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N to the rescue

Nitrogen applied either last fall or this spring may be lost from soils in areas experiencing a lot of rainfall in May and June. After excessive rains, corn growers need to reassess the available nitrogen supply in their fields.

Agronomists suggest growers walk fields to estimate N loss, looking for lighter green or yellowing corn. The pale plants are usually in low-lying areas where large amounts of rain have caused ponding, or on sandy soils with high leaching. Corn may show nitrogen deficiency symptoms across large field areas if loss conditions have been severe.

Key Points

Wet spring weather often results in nitrogen losses.

Find N loss with soil test, crop sensor or calculated estimate.

Supplemental N can maintain yield after heavy rainfall.

To estimate losses, use the late-spring soil nitrate test and apply extra N as a sidedress if needed. Using chlorophyll meters, active canopy sensors or aerial photos are also options. Although they don’t provide exact quantities, aerial images can help determine the extent of loss across a whole field. Aerial images work best if corn is waist-high or taller.

The canopy sensors mounted on nitrogen application equipment can also help determine field-wide corn N deficiency. Corn plants must be of adequate size and showing N deficiency before either aerial images or sensors can detect the deficiency. Canopy sensors work by emitting modulated light onto the plants and then measuring how much of that light is reflected back to the sensor. This measure of crop greenness and biomass correlates with plant N stress.

Options for applying N

Growers today have options to correct N deficiencies, notes Tracy Blackmer, research director for the Iowa Soybean Association’s On-Farm Network. Once corn is too tall to sidedress N with a toolbar applicator, rescue options are limited, but not impossible.

One option is to dribble nitrogen between the rows with high-clearance equipment or use aerial application. Another option is to broadcast dry urea. Broadcasting a urea-ammonium nitrate, or UAN, solution on taller corn can cause substantial yield loss due to severe leaf burn. Between-row applications are safer. These applications are most effective when made prior to silking, the R1 stage.

Is a late or “rescue” application of N worth the extra cost and effort? Studies show positive yield responses to in-season N application only if the available nitrogen supply is not meeting crop needs and the crop can respond with enhanced growth, says John Sawyer, Iowa State University Extension soil fertility specialist. Corn typically takes up nearly 40% of its total nitrogen needs after tasseling. Correcting N deficiencies before tasseling ensures that plants can meet their needs throughout grain fill, and the earlier that the N is applied during the vegetative growth stage, the better it helps reduce lost yield potential.

What if you used a nitrification inhibitor in the nitrogen fertilizer applied last fall? With warmer temperatures and significant amounts of rain, even growers who used inhibitors may lose N, says Sawyer. Nitrification inhibitors help delay the conversion of ammonium to nitrate. Corn standing in water for several days or in saturated ground loses nitrate to denitrification. It all depends on the conversion rate of ammonium to nitrate, and the timing of excess rainfall and wet soils.

How long should you wait to know whether there is sufficient N? You should be able to see a substantial difference after the rooting zone re-aerates, perhaps a week after the soil has been dry enough to walk on, the agronomists say. If after that time you still see a light-green canopy, you know with pretty much certainty your corn needs additional N.

How much is needed?

Realize that areas showing N deficiency have lost some yield potential, so a full-rate application isn’t the best alternative because the plant won’t be able to use all that N to make yield. Also, the sooner N is applied, the better response you’re likely to see, says Sawyer. You can see a yield response by applying N up until tasseling. Some studies have shown when corn is severely limited by nitrogen availability, the crop has great capacity to use a rescue N application, and it produces an increase in yield until silking.

Keep in mind that areas within fields needing a rescue nitrogen application are most often patchy, so targeted applications rather than even applications across the field can help minimize the cost and potential N loss to the environment, and increase the return on investment.

How’s it looking?

The appearance of a corn crop is a good diagnostic tool for nitrogen deficiency. Corn that is yellow-green or light-green is most likely nitrogen-deficient, although sulfur deficiency also shows as “less green” plants. Keep in mind, if corn is growing in waterlogged soil, yellow plants will show up even if there is plenty of nitrogen there, due to the plant response to the waterlogged soil conditions, says ISU Extension’s John Sawyer.

He adds, “If plant-available N is present, the yellow corn will green up when the soil eventually dries out, and no additional N may be needed. Careful evaluation is necessary to make sure plants will grow normally after the wet conditions subside; otherwise, additional applied N will be wasted.”


NEED NITROGEN? After heavy rains, many cornfields in Iowa last June showed areas with light-green color next to good-looking plants that were often further along in development. Farmers wondered whether to apply additional nitrogen to these patchy fields.

This article published in the June, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.