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Issue No. 1, Article 8/March 26, 2010

Nitrogen Fertilization in Light of Carryover Challenges from Last Year

We would venture to say that most Illinois farmers are hoping for better weather conditions this spring than in 2009. Last year's spring conditions were challenging enough, and then we didn't get the benefit of a good end to the season to help us forget the earlier challenges a little. Last fall's harvest was late, and in some cases it extended into the middle of winter. It was not unheard of to see combines going through snow-covered fields during very cold days in January and February. All those challenges have unfortunately combined to create new challenges for the 2010 growing season before it even starts.

Crop residue, compaction, and unfertilized fields. The late harvest and wet field conditions left many fields with a lot of crop residue on the surface because tillage could not be performed. These conditions could slow soil warming and drying this spring and potentially delay planting in some situations. Trying to do superficial tillage to bury some of the crop residue could be beneficial in allowing soils to get to better planting conditions sooner. However, light equipment should be used, such as a disk harrow, to prevent creating compaction.

Compaction is actually a problem many producers will already be inheriting from last year. Substantial compaction and ruts due to harvest traffic in less-than-ideal soil conditions may have added to compaction already present from spring corn planting in wet soil. This compaction could restrict root growth and limit soil exploration for water and nutrients. Unfortunately, we probably will not be able to deep-till before planting to alleviate the problem because soils will be too wet to produce any shattering of the compacted layers. If we receive adequate moisture during the growing season, it is unlikely that last year's compaction will have detrimental consequences this year.

The fact that many fields did not receive any fertilizer applications last fall is forcing farmers to apply all the needed nutrients this spring. In terms of nitrogen (N) fertilization, some of the issues we need to sort through this spring include what nitrogen source, when and how to apply it, and at what rate.

N source. Applying N closer to the time when the crop will take it up generally improves efficiency by decreasing loss potential and by placing N in a way that plants can access it more easily. In years like this one, when people might have to decide on a different source of N than is typical, we start to hear claims that some sources are more plant-available than others or that plants favor certain forms of N. These claims are often tenuous and unsubstantiated by research. The truth is that most N fertilizers are or become ammonium soon after application. Urea and ammonia transform quickly to ammonium once in the soil, and three-fourths of urea ammonium nitrate (UAN) is already ammonium or transforms rapidly to it once applied in the soil.

Ammonium is converted to nitrate by bacteria as soils warm up, so the N available from fertilizer changes from mostly ammonium to mostly nitrate as the season progresses. By the time rapid N uptake starts in June, when plants are around V6 to V7 development stage, much of the N from early-applied fertilizers is usually in the nitrate form. While it is known that plants need some ammonium to thrive, we don't need to be overly concerned about supplying N in that form because as soils warm up, the process of mineralization releases N as ammonium from the soil organic matter. Trying to keep more of the N as ammonium, including the use of a nitrification inhibitor (such as N-Serve) in the spring-especially at or after planting-is often unnecessary given the short time between application and plant uptake.

Anhydrous ammonia is often a cost-effective form to use for sidedressing, 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. Switching from anhydrous ammonia to urea or UAN will be safer if there's a need to apply some of the N near the seed, as in strip-till application. N-Serve might be cost-effective if N is applied long before planting, in order to slow the conversion to nitrate. This would be especially true if we have late planting conditions like last year, when soils were wet and warm enough to support bacterial activity.

When and how to apply N. We know that in general delayed planting reduces corn yields, but as we have seen in the last two years, sometimes we can get above-average yields even when corn is planted late. On the other hand, early planting sometimes results in reduced yields. We would propose not rushing planting, but at the same time don't delay it if at all possible. Since many producers have not yet applied any N, they would want to do so as soon as possible this spring to avoid having to be applying N when they could be planting. If it comes to a decision between fertilizing and planting, we recommend planting first and applying N later. As long as N is applied before plants start to take up N rapidly (early June in a normal year), rarely would there be a yield penalty due to delayed application. If the plan is to do anhydrous ammonia applications well before planting, though, we would suggest checking soil conditions before starting the application. Waiting to apply N until soils are dry enough, even though it might mean a considerable wait, allows the ammonia to disperse better and pays in the long run.

