Submitted to: Agronomy Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/1/2011
Publication Date: 12/8/2011
Publication URL: https://www.agronomy.publications/aj/articles/104/1/148 doi: 10.2134/agronj2011.0172
Citation: Forrestal, P.J., Kratochvil, R.J., Meisinger, J.J. 2011. Late-season corn measurements to assess soil residual nitrate and nitrogen management. Agronomy Journal. 104:148-157. Interpretive Summary: A late growing-season evaluation of corn nitrogen management and soil residual nitrate could provide important management information for subsequent winter wheat crops, or for prioritizing sites for planting grass cover crops. Our objective was to evaluate the ability of several late-season corn based measurements to identify sites with elevated soil residual nitrate. The field research studies applied four levels of fertilizer nitrogen to corn at rates ranging from nitrogen deficient (no nitrogen or 60 lbs. nitrogen per acre) to a luxury nitrogen rate of 240 lbs. nitrogen per acre. These nitrogen response studies were conducted at multiple sites and multiple years, providing ten site-years in all and included sites in the Atlantic Coastal Plain and the Piedmont. The corn based measurements were taken for each nitrogen rate treatment at three reproductive stages and consisted of: the visible and near infrared reflectance from corn leaves, leaf chlorophyll measurements with a portable hand-held meter, and the number of green leaves per plant. In addition, the corn stalk nitrate test and the post-harvest soil residual nitrate content were measured for all treatments. The corn stalk nitrate test was positively correlated with residual nitrate nitrogen, although high residual soil nitrate was found at some sites when the corn stalk nitrate test indicated that excess nitrogen would be unlikely, these data were associated with drought conditions. Drought stress was a major factor affecting residual soil nitrate. Drought reduced the need for nitrogen fertilizer, which contributed to the surplus nitrogen that manifested itself as increased fall residual nitrate when compared to non-drought site-years. All corn based canopy measurements at the kernel milk- to dough-stage were effective indicators of drought stress. The success of the canopy reflectance measurements of visible and near infrared light is noteworthy because it can be measured remotely, thus allowing for identification of drought sites in late summer before harvest. This early identification of drought sites should provide opportunities for targeting cover crops to high residual nitrate sites, which should reduce potential winter nitrate losses to water resources in humid regions.
Technical Abstract: Evaluation of corn (Zea mays L.) nitrogen (N) management and soil residual nitrate (NO3-N) late in the growing season could provide important management information for subsequent small grain crops and about potential NO3-N loss. Our objective was to evaluate the ability of several late-season corn based measurements, which have been advocated to assess corn N management, to identify sites with elevated soil residual NO3-N. These crop based measurements were collected at three reproductive phases and included normalized difference vegetative index (NDVI) at ten site years and green-leaf number and chlorophyll (SPAD) meter readings at six of these sites. The corn stalk nitrate test (CSNT) and post-harvest soil residual NO3-N were measured at all sites. Four levels of corn fertilizer N were applied, ranging from N deficient (0 or 67 kg N ha-1) to 269 kg N ha-1. The CSNT was positively (p<0.001) correlated with residual NO3-N (r2=0.41), although residual NO3-N was not always low at CSNT values < 2.0 g NO3-N kg-1, where drought reduced production. Drought stress was a major factor influencing excess N rate and residual soil NO3-N. All canopy measurements at growth stages R3-R4, including NDVI, which can be measured remotely, were effective indicators of drought stress. Across sites, relative canopy readings best predicted relative grain yield when collected at R3-R4. Use of remotely measured NDVI would allow policy makers to identify drought sites in the late summer and target them for cover crop planting, thus reducing potential winter NO3-N losses in humid regions.