Nitrogen recommendations based on crop reflectance

Nitrogen fertilizer is usually applied in greater quantities to corn than almost any other crop. But when it’s applied in excess of requirements, loss of the excess fertilizer to the environment can contribute to degraded water quality. One of the challenges to making an appropriate nitrogen fertilizer recommendation is the potential variability in soil nitrogen availability that may occur spatially across a field and temporally from year to year.

A canopy reflectance sensor would provide an easy-to-use method for farmers to improve sidedress nitrogen applications to corn if canopy reflectance could be successfully correlated to nitrogen requirements. Scientists at the Pasture Systems and Watershed Management Research Unit of USDA-ARS and Penn State University compared crop canopy reflectance to more traditional approaches of making nitrogen recommendations for corn.

The success of an active sensor, Crop Circle ACS-210 (Holland Scientific, Lincoln, NE), was compared with the success of using a chlorophyll meter, presidedress nitrate test (PSNT), and a preplant assessment for making appropriate nitrogen recommendations. Results from the study were published in the July–August issue of Agronomy Journal.

The economic optimum nitrogen rate (sidedress) for corn was determined at eight different sites in 2005 and 2006. These sites were selected to represent a wide range of soil nitrogen availability, based on the previous crops of corn, soybean, or alfalfa, and some sites had a history of receiving manure applications.

At each site, three different preplant treatments were applied, including: a manure application, 56 kg ha–1 nitrogen as ammonium nitrate, and zero nitrogen fertilizer. Seven sidedress nitrogen rates increased from zero to 280 kg ha–1 nitrogen. The success of the different methods of making nitrogen fertilizer recommendations was compared with the observed optimum nitrogen rate.

The results from this study indicated that a reflectance index based on relative green normalized vegetation index (GNDVI) measured when the corn was at the six to seven mature leaf stage was as good an indicator of optimum nitrogen requirements as a relative chlorophyll index or the PSNT. These latter two methods are currently considered the most reliable ways for making a nitrogen fertilizer recommendation for corn in Pennsylvania.

“Reducing the overall amount of nitrogen fertilizer that is applied to corn will depend on accounting for the within-field spatial variability and year-to-year temporal variability,” says John Schmidt, one of the scientists conducting the study. “This research suggests that active sensors could play a significant role in developing appropriate nitrogen recommendations for corn in Pennsylvania and the mid-Atlantic region.”

This research is continuing in various geographic regions of Pennsylvania and is being evaluated at the field scale using precision agriculture technologies.

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