Some rain has fallen somewhere in Illinois nearly every day for the past 3 weeks, and rainfall totals for this period exceed 7 inches – two to three times normal – over more than half of the state (Figure 1). This has a lot of people wondering if enough nitrogen remains in the soil to supply the corn crop.
Figure 1. Illinois rainfall from May 26 through June 18, 2015.
Daily high temperatures have averaged close to normal over the past three weeks, while night temperatures have been 3 to 4 degrees above normal, so growing degree accumulation rates remain high. Sunshine amounts have been marginal, but growing conditions have been good enough to keep the crop coming along rapidly. Fields that were planted in April in central Illinois have 10 to 13 leaves, and the crop is 4 to 5 feet tall or more and growing rapidly, adding 3 or more inches of height per day and adding a new leaf every two days or so. With warm temperatures during June and plenty of water, we can expect corn plants to be tall this year.
Except where roots have been in water for a week or more, fields and parts of fields where crop roots are still supplied with oxygen continue to show good canopy color. Much of the early-planted crop is in or about to enter the rapid N uptake period – from about V9 through tasseling – during which the crop takes up as much as 6 or 7 lb of N per acre per day.
We are continuing to monitor soil N in the N-tracking study that I described here on May 30 http://bulletin.ipm.illinois.edu/?p=3241. The most recent samples for which we have numbers were taken last week, before the end of the current deluge. Table 1 gives rainfall amounts and changes in soil N between samples taken May 20-22 and those taken 18 to 20 days later. The 340 lb found following fall application on the first date at Urbana is 85 lb more than was found 9 days earlier, and is likely a result of sampling variability.
Table 1. Rainfall and changes in soil N between two dates at three Illinois sites in 2015.
We know these numbers are variable and that we need to be cautious in using them, but the fact that soil N didn’t decrease sharply, especially at Urbana where so much rain fell, provides some confidence that losses have not been very high. The crop at Urbana is ahead of those at the other two sites, and so has taken up more N; I would estimate N uptake by the time of the second sampling (June 12) at about 40 lb of N per acre.
The percentage of N found as ammonium increased over this period at DeKalb and Urbana, but not at Monmouth, where ammonium percentage was higher on the first date than at the other sites. This may reflect an increase in mineralization as soils warmed up, and could also reflect some loss of nitrate. Mineralization is the likely cause of the uptick in soil N in the zero-N checks as well.
As plants begin to take up N at a rapid rate, we can expect soil N to drop, though N losses and mineralization will both affect the rate of change. Over the next few weeks, we expect that the plants (canopy color) will be a better gauge of N availability to the plant than will amounts of N we measure in the soil. The question that remains, without a solid answer, is whether N levels might slip below those needed to maintain crop growth before uptake starts to slow. From what we’re seeing so far that seems unlikely, at least if rains slow before too many more days.
A more immediate question is whether the pale green or yellow spots in fields need N applied now in order to prevent serious yield loss. Rapid loss of color in places where water stands comes from loss of root ability to take up N, not from loss of N from the soil. We know this because parts of fields where roots are in aerated soil are not showing deficiency.
We won’t know the extent to which roots of plants in wet or flooded soils will recover until soils dry and re-aerate enough for root function to return. Adding N before then will do nothing for the plants, and if soils remain wet or flooded, some of this N will be lost before the plant has a chance to take it up. Aerial application of urea is not inexpensive, and while it can test our patience, waiting until soils dry out for a week or more before deciding that more N is needed is the best course of action. Our hope is that a lot of acres will return to green once soils dry out. Even if that happens, we expect such areas to have lost yield potential, and the larger the plants were when first flooded and the longer soils stay wet, the larger will be the loss. Adding N now will do nothing to fix this.
As plant size and leaf area increase, the ability of the crop to help move water from the soil to the air will increase as well, so the crop itself will help to dry the soils and will speed progress toward aerated soil conditions. Water loss in yellow, root-damaged corn that is standing in wet soils is very slow, so we won’t see much help there. Our best hope is to get two weeks of weather without much rain and with average temperatures to help get the crop back on track. It won’t be an easy wait.