1. A method of evaluating the current growing condition of an agricultural crop within a defined field area at a selected time during its growing season, comprising the following steps: using infrared photographic techniques to photograph the field area from an aircraft under known daylight conditions; producing a color photographic image representative of the infrared radiation emanated from the field area as photographed; electronically integrating and measuring the average color density values contained in the photographic image over the total defined field area for a plurality of selected colors to arrive at a measured density value for each selected color; and comparing the measured density value for each selected color to experientially derived density values for the respective colors representative of optimum conditions for the crop at a corresponding time during its growing season.