Many cornfields throughout Illinois are showing areas with light-green color next to good-looking plants that are often farther along in development, too. Almost without exception, the pale plants are in low-lying areas where this season's large amounts of rain have caused frequent ponding.

The appearance of the corn crop is an excellent diagnostic tool for nitrogen. Corn that is yellow-green or light green is most likely nitrogen-deficient. One thing to keep in mind, though, is that if corn is growing in waterlogged soil, nitrogen deficiency will show up even if there is plenty of nitrogen there. If nitrogen is there, the yellow corn will green up when the soil dries out, and no additional nitrogen may be needed. How long should you wait for green-up to know whether there is sufficient nitrogen? You ought to be able to see a substantial difference about a week after the soil has been dry enough to walk on. If after that time you still see light-green canopy, you can know with pretty good certainty that your corn needs additional nitrogen.

For some farmers the nitrogen-deficient areas represent a small fraction of the total surface area, whereas others have substantial deficient acres within a field. Many would like to fix the problem and wonder if it is too late to do something and, if not, how to go about it.

Realize that the areas showing nitrogen deficiency have also lost some yield potential, so a full-rate application is likely not the best alternative, simply because the plant won't be able to use all that nitrogen to make yield. The sooner you apply nitrogen, the better response you are likely to see. You are very likely to obtain a yield response by applying nitrogen until tasseling. Some studies have shown that when corn is severely limited by nitrogen availability, the crop has great capacity to use rescue nitrogen and produce an increase in yield until silking.

It is also important to keep in mind that the areas needing a rescue nitrogen application are most often patchy, so targeted applications rather than even applications across the field are key to minimizing cost and potential nitrogen loss to the environment and to increasing return on investment. Aerial photography or observation from an elevated area above the canopy is the best way to determine where the trouble spots are. In most fields, the crop is reaching heights that would make it too difficult to find the problem areas by "walking the field."

Aerial photographs can be converted into variable nitrogen rate maps to guide a variable rate applicator. Another way to use aerial photography is for creating yield loss maps that allow farmers to know how much to spend in rescue nitrogen applications to correct the deficiency problem. High-clearance equipment is probably about the only way you could put in the rescue application. Luckily, with a few challenging cropping years back to back, high-clearance equipment that allows injection or dribble nitrogen solution applications between rows is becoming increasingly available.

For rescue nitrogen, limit the application to between-rows dribble or injection of UAN solutions or urea. Another option would be broadcast urea or urea plus a urease inhibitor such as NBPT (Agrotain). Do not do a broadcast application of UAN due to the high probability of canopy injury. Finally, do not apply foliar products simply because the amount of nitrogen that can be applied is often very low and the cost per acre too high to make it profitable.