Persistent rains this year may force many growers to sidedress their nitrogen (N) in corn this year much later than what is considered normal. Other growers may be supplementing their earlier N applications to replace N lost from denitrification and leaching. The following are some suggestions from extension soil fertility specialists at Ohio State and Purdue University from past years that address various questions concerning N applications to corn after planting.
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. 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 are preferred in most situations, especially high residue fields. Granular urea can also be applied over the top in clean tilled situations, but those applications can be risky if rainfall does not come shortly after application. Urea stabilizers (Agrotain) should be considered in high residue situations. Granular urea 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.
HOW MUCH N SHOULD BE APPLIED? If the corn has gotten too tall to sidedress by this point (mid to late June), it has probably not been severely stressed and yield potential is still good. An example would be rotation corn after beans which had some starter or 28% applied with herbicides with good green color. Nitrogen rates should approach what was initially planned at the beginning of the season. Research at Ohio State shows that decreased rates can do well, but do not decrease rates by more than 10-15%.
CAN I BROADCAST 28-0-0 SOLUTIONS "OVER THE TOP"? Using broadcast applications of 28% N solution to sidedress N will cause some burn to foliar tissue of corn plants.
The severity of injury is determined by the plant's stage of growth, the amount of N used and form of N. If the plant growing point is at or below the soil surface (or when plant has six collared leaves or less), the extent of foliar injury caused by burn will usually be negligible if the N rate is kept below 50 lb/acre. Even with higher N rates at later vegetative growth stages the injury from leaf burn is normally not so severe that it outweighs the potential benefits received from the N addition. The degree of plant burning is less with urea granules than with other N products.
Dribbling 28% solution with drop nozzles as a narrow band on the soil surface is an alternative approach that can help reduce foliar burning. Dribbling 28% is also a more efficient use of N than broadcast surface application because it helps reduce N volatilization. Urea stabilizers may be considered for this application, but tillage is a major deciding factor on whether or not they are necessary. High residue corn fields may benefit from urea stabilizers, but low residue fields are less likely to benefit. At this point of the growing season, and the need for plenty of nitrogen by the crop, we do not recommend nitrification inhibitors. We also rarely see positive responses to nitrification inhibitors when N is sidedressed.
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.
Another article on this topic published by Purdue University in 2011 can be found here: http://www.agry.purdue.edu/ext/corn/news/timeless/CornRespLateSeasonN.html.