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The objective of this study was to evaluate seasonal differences in bermudagrass response to N fertilization and irrigation by using optical sensing. A second objective was to determine if optical sensing could measure N status when the turf response to N was confounded by differences in moisture status. Bermudagrasses (Cynodon dactylon L.) 'Rivera' and 'Yukon' were managed under three irrigation treatments and six N treatments during the growing seasons in 2003 and 2004. Turf quality, normalized difference vegetation index (NDVI), green normalized difference vegetation index (GNDVI), red light reflectance in relation to near infrared reflectance (R/NIR), and green light reflectance in relation to near infrared reflectance (G/NIR) were measured. Bermudagrass demonstrated a noticeable third-order polynomial seasonal trend in response to N and irrigation treatment, and this trend was characterized as early-, peak-, mid- and late-season responses. Normalized difference vegetation index and GNDVI demonstrated a better relationship with turf quality and N status than R/NIR and G/NIR. A comparison among the four indices showed NDVI to be more closely correlated with irrigation, N fertilization, and turf quality. Minimum acceptable and target NDVI were developed by seasonal period based on visual turf quality assessment. It was also found that NDVI response to N fertilization was not strongly affected by irrigation treatment and could be used as an indicator of N status and need regardless of irrigation treatment. |
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