Nitrogen Response Index as a Guide to Fertilizer Management

The efficiency with which fertilizer-nitrogen (N) is transferred to grain-N in cereals is usually less than 50% and averages 33% worldwide. Two long-term N fertility experiments were evaluated to examine temporal changes in nitrogen use efficiency (NUE) and their causes. Averaged over 30-yr, non-irrigated winter wheat NUE was 49% at a 22.4 kg N ha 1 yr 1 rate and decreased to 34% at a rate of 112 kg N ha 1 yr . The average NUE for a 15-yr irrigated corn experiment was 30.6% at the lowest N input (90 kg N ha ) and decreased to 18.3% at a rate of 270 kg N ha 1 yr . Low NUE values are a result of excess N present in the soil–plant system. The extent to which N is present in excess is determined by the potential yield and how much of that yield will be supported by non-fertilizer N, presumably mineralized from soil organic matter. For both experiments there was greater temporal variability in nonfertilized (check-plot) yields (CVof 31.4 for wheat and 34.3 for corn) than Contribution from the Oklahoma Agricultural Experiment Station. *Correspondence: G. V. Johnson, Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 74078, USA. E-mail: [email protected]. JOURNAL OF PLANT NUTRITION Vol. 26, No. 2, pp. 249–262, 2003 DOI: 10.1081=PLN-120017134 0190-4167 (Print); 1532-4087 (Online) Copyright # 2003 by Marcel Dekker, Inc. www.dekker.com 249 ©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc. MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016