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Much of northwestern and western Missouri received excessive precipitation in early May. The Missouri and other rivers topped numerous levees, flooding tens of thousands of acres of cropland. Many fields that did not go under water may still have received enough rainfall to cause nitrogen loss.

For corn fields that were kept, replanted to corn, or replanted to another N-demanding crop such as milo, producers need to make decisions about whether to apply additional N, and, if so, how much. Unfortunately, there is no way to make this decision that is simple, reliable, and convenient. However, the difference between a good decision and a bad decision can be quite a bit of money. I would suspect that there will be quite a few fields where applying additional N will be profitable.

I have, in another article in this issue (see 'Nitrogen Loss Scoresheet'), proposed a decision aid that is simple and convenient, but not necessarily very reliable. This is because the factors and processes that lead to nitrogen loss are complex. I do not put a lot of stock in any calculation- or score-based method of diagnosing N loss. However, they can be better than a seat-of-the-pants guess. And some broad principles in these systems are pretty solid, such as that spring-applied anhydrous ammonia is less vulnerable to N loss than other N source/timing combinations.

Using soil or plant measurements to diagnose the N status of a field is more reliable than any scoresheet method, but is either difficult or inconvenient or both. Given the heavy loads already on producers and service providers, it's going to be difficult to use diagnostic tests on very many acres. However, they may pay off for those who can find the time. Just sampling a few fields representing different scenarios may provide useful guidance for other similar fields. Some will be in the ammonium form in most fields, unless they were fertilized with dry or liquid N before April 1 or with anhydrous ammonia in early fall. If soil samples are taken, both nitrate and ammonium tests should be requested in most cases.

Interpretation can also be tricky, because producers should expect to find not only their fertilizer N but also N that was contributed by the soil, typically around 50 lb N/acre. A more detailed description of how to use soil tests to assess N availability can be found in University of Missouri Extension Guide G9177 (http://extension.missouri.edu/explore/agguides/soils/g09177.htm).

Another option is to use the color of the crop to diagnose how much N remains. This option is simple and reliable, but is inconvenient in that the diagnosis can't be made until the corn is at least a foot high. That means sidedress or possibly high-clearance N application equipment, or application through pivot irrigation.

To use the color of the crop as a guide requires knowing the color with sufficient N. I would recommend adding at least 100 pounds of additional N in a small area of the field. This can be done when replanting for fields that were flooded, or by hand or with sidedress equipment for fields where the crop was not lost. The high-N area should be marked with flags or using a GPS. Then, if you can see that the crop is darker in the area where extra N was applied, you know that the field will need additional fertilizer. The bigger the color difference, the more N is needed.

The fastest and most convenient way to assess corn color over many fields is with aerial photographs. Nitrogen loss is usually patchy, so it's hard to evaluate color in all parts of a field from the ground.

However, it's also hard to do a good job of diagnosis with aerial photos unless the soil is mostly covered by the canopy. With corn this doesn't happen until it's about waist high. Waiting until the corn is waist high to make a decision is unlikely to cause yield loss in our current situation, but applying N at that stage is practical for only a small minority of producers or service providers. But for those who have the high-clearance equipment or pivots with injection pumps, it's a great option.

For people who use other methods to make their decisions about nitrogen, there may still be a need to evaluate whether enough N was applied. Aerial photographs can be a good way to accomplish this.

I will be hiring a plane to take aerial photos of corn fields over the next four to six weeks. Anyone who's interested in getting aerial photos is welcome to contact me. I can probably provide a fairly large (but not unlimited) number of free aerial photos to interested producers or service providers. Those who are interested in using the photos as their main decision tool will get top priority. I can also arrange for an even more limited number of photos to be translated into maps of potential yield loss and recommended N rate. Aerial photos can also be obtained from Tom McMurren, John Deere Agri Services, 815-970-4897, McMurrenTom@JohnDeere.com, and from InTime (Derek Emerine, 573 670 2667). These companies will also provide maps defining nitrogen stress zones based on their aerial photos.

Peter Scharf
573-882-0777
scharfp@missouri.edu