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Issue No. 1, Article 7/March 21, 2008

Corn and Nitrogen Prices and Spring Nitrogen Management

The price of nitrogen (N) has increased considerably in recent months, while the price of corn has swung widely but has generally moved higher as well. While the high N price has people interested in using only the amount that the crop needs, the high price of corn also has producers interested in making sure that the crop isn't short of N. How can this dilemma be resolved?

While it would be great to have a method to determine exactly how much N corn needs before the season starts, there is none. We do have good confidence that a crop that yields more will take up more N--on average, about two-thirds of the N in the crop at maturity is in the grain. Grain that contains 9% protein has about 0.7 lb of N per bushel and would have about 0.3 lb of N in the stalks for each bushel of grain. So a 200-bushel crop contains about 200 lb of N per acre at maturity, plus some N in the roots.

Many N rate trials over the past 10 years have shown, however, that the amount of fertilizer N needed by a corn crop shows very little correlation with actual yield--high yields can require high N fertilizer rates or may need little N. A major reason for this is the wide variation in efficiency of fertilizer N use--the percentage of fertilizer N taken up by the crop--due to factors such as fertilizer placement, N loss, soil moisture, and root growth patterns.

A second reason is that soil organic matter is a good but unpredictable source of N; soils with organic matter of 3% or 4% can provide as much as half the N needed by the corn crop. This N becomes available as a result of microbial breakdown of organic matter, so the amount varies with soil conditions. Under cool spring temperatures and too much or too little water, the same soil might produce less than half the amount of N produced under better conditions. And plants that have difficulty growing well usually have problems taking up the N that is in the soil.

Fertilizer N needs to make up the difference between what the crop needs and what the soil supplies. This difference is unpredictable. In general, growing conditions that favor high corn yields also tend to favor more N production from organic matter and more N uptake by the plants. Unfortunately, we can't predict yields very well either. In addition, microbes that break down corn residue take up N from the soil, thus making less available to the next crop. So corn following corn tends to need more fertilizer N, unless there is carryover N that was not used by the previous corn crop. This amount of carryover can be large, especially when the previous crop yielded less than normal due to drought and when the weather stays dry through the fall and winter.

We can approach this problem by merging the results of many N rate trials in order to calculate the amount of N that will optimize corn yield "on average." The results of individual trials vary widely, with optimum N rates ranging from less than 50 lb to more than 250 lb N per acre, and these rates are independent of corn yield. So to be completely certain that the crop in a given field in a given year has no chance of having the slightest deficiency would require more than 250 lb of N. Obviously, such rates are not economically sound.

Finding the optimum rate of N--that amount that increases yield just enough to pay for itself--requires that we know or estimate the prices of both N and corn. The ratio between these two prices determines the optimum N rate. Even though both prices have increased a great deal, their ratio has not changed much over the past five years. When N was 20 cents per lb, corn was about $2 per bushel, and the ratio was about 1 to 10, or 0.1. Now N is at or above 50 cents and corn is at or above $5, and so the ratio is nearly unchanged. As long as this is the case, the optimum N rate stays the same, unless new data added to the database change the calculation.

We have a fairly large database by now, so adding more trials, while important in order to increase confidence, does not change the calculations much unless the responses are much different from previous ones. This was not the case in 2007, though some trials of corn following corn in central Illinois showed the need for more N than some previous trials had shown. For corn following corn in central Illinois we added data from 13 new trials to 53 already in the calculator, and this raised the optimum N rate by about 5 lb N per acre. Other areas will show little change as a result of adding data from 2007.

Using an N price of 50 cents per lb and a corn price of $5 per lb, the calculator provides guideline N rates for corn following soybean and corn following corn of 139 and 186 lb N in northern Illinois, 168 and 173 lb N in central Illinois, and 165 and 179 lb N in southern Illinois, respectively. The calculator also suggests ranges of about 20 lb N on either side of these numbers. Unless there is some reason to believe that N response in a certain field will be less than or more than the midrange value, there is little reason to move these rates toward either end of the range.

Expensive N suggests that care be taken to protect fertilizer N from loss and to make it more available to plants. In general, applying N closer to the time when the crop will take it up improves efficiency, by decreasing loss potential and by having plants present to facilitate N application and encourage injection. But claims that some forms of N are more available than others or that the plants "prefer" some forms are often shaky. Corn plant roots take up mostly nitrate because ammonia is converted in the soil into nitrate over time, and the mixture of forms in the soil is usually taken up by the plant with no problems. Attempts to keep more N as ammonium, including the use of N-Serve in the spring, are often unnecessary, given the short time between application and plant uptake.

Other forms being sold as superior to normal forms of N fertilizer include foliarly applied, slow-release forms of N, usually urea-processed to make it slow to break down. We have heard claims that such forms are "five times more effective" than other forms. Remember that corn roots are designed for, and are completely capable of, taking up nutrients like N. Leaves are not designed to take up N, though they can do so in small amounts if the N form does not injure or destroy delicate leaf cells. Plants that grow slowly or poorly usually do so because they have too much or too little water, they have insect or disease problems, or temperatures are unfavorable. Adding nutrients through the leaves will solve none of these problems, and the leaves often green up after such problems are relieved whether or not foliar N was applied. Getting adequate nutrients from the soil to the leaves is hardly ever a limiting factor.

Cautions about using expensive N include not applying it under the rows at or soon before planting and not applying very much N solution in contact with seeds. Some producers applied N as part of a spring strip-till operation in 2007, only to find serious 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 as it did in 2007, waiting a week or more after ammonia application to plant may not prevent damage. If it rains soon after application, damage is unlikely. Because we can't predict this, 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 in the spring.

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 is high if urea is 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 may be needed to preserve it if it was applied on the surface and not injected. There are also urease inhibitors that reduce the rate of breakdown, but they might be more costly than incorporation.--Emerson Nafziger

Author:
Emerson Nafziger

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