The rainfall that delayed corn planting across Illinois this year is also affecting nitrogen fertilizer management. For many, the rush to get the crop planted meant abandoning or modifying plans to apply N before or after planting or before tillage. Some ammonia was applied last fall or in early April, and some producers were able to broadcast UAN before or after planting. But in many cases planting took precedence, and where it remains wet it’s likely that a lot of acres still have not had N applied.
Later-applied N has little time in which to be lost before plant uptake begins. This means lower overall loss potential and so less need to apply “insurance” amounts of N. As an example, a plan to split-apply N – right after planting and again at sidedress – might, if the first application couldn’t be made before the crop emerged, now be modified to apply less total N but in a single application.
It is important to get N applied before the crop becomes deficient. But the chance of having N deficiency early in corn planted into warm soils is fairly small. One reason for this is that small plants take up little N – by the time the corn crop reaches V5 (five leaves fully emerged) it has taken up only about 10 lbs. of N, or so, per acre. The other reason is that mineralization – the production of plant-available N from soil organic matter – is getting underway as soils warm up. Hence we tend to see little sign of N deficiency when temperatures are warm early in the season, even where little or no N fertilizer has yet been applied.
As we can see in Figure 1, nitrate-N levels dropped from the fall to the spring, especially in the top foot. Ammonium-N levels tended to be close to what we might consider a baseline of about 5 ppm in both fall and spring.
If we calculate plant-available N as the totals (ppm x 4 for each foot of depth) of nitrate-N and ammonium-N in the top 2 ft., we find that N dropped from about 180, 150 and 180 lbs. N/acre in northern, central and southern Illinois last fall, to about 140, 105 and 112 lbs. N/acre this spring, respectively. This represents disappearance of about 23, 30 and 38% in northern, central and southern Illinois, respectively, or about 42, 46 and 68 lbs. N/acre.
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Should there be any adjustment of this year’s N rate based on the amount of N in the soil this spring? In an earlier article I suggested making adjustments only if there were more than 10 ppm nitrate-N in the top foot in spring samples. Most of the spring samples had less than 10 ppm, and it’s likely that even some of that N was moved out of the top foot by rainfall after sampling. Thus we doubt that any adjustments should be made based on amounts of soil N present this spring.
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