A Corn Nitrogen Status Indicator Less Affected by Soil Water Content

The SPAD‐502 chlorophyll (Chl) meter and the Dualex device can estimate crop N status based on the measurement of leaf Chl concentration and polyphenolics (Phen) concentration, respectively. However, soil water status may confound such assessment of N status. This study compared the sensitivity of SPAD, Dualex, and SPAD/Dualex ratio as indicators for assessing corn ( Zea mays L.) N status and the influence of soil water content (SWC) on the indicators in their absolute and nitrogen sufficiency index (NSI) expression. A greenhouse trial was conducted with five N fertilizer application rates (0, 50, 50+75, 50+150, and 200 kg N ha −1 ) and three SWC levels (high, medium, and low). A field trial also was performed with six N rates (0, 20+40, 20+80, 20+120, 20+160, and 250 kg N ha −1 ) and spatially variable SWC as a covariate. The responses of SPAD, Dualex, SPAD/Dualex ratio (and their corresponding NSI) to N rates and SWC levels were compared. The results showed that SPAD, Dualex, and SPAD/Dualex ratio were all influenced significantly by N rates and by SWC levels. When expressed as NSI, however, the parameters' relationships with N were essentially decoupled from SWC. The NSI SPAD was more affected significantly by interactions among N, SWC, and DAS (days after sowing) than were 1/NSI Dualex and NSI SPAD/Dualex . The latter showed a greater sensitivity to N fertility levels than the other indicators, resulting in a better discrimination of N treatments and under variable SWC conditions in the two trials.