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Issue No. 20, Article 5/August 8, 2008

Nitrogen Deficiency in Corn After Pollination

More than 90% of the corn crop has pollinated, and the pollination is reported to have gone well in most fields. There have been few insect problems, and the moisture supply has been good. Silks emerged earlier than the start of pollen shed in many fields, which may have contributed to good seed set in most fields. Favorable temperatures this week should help plants keep the kernels that were fertilized, and filling of kernels is getting underway in most fields.

After pollination, the success of the seed-filling process depends both on the weather--the crop needs sunshine, water, and favorable temperatures--and on the health and completeness of the crop canopy. I have made the point often that sunlight that gets through to the soil surface is simply wasted, and that the minimum yield loss we can expect from "missed" sunlight is 1% yield loss for each percent of light not intercepted by the leaves. The very best canopies will intercept 97% to 98% of the sunlight; underneath such canopies, it should be as dark as a forest floor in thick woods, and there should be only a few small spots of sunlight hitting the ground.

Near-complete light interception beneath a healthy corn canopy.

We have received a number of reports about light green upper leaves and firing of the lower leaves, both symptoms of nitrogen deficiency. When there is not enough N in plants at this stage, the lower (older) leaves mobilize it by breaking down proteins, including those involved in photosynthesis. This is first noticeable as V-shaped yellowing that starts on lower leaves, moving from the tip of the leaf toward the base. Such leaves start to die once they have lost most of their N.

Symptoms of N loss from lower corn leaves, including death of lowest leaves.

Lower leaves are usually shaded and so have limited ability to receive sunlight, at least in those canopies formed by a crop with a relatively good upper canopy. But loss of N from lower leaves is commonly associated with lighter color of upper leaves as well. If N deficiency has been present since early in the season, leaves of such canopies might also be smaller than normal. The result of having leaves smaller and lighter green than normal is a canopy that cannot intercept all of the sunlight it receives.

Light-colored and inadequate leaf cover resulting from N deficiency.

The visible response in our N-rate trials does not seem to be more pronounced than normal for this stage of crop development. This suggests that the supply of N from the soil may be near normal, even after the cool soil conditions in May that likely slowed the release of N from soil organic matter. The large amounts of rainfall in most areas after the soil thawed out last spring probably leached out much of the soil nitrate there at the time, at least in soils that have internal drainage.

Much of the N loss we are seeing is in fields that had N applied early and that included the use of N fertilizer containing nitrate. Conversion of reduced N (ammonium) to nitrate was slower than normal before soils warmed up in June, but it is likely that some, perhaps most, of the nitrate in fertilizer applied at or before planting was lost as large amounts of rainwater moved into and down through the soil in June and July. Compaction from tillage and planting might also have reduced the extent of root growth, thereby limiting the ability of plants to take up the N in the soil, especially N that had moved down.

There is little to be done to correct N deficiency in corn following pollination, especially when we have to count on uncertain rainfall to move fertilizer N into the soil so the roots can take it up. Root systems are also starting to decline in size and activity once ears reach the roasting ear stage. It is unlikely that 10 lb or so of N applied using a foliar-safe form will do much to cure deficiency this late. In hindsight, a supplemental application of N in June might have helped in some fields or parts of fields, but there was no way to anticipate the 12 or more inches of rain in June and July that led to this loss of N.

On the positive side, if the leaves above the ear retain their green color through the dent stage, the loss of lower leaves to N deficiency may not result in much yield loss. Watch the canopy this year, and consider how to apply N in ways that assure more retention when the spring and early summer turn wet. This is not easy--as we have seen this year, "best" management practices for N, such as applying preplant, might not have worked very well due to the unusual weather.

Corn fertilized with UAN and then planted late--in early June--tends to have better color and N status this year than corn planted (and fertilized) early. This lends support to the idea that the main problem this year was loss of early-applied nitrate rather than slow mineralization, though soils warmed up in June and started to contribute N to the crop. If it keeps raining and N nutrition is maintained in the late-planted crop, it might yet yield more than we expected in June.--Emerson Nafziger

Emerson Nafziger

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