Abstract: A variable rate injection system includes a paddle wheel flowmeter which produces a pulse train proportional to the amount of liquid flowing through an irrigation line. An accumulator circuit totalizes the pulse train and produces a triggering pulse to a flow control timer after a predetermined amount of fluid has passed through the irrigation line. The triggering pulse activates an adjustable time delay output on the flow control timer and its timed contact closure electrically operates a solenoid valve in an injection line. A constant displacement injection pump maintains an additive under pressure in the injection line and the solenoid valve, when opened, permits flow of the additive through the injection line and into the irrigation line. When the solenoid is closed, a pressure relief valve and relief line permit return of the additive to a reservoir. A safety shutdown includes a flow sensor mounted in the injection line between the normally closed solenoid and the main irrigation line.