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The Corn Crop and Sidedress Nitrogen

8/30/2017

Written by Emerson Nafziger    (View the U of I bulletin)

The weather has turned from cool and wet to warm and dry, with thoughts now turning to when it might rain next. The US Drought Monitor at http://droughtmonitor.unl.edu/ shows no drought in the Corn Belt, and water use is still low, but some plants whose roots are not growing well or are in compacted soil are starting to show afternoon leaf curling, and water demand is increasing as plant growth rates increase. We hope rainfall returns soon.

 As I have been reporting in recent posts, our soil N sampling is continuing to show that most of the N we applied to the crop earlier is still present. The amount of fall-applied N we recovered here at Urbana on May 31 was down only slightly from the amount recovered on May 17, and, at about 170 lb. N per acre (after applying 200 lb. last fall), is only about 15 lb. less than we found at this time in 2016. We recovered more than 240 lb. of N from NH3 applied in April, and more than 100 lb. of N from soil that hadn’t received any N fertilizer. These are also in line with what we’ve seen in early June in the past.

While the soil N supply seems to be holding up fairly well as soils dry, the crop in many fields is showing symptoms of the stress it’s been through. One of the most common is uneven growth. Our corn planted here on April 20 emerged fairly well, but in places where water stood temporarily, we see some lower stands and considerable variability down the row in plant size and growth stage. It’s hard to guess what caused this, but it’s likely that it will affect overall yield potential as plants that are behind now struggle to compete.

In areas in many fields where water stood, crop color continues to be paler than normal. This is related to both the effect of water on root health and growth, and perhaps to loss of some nitrate from the soil around the roots. We expect that color of these plants will improve some, but we can’t say with certainty that the plants still have their full yield potential, especially if it takes another week or more for the color to improve.

April-planted corn in central Illinois has now reached the V6 stage (6 leaves with collar visible) or beyond, and above-normal temperatures are helping growth accelerate as the stem begins to elongate. The need for water increases as plants get larger. Roots take up water near them and dry out the soil there, so root growth need to increase in order to maintain the water supply to the plant.

We normally consider some dry weather in June as a positive, since it encourages roots to growth deeper. But with the difficult start to the season this year, including low soil oxygen, cool temperatures, and water that likely moved nitrate down more than normal, roots may not be able to grow down fast enough to keep up with the demand for water and N. Plants in many fields are showing leaf curling by the afternoons during the current stretch of warm, sunny weather. More leaf area means more demand for water, and we can expect the crop to continue to struggle.

 Corncrop and sidedress.jpg

 

Managing sidedressed nitrogen

The best measure of the N supply to the crop right now is crop color. With the sunshine and warm temperatures, many early-planted fields or parts of fields where water didn’t stand are showing considerable improvement in crop color, with leaves now taking on the dark green color we hope to see.

If a field or part of a field is paler in color than plants of similar size in the same field or other fields, then it’s probably not getting enough N with the water it takes up, and as discussed above, it may not be taking up enough water.

A lot of questions have come up about how to manage N now that conditions are good for application and the crop is starting to take up N more rapidly. Here are some questions and answers on the topic:

If corn was replanted or planted late, should the amount of N applied be lowered to reflect lower yield potential?

Our research does not show that lower yields usually require less fertilizer N than higher yields. We think that’s because the causes of lower yields, which are typically stress from having less available water at critical times, often affect root growth, and so may make it harder for plants to take up the N that’s in the soil. If the plan was to apply the 160 to 180 lb. of N needed to produce the best return for corn following soybeans (200 to 210 lb. N for corn following corn) then stay with that amount. If the plan was to apply more than that, then cutting back would be reasonable.

Should I plan to apply sidedressed N more than once over the next month?

While the idea of “spoon-feeding” N has some appeal, we have found very little benefit to delaying some of the N until later during vegetative growth. As soils dry out, concern will increase about whether applied N is getting to the roots, and applying N more than once will bring that same concern each time. Chances for multiple applications to pay for themselves are low by now, and they’ll get even lower, especially if it remains relatively dry.

Should we use inhibitors with N applied now?

With the crop starting to take up N, there is simply no need to try to keep applied N in the ammonium form as long as possible, which is what nitrification inhibitors do. Plants take up mostly nitrate, but always have access to some ammonium. There’s no problem associated with this mixture, and there is no benefit to trying to increase the amount of ammonium. Using a urease inhibitor to slow the loss of urea (as ammonia gas) might be useful if applying urea or UAN, but only if urea is applied to the soil surface. Even then, a half inch or more of rain will carry surface-applied urea into the soil, which will capture any volatilized ammonia. There is no value in using a urea inhibitor with injected UAN. Finally, using extended-release forms of N is inappropriate when the crop has reached the stage of rapid uptake. The N needs to get into the soil and available to the crop as soon as possible. There is a time for inhibitors, but it is not during sidedress unless there’s no alternative to surface placement of urea and the weather is in a dry pattern.

What about N placement?

It is important to get sidedressed N into the soil near the roots as soon as possible so uptake can get underway. Nodal roots take up nearly all of the N, and these roots originate at the lowermost nodes of the plant – they are shallow near the plant and deeper farther away. When the surface soil dries out, roots in the top several inches of the soil may not be actively taking up water, so aren’t taking up N. In that case, applying UAN solution near the row may improve access of the plant to the N, compared to shallow placement in the row middles. But soils near the plant typically dry out first, and roots may be more active farther away from the rows but several inches deep. It may be worth increasing application depth if UAN is injected between the rows. Anhydrous ammonia should also work well, but roots of V6 plants are well out into the row middles, and may be damaged slightly by injection of NH3.

Is nitrogen management the key to bringing this crop back to full yield?

History tells us that the water supply will be the key to how the 2017 corn crop does. The difficulty, of course, is that we can’t do much about the water supply; but we can do something about nitrogen, including adding more N to compensate for what we think might have been lost. We do need to use sound management in applying any N that still needs to go on, and it needs to go on quickly in early-planted fields. But it’s unlikely that making extra trips or applying a lot more N than we had planned to apply is going to be profitable.