Variable Rate Nitrogen Using Sensors

The top presentation last week in Springfield was by Peter Scharf of the University of Missouri in Columbia.  He points out that the need for an approach to variable rate nitrogen fertilizer is need for obvious reasons as simple as looking for yellow areas in the corn field.  He has looked at a number of ways to adjust nitrogen rates in the field and has found most of them lacking in some respect.

Rates based on historical yield data seem to leave lower yield areas short of nitrogen, which is not really a surprise because I am betting many of those areas are wet areas that could benefit from added N during the growing season. 

He finds that rates based on soil nitrate tests are not great because nitrates go up and down based on moisture and aeration.  We use the soil nitrate test for assessment and recommendations.  In general, I think we make good recommendations on a field basis, but we have not tried to do variable rates.  A short come that I can see is that in order to make variable rate maps, the soil testing would prove costly.  I have seen some expatriation with 1 acre grids.  That is expensive and time consuming to do it right. I will say that if yo want to find out what is going on with nitrogen in general then soil testing is the way to go. 

Beck's Seeds has been using a system of Aerial photography and reference strips to develop nitrogen rate zones.

Scharf has been working on his system for a number of years.  He has had the most success with  using reference strips for calibration and using a tractor mounted Greenseeker.  the short coming of the Greenseeker  is that in needs to be calibrated about every 2 hours.  He preferred it over the OptRX.  In addition to the reference strip, one of Scharf's recommendations was to put on about half of your nitrogen rate early.  He also cautions against use of starter nitrogen.  He shared a very complete paper called Managing Nitrogen with Crop Sensors.  I could not really repeat all his information in this space.

It looks like very promising technology that is ready to try.  Before you jump into it you need to get the details from Scharf.  Another well written paper out of Iowa State University references Scharf's work as well. I should close by saying that i am not overly fond of using refence strip because it adds work, but it may be the price yo pay for acculturate variable rate N.