Q. I have heard that corn leaf and plant samples are coming back low in nitrogen. Do I need nitrogen on my corn even though my corn looks good?

A. Whether a specific field has significant nitrogen this year depends on many factors. Wide areas of the state have had so much rain that nitrate leaching is a good possibility. Where water stood or the ground has been saturated for a week or more, there also may be denitrification losses
(nitrogen lost to the atmosphere).

Determining Nitrogen Status

Maximum nitrogen uptake occurs ...

in periods of maximum growth (in corn roughly between vegetative growth stages V9 and V18, or from knee-high to just before tasseling). Risk of N loss is low during this period, but both leaching and denitrification losses can occur. 

You could take a soil sample to determine where the nitrogen is, but to get an accurate assessment you would need to go to 5 ft and this takes some time and effort.

Crop canopy sensors also could play an important role in making these determinations. Our recommendation for their use is to include strips of fully fertilized areas (maybe 50 lb more than usually needed) that are used for a comparison. While such strips may not be common, if they're available, comparing the normal and the fully fertilized areas will help determine if additional nitrogen is needed. The UNL NebGuide, Using a Chlorophyll Meter to Improve N Management (G1632), suggests that when the readings on a normal nitrogen strip drop to 95% or less than the fully fertilized strip, additional nitrogen is recommended.

If you don't have any sensors and you don't have a comparison area, you can observe the corn carefully in the field. Walking through the field and examining the lower leaves can help indicate nitrogen deficiency. The lower leaves will yellow from the tip to the stalk in a V-shaped pattern.  At this time of year the whole plant should be green.

Nitrogen can be applied at any time. Corn nitrogen is just over 60% of the total nitrogen is taken up by R1 (pollination). This is for corn that is growing normally. If it is nitrogen deficient, partial remediation is possible, but the longer you wait the less response you'll get.

After R1 the nitrogen in the leaves moves to the grain, so some yellowing of the bottom leaves is normal. Corn grown for grain whose lower leaves stay green until physiological maturity probably has been over fertilized.

Charles Shapiro, Extension Soil Scientist – Crop Nutrition
Haskell Ag Lab