Comparison of True-Color and Color-Infrared Digital Photography for Monitoring Crop Growth At Different Levels of Applied Nitrogen.

Two basic responses of crop growth to different amounts of available nitrogen are reduced growth and reduction of leaf chlorophyll concentration.  Remote sensing may be used to monitor these responses: (1) reduced growth may be measured using vegetation indices with either passive or active sensors; and (2) reduced chlorophyll concentrations may be measured with either chlorophyll meters or indices that quantify absorption by chlorophyll.  Digital photography combined with global positioning system data may be a low-cost alternative for nitrogen management.  Replicated plots of corn and rye were fertilized with different rates of applied nitrogen.  Nadir true-color and color-infrared photographs were acquired 2-m over the plots at various dates after planting.  Digital numbers (DN) of random points were obtained from both sets of photographs using the SamplePoint program.  DN's from sunlit leaves (without specular reflection) were used to calculate the Triangular Greenness Index (TGI) and the Green Normalized Difference Vegetation Index (GNDVI).  There were good correlations between biomass and GNDVI, and between chlorophyll meter values and TGI.  However, there was considerable overlap in crop growth and leaf chlorophyll concentration among the treatments, so that correlations with either index were weak.  Monitoring both biomass and chlorophyll concentrations may increase the reliability of both ground and remote sensing techniques for nitrogen management.