Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 1, 1996
Publication Date: N/A
Interpretive Summary: Dark green leaves of corn are intuitively associated with good growing conditions. The nitrogen status of corn can be measured with a hand- held chlorophyll meter, but that instrument measures the absorbance of red light passing through a single leaf blade. Information was needed to determine how the greenness of a corn canopy related to the nitrogen (N) status of individual leaves. An instrument that measures the intensity of the various colors of reflected light was used to characterize reflectance from a corn canopy. Measurements were made using four popular corn hybrids grown at five N fertilizer rates. Results showed that reflectance in the green band (550 nm) and another narrow band at the upper edge of the red band (710 nm) were better correlated with crop N status than the chlorophyll absorbing blue (450 nm) or red (650 nm) bands. A ratio of reflectance in the red band (sensitive to N) and the near infrared band (insentivie to N, but sensitive to plant biomass) was also highly correlated with crop N status. An inexpensive photocell with sensitivity in the green band was also found to be sensitive to crop N status. These results indicated that it may be possible to use inexpensive commercial sensors to monitor crop N status.
Technical Abstract: Techniques that measure the N status of corn can aid in management decisions that have economic and environmental implications. This study was conducted to identify reflected electromagnetic wavelengths most sensitive to detecting N deficiencies in a corn canopy with the possibility for use as a management tool. Reflected short-wave radiation was measured from an irrigated corn N response trial with four hybrids that were fertilized at five N rates. A portable spectroradiometer was used to measure reflected radiation (400 to 1100 nm in 1992 and 350 to 1050 nm in 1993) from corn canopies at approximately the R5 growth stage. Regression analysis revealed that reflected radiation near 550 and 710 nm was superior to that measured at 450 or 650 nm for detecting N deficiencies. The ratio of light reflectance in a band from 550 to 600 nm compared to that measured from 800 to 900 nm also provided sensitive detection of N stress. In 1993, an inexpensive photocell, which has a peak sensitivity to light centered at 550 nm, was also used to measure reflected radiation from a corn canopy. Photometric cell readings correlated with grain yield (r2=0.74), but more research will be required to develop procedures to account for varying daylight conditions. These results provide information needed for the development of variable-rate fertilizer N application technology.