Abstract: An inventive apparatus and method is disclosed for the automatic injection of a liquid additive (e.g., liquid fertilizer, liquid herbicide, liquid pesticide, liquid fungicide, a wetting agent solution or a mixture thereof) into a horticultural liquid dispensation system (e.g., a conventional sprinkler system). The invention can be readily packaged/installed with conventional watering systems or readily implemented with existing, installed watering systems. In one arrangement, the invention includes a programmable controller and an injection assembly comprising at least one solenoid valve and an injector. The controller senses which of a plurality of watering zones is currently being watered, and effects the transmission of electrical pulses to the solenoid valve. Such pulses cause the valve to open/close and thereby produce stroking of a spring-loaded piston within the injector. As a result, a predetermined amount, or “slug”, of liquid additive is injected into the watering system on each stroke.