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N sensors shine through Reflected light gauges plant health and nitrogen needs

Agriculture.com Staff 12/02/2006 @ 1:46pm

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.

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.

When the sensors are mounted on an N-injection rig or sprayer and tied into a rate controller, you can simultaneously apply N at variable rates as you cross the field.

Using an N sensor paid off last season for Miami, Oklahoma, wheat grower Brent Rendel.

Harmon plans to put the sensors on his spray boom and map other fields when he sprays in late May or June. "Then I can look at the maps and see if there are problem areas."

"The learning curve has been slow, but we think nitrogen sensors have a place," says Higginsville, Missouri, farmer Gary Riekhof. He used two of the sensors in front of his 16-row liquid sidedress applicator.

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