Research hypothesis: Variable-rate nitrogen application systems that incorporate within-season crop reflectance sensing will improve profitability and reduce the environmental risks associated with nitrogen application.

Applying nitrogen (N) in excess of crop needs results in unused soil nitrate and increased potential for N loss. Unused nitrate can move to groundwater, to surface waters, or denitrify from the soil into atmospheric greenhouse gases. Farmers want to be good environmental stewards; however, many ideas proposed for reducing N loss from fields create the risk of reduced productivity and profitability. To reduce N loss while maintaining productivity, several research groups have recently explored variable rate N fertilizer application. Advances in reflectance sensor technology have allowed development of sensing/applicator systems for assessing N needs and automatically adjusting N application rates based on the principle that plants under N stress are lighter in color and reflect more light than plants with sufficient N.

Researchers in Missouri and other states have been working cooperatively to develop recommendations for N fertilizer application on corn based on reflectance measurements. Research is underway at the Delta Center to apply the same approach to cotton. Research plots with N application rates ranging from 0 to 200 lb N/acre were established on a Tiptonville silt loam at the University of Missouri (MU) Lee Farm near Portageville and a Bosket fine sandy loam at the MU Rhodes Farm near Clarkton. Reflectance readings were made at approximately two-week intervals beginning shortly after the first square growth stage and are expected to continue through physiological maturity. Supporting measurements include soil N content, leaf chlorophyll content, and plant growth and maturity indicators. Similar measurements were also made in an ongoing N study on a Portageville clay at the MU Lee Farm.

The goal of this research is to develop recommendations for reflectance-based N application. In this way, optimal rates can be applied to every part of the field, rather than making field-wide blanket applications and knowing that some areas of the field will receive too much N while others don't receive enough. In this way, both the economic impact on the grower of applying more N fertilizer than needed and the environmental side-effects of excessive N application will be reduced. In addition, indirect effects on growth and maturity resulting from excessive or deficient N can be better managed.

2006 Field Day Report