Radiometric Field Measurements of Maize for estimating Soil and Plant Nitrogen

Conventional recommendations of nitrogen (N) fertiliser are based on composite soil samples. This may result in either underfertilisation or overfertilisation due to the neglected spatial variability. This paper shows a procedure to estimate in season plant and soil nitrogen by using remote sensing. The study was conducted on two experimental sites. The first experiment consisted of six non-replicated fertiliser plots. These data were used to develop the relationships between reflectance data and the plant and soil N. The second experiment had four plots with various N and water treatments on which the developed relationships were verified. Plant N, soil N and reflectance data from both nadir and 75° view angles were collected almost weekly. The nitrogen reflectance index was employed to estimate plant N. Regression analysis was used to develop relationships. Spatial variability was mapped in ArcView geographical information system. Regression analysis between reflectance data and plant N, and soil N and plant N showed good relationships at various stages. The nadir view tended to underestimate the plant and soil parameters, whereas the 75° view slightly overestimated. Geographic information system mapping of measured and estimated soil N showed an agreement except in locations where high N spots were encountered.