Source: University of Illinois at Urbana

The price of nitrogen (N) has moved rapidly in recent months. The price of corn has changed as well, and swings continue. As a result, we suggest that Illinois producers who have yet to apply N take a look at the rate calculator at as they set rates for this spring. The numbers for Illinois used in the calculator have been updated to use the 2008 data, so they represent the best information we have on N response for Illinois producers.

Table 3 has guideline N rates and ranges for corn following corn and corn following soybean in northern, central, and southern Illinois, based on an anhydrous ammonia price of $600 per ton and a corn price of $3.80 per bushel.

These numbers are different than they were when we ran the calculator last fall, due to the addition of new data (and some deletion of old data) and to the fact that the corn-to-N price ratio has changed. The changes have been modest in northern and central Illinois, but following the wet weather and highly responsive N rate trials in southern Illinois in 2008, plus the addition of some new data from previous years, guideline N rates increased in southern Illinois. Remember that the ranges given for each rotation and region provide support to adjust N rates based on experience and knowledge of the field and crop.

Cautions regarding spring application include not applying ammonia under the row shortly before planting, and not applying very much N solution in contact with, or very close to, seeds. Some producers have applied anhydrous ammonia as part of a spring strip-till operation in the past, only to experience damage as soils dried out after planting and ammonia was able to move up into the rooting zone, burning off roots or even killing small plants. When it stays as dry after planting, waiting a week or more after ammonia application to plant may not prevent damage.

If it rains soon after application, damage is unlikely. Because this can't be predicted, it is safer if producers can use GPS and assisted steering to apply spring ammonia between the corn rows, or between where the rows will be once they're planted. Waiting to apply N until soils are dry enough to allow the ammonia to disperse better will also help, but this can mean a considerable wait.

Applying N closer to the time when the crop will take it up generally improves efficiency, by decreasing loss potential and by allowing N placement so that plants can access N easily. But claims that some forms of N are "more available" than others, or that the plants "prefer" some forms, are often shaky. Urea converts to ammonium quickly in the soil, and between urea, ammonia, and UAN (in which one-fourth of the N is nitrate), we apply mostly the ammonium form as fertilizer. Ammonium is converted to nitrate by bacteria as soils warm up, so the mix available to plants changes from more ammonium early to more nitrate over time. By the time rapid N uptake starts in June (plants with 6 or 7 leaves), much of the N from early-applied fertilizer is usually in the nitrate form. The supply of N released from soil organic matter also increases with soil temperatures, and this N is initially in the ammonium form. We know that plants don't thrive when provided only nitrate, but the mixture of N forms in the soil is usually taken up and used by the plant with no problems. Attempts to keep more of the N as ammonium, including the use of N-Serve in the spring - especially at or after planting - are often unnecessary, given the short time between application and plant uptake. Anhydrous ammonium is often a cost-effective form to use for side-dressing, but it's not likely to be more effective than UAN or other nitrate-containing forms based on the fact that it changes the mixture available to the plant toward ammonium. N-Serve might be cost-effective if N is applied long before planting, in order to slow the conversion to nitrate.

Finally, urea is gaining in importance as an N source, and we need to manage it properly. The risk of loss from decomposition to ammonia and loss into the air is high if urea stays on a warm, dry soil surface for a number of days, especially if the soil surface has a lot of residue. This is true with both dry urea and for the half of UAN solution that is in the form of urea. Rainfall within a few days after application usually moves the urea into the soil and limits losses, but if it stays dry, incorporating urea that remains on the soil surface may be needed to preserve it. There are urease inhibitors that reduce the rate of breakdown, and also ESN, which is polymer-coated urea that is physically protected for some time against breakdown. The added cost for such protection needs to be weighed against other methods of protecting urea N from loss.