SOIL AND PLANT TESTS TO PREDICT OPTIMUM NITROGEN RATES FOR CORN1*

Abstract

Optimum fertilizer nitrogen (N) rates are highly variable from one production corn (Zea mays L.) field to another, but producers usually use about the same N rate over their whole farm. Matching N application rates to actual crop need could have both economic and environmental benefits. The objectives of this research were to evaluate soil and plant tests for their ability to predict optimum N rates across a range of production cornfields in Missouri, and to evaluate the economic performance of recommendation systems based on these measurements. Yield response to N was measured in each experiment, along with soil mineral N measurements (planting and sidedress), tissue N at sidedress time, and chlorophyll meter reading at sidedress time. Optimum N fertilizer rates were fairly evenly spread from 0 to 235 kg N ha−1 in these sixteen experiments. Soil N measurements, tissue N at sidedress, and chlorophyll meter reading at sidedress were all clearly related to optimum N rate—the higher the test result, the lower the optimum N rate. Plant measurements (tissue N content and chlorophyll meter reading) were more strongly related to optimum N rate than were any of the soil measurements. Only two of the recommendation systems tested significantly increased profit (by $17 to 20 ha−1) relative to N rates used by producers in these fields: 1) subtracting both traditional credits (for manure, alfalfa [Medicago sativa L.], or soybean [Glycine max L.]) and a preplant soil nitrate credit from current University of Missouri recommendations, or 2) using sidedress tissue N content to predict sidedress N rate. Three other systems reduced N rates significantly relative to producer N rates without reducing profits: 1) chlorophyll meter to predict sidedress N rate, 2) sidedress soil nitrate test with the critical value adjusted for wet springs, and 3) University of Missouri recommendations with history credits only. *Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 12,767.