Getting N Rates Right: Crop Sensors to Guide Variable-Rate N

Soils vary widely in how much N they can supply to a crop. Missouri research in 2006-2007 showed a wide range in the best N fertilizer rate for cotton, from 0 to 200 lb N/acre. This variation can be wide even within a field, and from year to year for the same field. Producers do a good job of supplying the ‘average’ fertilizer N needed by a crop, with a little extra thrown in for risk management. This means that most areas and fields are over-fertilized and some are under-fertilized. Diagnosing the actual N needed by your crop is more accurate than fertilizing for the average. Crop sensors show promise for judging how much N is needed by cotton at the mid-square (7 to 10 days after first square) to early flower stages. Several fields have been managed using real-time sensor-based N applications at mid-square in Missouri in 2008 and 2009, with promising results. In the 2008 demonstration field, an average of 45 lb N/acre was saved using sensors, and defoliation was more complete in these areas.