Frequent and heavy rains have been common this spring across the Corn Belt.

Fields were flooded and saturated for days at a time. Temperatures were generally cooler in April and early May, but temperatures increased in mid-May. Nitrogen loss due to denitrification does become a factor in saturated fields in these conditions. Many growers are asking, Are there ways to determine nitrogen loss and what can we do about it?

There are quantitative methods to determine N contents in the soil, such as pre-side dress nitrogen test in fields where manure was applied or other tests for testing nitrate levels. In many cases it is not convenient or possible to do these tests in all the affected fields. There are methods to estimate nitrogen loss. One method proposed by Peter Schaf at the University of Missouri is a score sheet developed with assessing major factors that cause nitrogen loss. Use of the following score sheet can help with determining the possible need for added nitrogen applications where possible, due to stage of crop development on your farm. Also included is an article by Peter Thomison and Jay Johnson from Ohio State who discuss the options for late application of nitrogen on corn.

Click here to use the Nitrogen Loss Scoresheet

Frequently Asked Questions on Late Applications of Nitrogen to Corn – Peter Thomison and Jay Johnson

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, 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.

How Much N Should Be Applied?

If the corn has gotten too tall to sidedress by this point (late June and early July), 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. Supplemental N rates at this point should probably be in the 0.5 to 0.7 pounds N per bushel of expected yield.

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.

Can I Broadcast Urea and 28-0-0 Solutions “Over the Top”?

Using broadcast applications of urea and 28% N solution to sidedress N will cause some burn to foliar tissue of corn plants. Damage results when urea granules or 28% UAN solution get inside the leaf whorl 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 fully expanded 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 at higher N rates and later vegetative growth stages (up to V6) the injury from leaf burn is normally not so severe that it outweighs the potential benefits received from the N addition. The degree of this 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.

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.

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. Even at higher N rates and later vegetative growth stages the injury from leaf burn is normally not so severe that it outweighs the potential.