USING SITE-SPECIFIC NITROGEN FERTILIZER APPLICATION TO REDUCE NITRATE LOSSES

J.G. Davis and D.K. Shannon
University of Missouri
Columbia, MO, USA

N.R. Kitchen and K.A. Sudduth
USDA-Agricultural Research Service
Columbia, MO, USA

ABSTRACT

The optimal crop requirement for nitrogen (N) varies widely from year to year, and within farm fields. The objective of this work was to determine the site-specific applications of N that are adequate to meet crop needs that decrease environmental N loss with no yield loss. In this study uniform N rates were applied in strips across the fields. Within-season crop N status was monitored using a chlorophyll meter. Chlorophyll concentration in leaves is often a good indicator of N sufficiency for optimum crop growth. Sampling transect positions for chlorophyll concentration sampling were selected by examining maps of soil electrical conductivity. Soil electrical conductivity has been correlated to soil properties in previous Missouri studies. Grain yields were measured using a DGPS-equipped yield monitor combine. Using side-by-side yield response comparisons the optimum N rate was determined. There was a wide variation in the optimum N fertilization rate. Chlorophyll concentration and optimum N rate determinations did not always agree regarding the true optimum fertilization rate, probably because of lower N use efficiency of added N at higher fertilization rates.

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