This growing season it is difficult to talk about nitrogen (N) management in general terms because of the variability in amounts and frequency of precipitation across Illinois as well as the large range of corn planting dates and growth. Traveling across the state last week I saw fields with corn just emerged and others with corn waist-high. Regardless, the common questions occupying farmers' minds are "Have I applied all the N needed to avoid shortchanging the crop?" and "If I need to apply N, what is the best way to do it?"
This year, fall applications were subject to some N loss. Luckily, fall applications done correctly (in terms of geography, soils, and soil temperature) were not subject to large N loss because a lot of the rain fell early in the spring, when soils were still cool and much of the N had not transformed to nitrate. Areas that had a lot of rain later, when soils were warm, might have had greater N loss. In some parts of the state, torrential rains have caused water to puddle in low-lying areas of fields, damaging crops and causing N loss through denitrification. Spring preplant anhydrous ammonia applications have been less subject to loss because most of the N stays in the ammonium form for a while and is not subject to loss even if soil conditions turn wet after application.
Plant tissue testing at early development stages is mostly useless to determine future N needs because it assesses N sufficiency only at the time of collection; it tells nothing about how much N there will be later in the season, when the crop's N needs are high. Also, no soil tests can provide very reliable information to determine how much N will be available and whether additional N is needed at sidedress time.
The pre-sidedress soil nitrate test (PSNT) can help determine how much organic N will be available to the plant through mineralization. The test is useful only in cases where manure was previously applied or a corn crop follows a pasture. However, accurate collection of soil samples is critical to obtaining reliable test results. Samples should be collected when corn is in the 4- to 6-leaf stage to a soil depth of 12 inches, and each sample should be a composite of at least 10 cores (15 to 25 are recommended).
Due to the difficulty in obtaining adequate samples, the PSNT is not being used extensively. If results are greater than 25 ppm under typical growing season conditions, the chance of increasing yield with additional N is very unlikely. Most important, be aware that at this time of year, test results will likely be higher than if samples were collected in mid- to late May, the timing typically recommended for the test. A more practical approach to determining whether additional N is needed is to perform strip applications in a field to see if there is a response in growth or level of greenness.
If you need to apply additional N, either because you have determined that parts of your field lost a lot of N or because you planned a split application (with some N applied at sidedress), here are some issues to consider.
When should I apply N? Not much N is needed by the crop from early vegetative stages through about the 5-leaf stage of development. The largest portion of total N taken up by corn occurs from the 8-leaf through VT (tasseling) stages. Uptake is mostly done shortly after pollination, so applying N before V8 is best. Research has shown that if applications are done around the V6 stage, it is rare to see yield loss due to N stress. This is because most soils in Illinois can provide sufficient N to satisfy the demands of young corn plants. Of course, if a portion of the total N was applied preplant or at planting, a delay in application of supplemental N is not likely to cause plant N stress. In cases where no N was applied or the supply is very low, make it a priority to try to apply early (preferably before V6) to avoid potential yield loss.
What are the best options for sidedressing? The best way to sidedress N is either injection into the soil or dribbling fertilizer between rows; these applications can reduce volatilization of urea and protect the crop from foliar damage. If ammonia is used, it is important to watch soil conditions; you want to ensure that the knife track closes properly to avoid damaging foliage through the escape of free ammonia into the atmosphere.
When injecting and dribbling are not viable options, broadcast application is the next choice. Urea granules will have the least impact on leaf burn compared with UAN or dry products such as ammonium nitrate or ammonium sulfate. To minimize adhesion of dry products to leaves, apply when foliage is dry. Remember, though, that urea is subject to volatilization if rain does not fall within 3 or 4 days after application. As much as 30% of broadcast urea can volatilize if there is no rainfall within approximately 10 days after the application.
If UAN solution is broadcast over corn when plants are small (about 6 inches), damage will not likely result in yield loss. Even when plants are bigger (V4), foliage damage caused by a rate as high as 90 to 100 lb N/acre typically does not cause significant yield reduction. One way to reduce damage from UAN is to apply before rain. If rain falls within a few hours after application, it will both wash the fertilizer off the foliage and reduce the potential for volatilization of urea.
If a broadcast application of UAN is the only option available, try to do it as soon as possible, because the smaller the plant, the less the potential for foliar damage. If you plan to include herbicide with your UAN application, make sure you read the label first to confirm that such application is allowed. Also, be aware that including herbicide with the UAN solution can intensify leaf burn. In Minnesota, adding 2 lb atrazine/acre at a rate of more than 90 lb N/acre at V3 stage caused severe leaf burning. Applying 2 lb atrazine/acre at 60 lb N/acre causes leaf burning similar to applying 120 lb N/acre with UAN alone.
If my crop is too tall already, can I apply N "over the top"? In some fields crops are getting tall, and some still need additional N. Applying dry products, such as ammonium nitrate and urea, over the top can result in foliar damage, observed as small lesions when the granules fall into the whorl or leaf axil of the corn plant. Also, as the leaf emerges from the whorl, the margin might be white due to excess N in the leaf. Typically, though, this damage is an aesthetic concern that rarely translates into yield reduction.
Over-the-top applications of UAN are the least desirable method of applying N. However, if this is the only alternative and the plant needs more N, the yield benefit from the additional N will likely outweigh the leaf burn caused by the application. Research has shown yield reduction when a rate of more than 60 lb N/acre was applied at V8. To avoid extensive foliage damage, when N application is needed later than V8, it is very important to fit the high-clearance equipment with drop hoses so that UAN is applied directly on the soil surface without touching the crop canopy.
In summary. If you need to apply additional N, sidedress earlier rather than later in crop development if at all possible. If you have options for how to apply N at sidedress, first choice is injection or dribbling UAN solution between rows, second is broadcast of solid ammonium-containing fertilizers, and third is broadcast UAN solution.