Nitrogen Response Index as a Guide to Fertilizer Management

Abstract

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⁻¹. The average NUE for a 15‐yr irrigated corn experiment was 30.6% at the lowest N input (90 kg N ha⁻¹) and decreased to 18.3% at a rate of 270 kg N ha⁻¹ yr⁻¹. 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 non‐fertilized (check‐plot) yields (CV of 31.4 for wheat and 34.3 for corn) than in plots where maximum yields were obtained from N‐fertilizer (CV of 20.1 for wheat and 15.7 for corn). The yield of unfertilized plots was not related to the maximum yield of fertilizer plots over time. A response index (RI) was calculated by dividing the maximum yield of fertilized plots by the yield of unfertilized plots to determine the extent of fertilizer need and response for a given year. For both wheat and corn the RI was unpredictably variable (CVs≈35) over time, and ranged from lows of about 1.1 to a high of 4.1 for wheat and 3.5 for corn. In general, low RI values resulted when unfertilized yields were high, which even occurred after 10 to 30 yr without N fertilization. Low RI values may be more common in farmer's fields where N is applied annually. Except at the lowest N rates, NUE increases for each rate as RI increases because N inputs are less likely to be excessive. Nitrogen management strategies that increase NUE may only be possible to evaluate for site‐years when RI is substantially greater than 1 (e.g. >1.5). Since response to N fertilizer is strongly dependant on supply of non‐fertilizer N in a given year, any N management strategy that includes a reliable in‐season predictor of RI should dramatically improve NUE in cereal production.

^#Contribution from the Oklahoma Agricultural Experiment Station

Acknowledgments

The authors are grateful to Drs. R. L. Westerman and B. B. Tucker (Emeritus), former faculty of the Plant and Soil Sciences Department of Oklahoma State University, and Professor R.A. Olson (deceased) former faculty member in the Department of Agronomy of the University of Nebraska for their foresight to establish and continue the long‐term N fertility studies that made this work possible.

Notes

^#Contribution from the Oklahoma Agricultural Experiment Station