Response of Canopy Nitrogen Stress Indices to Changes in Cultivar and Available Potassium.

Response of Canopy Nitrogen Stress Indices to Changes in Cultivar and Available Potassium

Taylor D. Coomer, Derrick M. Oosterhuis, Tyson B. Raper, Cristiane Pilan, and James Burke

The spectral response of crops to nitrogen (N) stress has been examined in detail; however, the spectral responses of crops to varying potassium (K) quantities has not been defined. Subsequently, sensitivities of indices such as the Normalized Difference Vegetation Index (NDVI) and the Canopy Chlorophyll Content Index (CCCI) to variables other than N deficiency have been shown to result in over application of N. Responses of each index to variables other than N deficiency must be considered to prevent over application of N and resulting environmental and financial repercussions. Using a randomized complete block study  with five replications, a two-year field study was conducted at the Lon Mann Cotton Research Center in Marianna, AR. The aim was to examine the response of the NDVI and CCCI indices to changes in cotton (Gossypium hirsutum L.) cultivar and plant available K. Treatments consisting of 0, 30, 60, and 90 K₂O/acre applied to three cotton cultivars. Reflectance was measured at two-week intervals after flowering with tissue K and reflectance analyses concentrated on dates in which differences in visible characteristics were observed. First year results show a significant (p<0.05) interaction between cultivar and K quantity, and that K supply increased yield in two of three cultivars. Interaction of available K₂O and cultivar on NDVI was significant (p<0.10), while CCCI was only significantly (p<0.10) affected by cultivar, not K quantity. In the second year, results show there were no interaction effects on available K and cultivar or reflectance index, however, NDVI was affected by cultivar at later dates while CCCI was affected by cultivar both early and late dates. Therefore, CCCI algorithms are preferred over NDVI due to less calibration and earlier detection of K deficiency.