If soils are too wet, the soil might not close properly, and this could result in N loss as free ammonia goes off to the atmosphere. Also, when applying on wet soils, ammonia does not disperse very well into the soil and tends to stay concentrated at the point of injection. When soils dry up they tend to crack along the knife track, causing N loss to the atmosphere, and if the crop is already growing, injury can result. Similar injury problems have been seen when soils dry up after application of anhydrous ammonia as part of a spring strip-till operation. For spring applications of anhydrous ammonia, it is worthwhile applying between the corn rows if you have access to autosteering or applying between rows once the field is planted rather than risk seedling injury by planting on top of the knife track. Because seedling roots do grow into the soil, you don't have to get N as close to the seed as possible. Anhydrous ammonia should not be applied closer than 6 inches or so from the row. There is little indication that applying anhydrous or other forms of N halfway between the rows makes N inaccessible to the crop. Remember that most of the N is taken up from the time corn is a foot tall up to pollination, and roots fill the middles quite well during this period. Additionally, application of dry forms of N in bands can be done safely, but be sure that this N can't get into the seed furrow or very near to the seed if a high rate of N is applied this way.

In addition to avoiding ammonia application under the row shortly before planting, another caution regarding spring application is to not apply very much N solution in contact with or very close to seeds. We have been bombarded with the idea that we need to "split" N applications, making sure that there is some N immediately available when the seeds germinate but putting some on earlier or later as well. Once soils are warm enough to get germination to occur, they are starting to release some N from mineralization, so the amount of N available is likely to be more than zero, even without spring application. This is one reason why starter response has been inconsistent in Illinois studies. We have also seen cases where plants were N-deficient early in the season (usually due to soil conditions), with little if any effect on yield potential. But for those really committed to having small plants be as dark green as possible, a urease inhibitor-protected application of UAN after planting (with herbicide, probably) should provide enough N. The nitrate in UAN will get down to the seed level with some rain. Starter fertilizer N will have the same effect.

If you decide on a pop-up application (fertilizer with the seed), no more than 10 lb per acre of N plus K2O is recommended to avoid salt injury. If you have the choice of applying fertilizer as a pop-up or as a starter in a 2x2 placement (2 inches to the side and 2 inches below the seed), we would recommend the latter. If there is a benefit to be gained from a starter fertilizer, it will exist regardless of method, but the 2x2 placement is less risky (especially if the spring is dry) and allows higher rates of fertilizer application.

Finally, since most producers will have to apply all their N this spring, they might need to resort to using other N sources with which they are less familiar. One such source may be urea. The risk of volatilization loss from decomposition to ammonia is high if urea stays on a warm, drying soil surface for several days, especially if there is a lot of surface residue. This is true with both dry urea and for half of the N in UAN solution that is in the form of urea. Rainfall within a few days after application usually moves 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 (such as Agrotain) that reduce the rate of breakdown and also polymer-coated urea (such as ESN), which physically protects urea for some time against breakdown. While these are viable options, the added cost for such protection needs to be weighed against other methods of protecting urea N from loss. A good option for broadcast applications that cannot be incorporated and where there is a lot of residue present is ammonium sulfate. This source is not subject to volatilization losses if it has to sit on the soil surface for a while before it gets incorporated by rain.

N rate. Determining N rates this spring will not be different than in previous years. The approach used with the corn N-rate calculator (extension.agron.iastate.edu/soilfertility/nrate.aspx) works very well for spring applications. These calculations for Illinois are based on more than 400 site years of data collected from recent N rate trials set up to minimize N losses. Nitrogen in these trials was applied as preplant or sidedress application and would represent the way most N will be applied this year in Illinois. Some producers are concerned that there may be very little N carryover from last year and that the rates from the calculator should be adjusted upward. Our data has not indicated that this is necessary. However, if it helps you sleep better at night, one simple way to determine the need for additional N is to establish a reference strip (or preferably a few strips) across the field with 10% to 20% more nitrogen and compare greenness levels between the field and the reference strip(s). If there is no color difference between the two, that would indicate that there is no need for additional N.--Fabián G. Fernández and Emerson Nafziger

Authors:
Fabián Fernández
Emerson Nafziger

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