Estimating nitrogen levels in late spring
“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.