Uncalibrated Late-Season Digital Aerial Imagery for Predicting Corn Nitrogen Status within Fields.

Using uncalibrated digital aerial imagery (DAI) for diagnosing in-season nitrogen (N) deficiencies of corn (Zea mays L.) is challenging because of the dynamic nature of corn growth and difficulty of obtaining timely imagery. Digital aerial imagery taken later during the growing season is more accurate in identifying areas deficient in N. Even so, the quantitative use of late-season DAI across many fields is still limited because the imagery is not radiometrically calibrated. We tested whether spectral characteristics of corn canopy derived from uncalibrated late-season DAI could be used to predict corn N status within fields. Color and near-infrared DAI was collected in late August or early September from 602 corn fields in 2006 and 690 fields in 2007 across Iowa. Four sampling areas (one within a target-deficient, lighter color area on the imagery) were selected within each field for the late-season corn stalk nitrate test. The imagery was enhanced to increase the dynamic range and normalize reflectance values across all fields within a year. The reflectance values of individual bands and three vegetation indices were used to predict corn N status expressed as Deficient or Sufficient (a combination of the marginal, optimal, and excessive stalk nitrate test categories). The lighter color on the imagery indicated N deficiency in about 50% of samples in 2006 (dry summer) and 70% in 2007 (wet spring). The green band correctly identified Deficient and Sufficient N status in about 70% samples in 2006 and 64% samples in 2007.  The results showed that with the appropriate enhancement method, it is possible to use late-season uncalibrated DAI to predict corn N status within fields.