Abstract

A soil test for improving N management is greatly needed in much of the Corn Belt. Relationships between yields of corn (Zea mays L.) grain and concentrations of nitrate in the surface l-ft layer of soils when corn plants were 6 to 12 in. tall were studied across a total of 756 plots at several locations during 1985 and 1986 to evaluate the late-spring soil test for nitrate. The time of sampling for this soil test represents a compromise between the need to sample late enough to reflect the effects of spring weather conditions on N availability but early enough so that supplemental N can be applied as a sidedressing if needed. A linear-response-and-plateau (LRP) model showed that nitrate concentrations could explain 82% of the variability in relative yields (yields expressed as percentages of the highest yields observed within rotation-site-years) across all data collected. Although this model indicates that 21 ppm nitrate-N in the surface l-ft layer of soil is adequate to attain maximum yields, we suggest that a range of 20 to 25 ppm should be considered optimal. These findings indicate that this soil test offers great potential for improving N management in the Corn Belt. Amid mounting concerns that many farmers are applying more fertilizer than is desirable for environmental and (or) economic reasons, the most important use of the soil test may be to reduce excessive applications of fertilizer by showing when additional N is not needed.

Copyright © 1989. . Copyright © 1989 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, 5585 Guilford Rd., Madison, WI 53711 USA