Estimating nitrogen levels in late spring
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.”
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