An improved system for controlled aerial distribution of substances such as chemicals upon crops in a select field, including coordinating the simultaneous independent application of two different substances. The system may include a pair of sub-systems, one of which includes a computer controlling a variable flow valve capable of varying the release of a substance from the first subsystem. There may also be a second subsystem having a pump activated by a lever that also opens and closes the fluid communication of the first subsystem, in this basic version of the invention, manually positioning the lever to open the spray valve also commences continuous release of a second substance from the second subsystem at a constant rate, until the lever is positioned to close the first fluid communication. In more sophisticated versions, the second sub-system may include a second computer controlling a second variable flow valve capable of varying the release of a substance from the second subsystem, or the second variable flow valve may be controlled by a separate output from the first computer.