Early detection of cotton leaf nitrogen status using leaf reflectance

Nondestructive assessment of cotton (Gossypium hirsutum L.) nitrogen (N) nutritional status could aid fertilizer recommendations by providing a rapid, easy, and inexpensive method of detection. A greenhouse experiment with fertilizer N rates equivalent to 40, 80, 120, and 160 kg ha(-1) was conducted to determine the response in leaf reflectance and chlorophyll meter readings to varying N nutritional levels. A 50/50 blend of masonry sand and a Marietta soil (fine-loamy, siliceous, thermic Fluvaquentic Eutrochrepts) was used as potting media. Measurements were taken the second week of squaring and the second week of flowering. Reflectance and chlorophyll readings were found to be significantly (p < 0.05) correlated with leaf N content. A deficiency of N increased leaf reflectance and decreased chlorophyll readings. However, prediction models based on light reflectance measurements at 550 nm wavelength (lambda) provided better estimates of cotton N status at squaring (r(2) = 0.60, p = 0.0032) and flowering stages (r(2) = 0.89, p = 0.0001), than chlorophyll meter readings (r(2) = 0.36, p = 0.0396, and r(2) = 0.87, p = 0.0001, respectively). Prediction models based on red edge wavelengths improved N concentration estimates only at flowering (r(2) = 0.92, p = 0.0001). Leaf reflectance as an indicator of leaf N status may be a feasible tool to quickly estimate the N nutritional status of cotton when N alone is the limiting nutrient.