Red Edge Detection of Cotton Growth and N Status

Cotton reflectance collected from a tractor mounted sensor has the potential to direct fertilizer N applications.  However, there is a need for a more precise definition of the relationship between sensor-observed reflectance, cotton plant height, and cotton N status.  The objective of this study was to examine NDVI, GNDVI and REIP calculated from the YARA N Sensor (Yara International ASA, Oslo, Norway) for their effectiveness in differentiating cotton (Gossypium hirsutum L.) plant height and leaf N.  Field trials were conducted in 2009 and 2010 at the Plant Science Research Farm, Mississippi State, MS. Fertilizer N rates of 0, 40, 80, and 120 lb N/acre were applied to establish wide growth differences.  The Yara N Sensor was used to collect canopy reflectance across 20 visible and near-infrared wavelengths at several physiological stages.  Plant height and leaf N were also determined at these stages.  NDVI, GNDVI, and REIP were analyzed relative to sensitivity of cotton N status. Derivative analysis of the spectral reflectance signatures showed consistent shifts in red edge inflection point (REIP) across N rates, suggesting REIP to be an indicator of early season cotton N status. Strong relationships for GNDVI and NDVI occurred at later physiological stages which would be less ideal for applying side dress fertilizer N. First week of square GNDVI and NDVI relationships with leaf N were weaker than REIP relationships with leaf N at the same sampling date in both years.   Although GNDVI and NDVI readings may be restricted to directing sampling or N applications in the following growing season, REIP has the potential to be an indicator of cotton N status early enough for efficient use of side dress applied fertilizer N.