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Rising costs and increased environmental restrictions on the application of fertilizers and herbicides have resulted in the need for more selective methods of application to be developed. Studies at Oklahoma State University (OSU) have shown that a plants nitrogen uptake and uptake rate in the early stages of growth are closely correlated with the amount of fertilizer the plant will need throughout the growing season. A machine that can accurately assess and apply a plants fertilizer requirement would increase the efficiency of nitrogen use in crops. OSU and NTech Industries Inc. have developed a field-scale machine that senses wheat canopy vigor, estimates plant nitrogen requirements, and applies top-dress fertilizer. The fertilizer application system can be adapted to common spray applicators. Active sensors detect light reflectance from a plant canopy to determine a vegetative index, which is used to calculate the desired application rate. Desired application rate is achieved by modulation of binary valves and through pressure control. The machine also utilizes boom height control and valve attitude adjustment to increase application accuracy. Benefits of the commercially viable fertilizer application system are both economic and environmental. Return to producers using the system has been estimated conservatively $24.7 per hectare ($10 per acre) in wheat, and machine costs can be paid back in approximately two years for typical systems.