Estimating nitrogen levels in late spring
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.
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