Spectral measurements were made over cotton which had received five different preplant nitrogen fertilization rates for the third consecutive year in 2000. Half of the plots received only rain (WL) and the other half were fully irrigated to replace potential evapotranspiration (WH).Multispectral reflectance measurements were made to determine when seedling emergence could first be detected, and later in the season to detect plant size differences due to water and nitrogen treatments, and to estimate other crop agronomic factors. Spectral reflectance values were calculated for the blue(0.45 -0.52 µ), green (0.52 -0.60 µ), red (0.63 -0.69 µ), and near infrared (0.76 -0.90 µ) wavelengths and related to various plant characteristics. The presence of emerged seedlings was detected seven days after planting (one day after initial emergence) on DOY 143 by spectral reflectance in the red wavelength and by the normalized difference vegetation index (NDVI). At this time leaf area in the target areas averaged
12.1 cm ² , the leaf area index (LAI) was 0.02, and the ground cover was
1.3%. By 15 days after planting on DOY 151, seedling leaf area was linearly related to the NDVI values. The red wave band was the most sensitive individual band during stand establishment for detecting the presence of cotton seedlings. The NDVI was equally sensitive, but its sensitivity was due to the red band since the NIR wave band did not detect the presence of cotton seedlings between DOY 143 and DOY 158.

Between first square and peak bloom, NDVI was not sensitive to nitrogen fertilization level. Leaf area index differences between water levels was positively related to NDVI. Plant height was also positively related to NDVI across sampling dates, but within sample dates only at peak bloom. Similarly NDVI was negatively related to leaf nitrogen content in the WH treatment across dates. NDVI values separated plots into two progressively distinct groups as sampling proceeded from first square to peak bloom and these groups coincided with the WL and WH treatments. NDVI at peak bloom was significantly correlated with lint yield of the WH treatment, but not the WL treatment yields which were low and showed little variation.