Fortunately, several tools are now available to assess the nitrogen status of your management system. The most rudimentary but yet invaluable tool is the soil sample. It may be a lot of work, but taking composite soil samples at different depths (0-8, 8-24, and 24-48 inches) and having them analyzed for ammonium and nitrate would give more confidence as to how much nitrogen is currently in the system, and where it is located. Is the bulk of the nitrogen in the lower depths? This should raise concerns for future losses.

Other tools are also available, such as chlorophyll meters, active crop canopy sensors and aerial imagery. These tools all require a growing crop and work best after the V8 growth stage, which is just beyond the window of most toolbar sidedress options. These tools are accurate if calibrated: They require nitrogen-rich strips to be used as the comparison. Sensors have been shown to be an effective management strategy for varying nitrogen rate throughout the field via high clearance N applicator. (See UNL Project SENSE introduction and 2015 results.) We realize not everyone will have access to these machines, and so what other options are there?

If the sensors need to be used beyond the point of when you can sidedress, think about using the PSNT, which is a 1-foot sample taken the first week of June. The ppm nitrate in this sample have been calibrated for Iowa, but not Nebraska. Consider how much N is currently in your system, and use the UNL N algorithm in Fertilizer Suggestions for Corn or use modeling tools like Maize-N to help estimate future N needs based on your field conditions.

Last but not least, some growers are set up to fertigate. Many shudder at the thought of putting more water on a field that is sopping wet, but later in the season, fertigation allows a grower to respond to potential deficiencies by delivering a timely application of N to the crop. We do not currently have guidelines specific to fertigation (we’re working on it), but by using the Nebguide Using a Chlorophyll Meter to Improve N Management, you can get a reasonable estimate of when and how much N to apply.

Of course all of these methods still require continuous monitoring of your corn crop and its response to changing weather and field conditions. Nitrogen is a tricky nutrient and needs to be watched carefully to prevent losses to both your bottom line and the environment.

(Source – http://cornandsoybeandigest.com/fertilizer/nitrogen-management-wet-spring?page=1)