Originally published in Purdue Pest Management & Crop Production Newsletter (29 June 1996)

Some Questions and Answers on Applying Nitrogen to Corn After Planting


David Mengel, Agronomy Dept., Purdue Univ., W. Lafayette, IN 47907-1150
Internet address: dmengel@dept.agry.purdue.edu

Many corn fields receive nitrogen fertilizer each year in Indiana after the corn has emerged. Traditionally this means sidedressing anhydrous ammonia when the corn is 6 to 18 inches tall. A number of questions arise each year as to what fertilizers should be used, how the material should be applied, etc. The following are some of the common questions received each year and the answers normally provided.

What are the advantages to sidedressing or delaying N application until after the crop is established? There are two primary advantages to a delayed N application; increased utilization of the fertilizer by the crop and the opportunity to use the time required for preplant applications for corn planting. This is especially important in wet springs where time is critical and N loss may be high.

Does it always pay to sidedress? No, it only pays to sidedress in years and on soils where N is lost during late April, May and June. If there is no early season N loss, then there would be no advantage to sidedressing. Year in and year out, sidedressing pays on excessively well drained sandy soils and on heavy textured, poorly drained soils.

Are there any disadvantages to delayed applications? Yes, the primary disadvantage is risk of not getting through the field in a timely manner. We generally have a window of about 2 to 3 weeks to sidedress, cultivate and put on post-emerge herbicides. If it gets wet, the corn may grow too tall to move through the field without breaking plants off and causing other injury. The more poorly drained the soil the greater the risk. For this reason, many people choose to preplant anhydrous ammonia with NServe, a nitrification inhibitor, on heavy textured, poorly drained soils when soil conditions allow early in the spring. A second risk is that the weather can turn dry and surface applied N will not be available to the crop. This problem is most likely to occur in late planted corn or when N is applied to corn near tasseling.

How early can N be sidedressed? Essentially as early as you can go through the field without injuring the young corn. With traditional buggy applicators or ammonia tool bars this normally means one can start when the corn is 4 to 6 inches tall. Ideally, corn would be in the 4 to 8 leaf stage.

How late can N be applied? Corn utilizes large quantities of N during the grand growth stage. From the 8 leaf stage through tasseling N uptake is 4 to 8 pounds per day. Ideally the N should be applied just prior to this period of high N demand. For most corn hybrids N uptake is complete shortly after pollination. So, most of the N should be applied prior to the 10 leaf stage, with any supplemental applications complete by or shortly after tasseling. Under conditions of severe N deficiency, some response would be expected to low rates of N (30 to 60 pounds) as late as three weeks after pollination.

What is the best N source to use? Ammonia or N solutions knifed in, or ammonium nitrate over the top are preferred in most situations, especially high residue fields. Granular urea can also be applied over the top in clean tilled situations. Both granular urea and ammonium nitrate broadcast in standing corn will cause some foliar burn when granules fall into the whorl. While it may appear unsightly, little yield decrease normally occurs if the fertilizer is applied prior to the 10 leaf stage. N solutions should not be sprayed directly on the foliage as this will cause extensive burn.

Can I spray 28-0-0 solutions "over the top"? No, N solutions should not be sprayed directly on the foliage as this will cause extensive burn. Research in Minnesota has shown significant yield reductions from "over the top" applications of N solutions after the 4 to 6 leaf stage. If N solutions are used they should be knifed in, or applied with drops or as a dribble band, keeping the fertilizer off the vegetation.

Can I apply N to every other row? Research in Indiana, Illinois and Iowa has all shown that farmers can knife ammonia or N solutions in every other row middle ( 60 vs. 30 inch spacing) with no reduction in yield. The only caution is that extra attention must be paid, especially in wet conditions, that no knives plug with soil. A plugged knife in 60 inch spacing gives 4 rows with no N and will seriously reduce yields.

How can I apply N to four foot tall corn? Tall corn requires the use of high clearance ground equipment such as a high boy. Care must be exercised to ensure that N solutions applied are not sprayed directly on the foliage as they will cause extensive foliar burn. Fitting the machine with drop hoses which deliver the fertilizer in a solid stream on the ground is preferred. Hoses should be long enough to drag on the ground. A weight attached to the hose helps it to drag true through the row.

What about aerial application? Aerial applications of granular fertilizers such as urea or ammonium nitrate can be made to growing corn. Unsightly burn spots will result where fertilizer granules fall into the whorl. However yield reductions will be minimal. N solutions should not be applied by air due to the problem of foliar burn.


Return to the the Chat 'n Chew Cafe.

The Corn Growers Guidebook , a WWW resource for corn management systems in Indiana and the eastern CornBelt.

Purdue University Agronomy Extension WWW Home Page.

Purdue Agronomy On-Line! , Purdue's Agronomy Department WWW Home Page.


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