When nitrogen (N) was relatively cheap and water quality wasn't a big issue, many of us applied high rates of N before planting to make sure the crop never ran short. Ironically, putting a high rate of N on early in the season is an important part of a new approach to N management. The twist with this new system is you will only put a high rate of N on a representative area of each field. Then you will use N sensors to compare that N-rich strip to the rest of the field. The sensors let the crop tell you when it has enough N to meet its yield potential and when it doesn't.

Using light reflected off the crop, the sensors compare the color of the crop in the N-rich strip to the color of the crop in the rest of the field.

Choosing the right N rate before the crop is even planted is more a matter of luck than science. It's like predicting the weather, with a slew of other factors thrown in.

N sensors promise a way to gauge crop status midway through the season. For corn, that means after spring and early summer N losses, and also after the soil has had a chance to release N for the crop.

The sensors also provide a means for measuring -- and managing -- the variability across a field. Oklahoma State University (OSU) researchers did a lot of the development work on the GreenSeeker N sensor, and they have done a lot of research on small-scale variability in fields. As an example of that variability, they cite "distinct differences in yield potential" between two small areas in a field that are less than 3 feet apart. Those differences exist even though the two areas have been tilled and fertilized the same for over 30 years.