid-May, and that much of the fall-applied nitrogen was still present, mostly in the nitrate form, by May. “The fact that more nitrogen was recovered than had been applied is not unusual; mineralization and carryover nitrogen contribute to the amount that is there,” he said.

While many soils are moist or even wet in mid-June, the threat of nitrogen loss is far higher where water has stood, or is standing, than where water has not stood for more than one to two hours. “When water stands long enough for the crop to begin to lose some of its green color – typically three to four days at warm temperatures – that’s a signal that soil oxygen is becoming depleted, Nafziger said.

“Two negative consequences of lack of oxygen are the start of denitrification (conversion of nitrate to gaseous forms of nitrogen) and nitrogen loss, as well as the beginning of root damage, some of which may be permanent. Most of those fields have recovered well.”

Nafziger said that it is likely most fertilizer nitrogen is in the nitrate form by now, though some applied as sidedressed NH3 or as sidedressed urea-ammonium (UAN) may still remain as ammonium. “Having most of the nitrogen present as nitrate in mind-June is typical. It means, though, that the nitrogen is subject to denitrification and, in lighter-textured or tile-drained soils, to moving out of the rooting zone,” he added.

Previous studies have shown that, at soil temperatures in the 70s, as much as 7 or 8 percent of the nitrate present can be converted to gas and lost for each day saturated conditions persist. Conversion rates may be lower than this with lower temperatures deeper in the soil and at night, if some of the nitrogen is still in the ammonium form, and if soils still have some oxygen present.

“There are indications that denitrification losses may be less than expected in some fields, but if plants are badly damaged by standing saturated soils, loss of nitrogen may be a smaller problem than the loss of yield potential from plant damage,” Nafziger explained.

It is rare that whole fields remain saturated for days, so in most fields, the risk of nitrogen loss by leaching or by movement out of the field through field tiles is greater than the risk of loss by denitrification. According to Nafziger, tiles began running relatively late this spring, which helped keep nitrogen in the fields. In many fields, the first water to reach the tiles came from rainfall before nitrogen was converted to nitrate or before nitrogen had been applied and may have carried relatively little nitrate, he said.

“But, by June, it’s not unusual for water from field tiles to have nitrate-nitrogen levels of 10 to 20 parts per million. At 15 ppm N, one acre-inch of water leaving the field carries with it about 3.8 pounds of nitrogen.”

In fields where all of the nitrogen has been applied (especially if some was applied in the spring as NH3) and crop color has remained or returned to healthy green, and where there is no standing water now, it’s reasonable to assume that it’s not necessary to add more nitrogen, Nafziger said.

In fields where all of the nitrogen has been applied but where water stood long enough for the crop to lose much of its green color, adding supplemental nitrogen will increase yield only if plants can grow enough new roots to take advantage of the added nitrogen. Chances of such recovery are much greater when the water comes in early vegetative growth like it did this spring than when it comes later.

“In fields that still need sidedressed nitrogen, or where plants stood in water to the point of turning pale green but now seem to be recovering, nitrogen should be added as soon as practicable,” he said. “Where nitrogen is needed, applying it as late as tasseling time will often provide a yield benefit if there’s enough rainfall to carry it to the plant roots.”

While it’s good to apply supplemental nitrogen (if it’s needed) or planned sidedressed nitrogen as soon as we can, the yield cost of further delays depends on the nitrogen available to the plant now, Nafziger explained.

“The best way to know how much nitrogen is available to the crop now is to observe canopy color; as long as leaves remain a reasonable shade of green, the plant is not deficient, or not deficient enough to cost yield as long as final nitrogen supply is adequate. In that case, some delay in applying nitrogen may not cost any yield. If it turns dry after surface application of nitrogen, uptake will be delayed and the risk will increase of having the crop run out,” he added.

Putting all this in perspective, Nafziger said the 2014 season has not been one of above-average nitrogen loss potential, except in areas that had heavy downpours. “Remember that mineralization of soil organic matter is contributing substantially to the nitrogen supply in the soil now, helping to counter some nitrogen loss from tile lines. This is not the case in saturated soils, where mineralization is slowed as denitrification speeds up,” he said.

“Still, if good rainfall and temperatures continue, the nitrogen supply, even if reduced some by loss, is unlikely to limit yield. In fact, most of the highest yields we have seen in several hundred nitrogen rate trials over the last 20 years have come at modest nitrogen rates. I think this happens because good root systems mean good uptake of water and nitrogen and that conditions that are ideal for yield also tend to be very good for soil nitrogen supply,” he said.

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