Abstract: An agricultural spray is controlled by an apparatus having a sensor (15) for determining the irradiance in the red and near infrared wavebands of the electromagnetic spectrum and a plurality of sensors (13) for determining the radiance of a target area to be sprayed in the red and near infrared wavebands. A controller (14) which controls the individual sprays (12) of the agricultural spray compares the measurements taken by the sensors (15 and 13) and compares the ratio of these values (reflectance) to determine if the spray for the particular target area should be turned on or turned off. The determination may be made by simply looking up tables of reflectance values or by carrying out the calculations of non-linear decision algorithms.