Originally published in (29 June 1996)
David Mengel, Agronomy Dept.,Purdue Univ., W. Lafayette, IN47907-1150Internet address: dmengel@dept.agry.purdue.edu
Many corn fields receive nitrogen fertilizer each year in Indiana afterthe corn has emerged. Traditionally this means sidedressing anhydrous ammoniawhen the corn is 6 to 18 inches tall. A number of questions arise each year asto what fertilizers should be used, how the material should be applied, etc.The following are some of the common questions received each year and theanswers normally provided.
What are the advantages to sidedressing or delaying N applicationuntil after the crop is established? There are two primary advantages to adelayed N application; increased utilization of the fertilizer by the crop andthe opportunity to use the time required for preplant applications for cornplanting. This is especially important in wet springs where time is criticaland N loss may be high.
Does it always pay to sidedress? No, it only pays to sidedress inyears and on soils where N is lost during late April, May and June. If there isno early season N loss, then there would be no advantage to sidedressing. Yearin and year out, sidedressing pays on excessively well drained sandy soils andon heavy textured, poorly drained soils.
Are there any disadvantages to delayed applications? Yes, theprimary disadvantage is risk of not getting through the field in a timelymanner. 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 growtoo tall to move through the field without breaking plants off and causingother injury. The more poorly drained the soil the greater the risk. For thisreason, many people choose to preplant anhydrous ammonia with NServe, anitrification inhibitor, on heavy textured, poorly drained soils when soilconditions allow early in the spring. A second risk is that the weather canturn dry and surface applied N will not be available to the crop. This problemis most likely to occur in late planted corn or when N is applied to corn neartasseling.
How early can N be sidedressed? Essentially as early as you cango through the field without injuring the young corn. With traditional buggyapplicators or ammonia tool bars this normally means one can start when thecorn 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 Nduring the grand growth stage. From the 8 leaf stage through tasseling N uptakeis 4 to 8 pounds per day. Ideally the N should be applied just prior to thisperiod of high N demand. For most corn hybrids N uptake is complete shortlyafter pollination. So, most of the N should be applied prior to the 10 leafstage, with any supplemental applications complete by or shortly aftertasseling. Under conditions of severe N deficiency, some response would beexpected to low rates of N (30 to 60 pounds) as late as three weeks afterpollination.
What is the best N source to use? Ammonia or N solutions knifedin, or ammonium nitrate over the top are preferred in most situations,especially high residue fields. Granular urea can also be applied over the topin clean tilled situations. Both granular urea and ammonium nitrate broadcastin standing corn will cause some foliar burn when granules fall into the whorl.While it may appear unsightly, little yield decrease normally occurs if thefertilizer is applied prior to the 10 leaf stage. N solutions should not besprayed directly on the foliage as this will cause extensive burn.
Can I spray 28-0-0 solutions "over the top"? No, N solutionsshould not be sprayed directly on the foliage as this will cause extensiveburn. Research in Minnesota has shown significant yield reductions from "overthe top" applications of N solutions after the 4 to 6 leaf stage. If Nsolutions are used they should be knifed in, or applied with drops or as adribble band, keeping the fertilizer off the vegetation.
Can I apply N to every other row? Research in Indiana, Illinoisand Iowa has all shown that farmers can knife ammonia or N solutions in everyother row middle ( 60 vs. 30 inch spacing) with no reduction in yield. The onlycaution 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 rowswith no N and will seriously reduce yields.
How can I apply N to four foot tall corn? Tall corn requires theuse of high clearance ground equipment such as a high boy. Care must beexercised to ensure that N solutions applied are not sprayed directly on thefoliage as they will cause extensive foliar burn. Fitting the machine with drophoses which deliver the fertilizer in a solid stream on the ground ispreferred. Hoses should be long enough to drag on the ground. A weight attachedto the hose helps it to drag true through the row.
What about aerial application? Aerial applications of granularfertilizers 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 byair due to the problem of foliar burn.
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