Using, as raw material, solid fertilizer accommodated in a sealed container (1) with hermetic closure, and having a container holder for accommodating, in a tight-fitting manner, said container, connected to a bypass (10-11) of the general irrigation piping (13), such that the water diverted via said bypass enters into the container (1), dissolves the fertilizer and exits in order to be re-directed back to the irrigation piping, the invention is based on the provision of a pressure-regulator unit, based on a pressure controller (23), a check valve (24), a safety valve (25) and small piping that opens out in a normally closed depressurizing valve (26), said pressure regulator supplying an injector element (27) with fertilizer solution, same being of the " Venturi" type, which is likewise connected to the upper part of the container holder for the evacuation of air (39). In this way, a constant working pressure is maintained in the course of injection, and it is possible for the injection flow rate to be regulated as desired by means of an injection regulator valve (12) fitted in the segment of the irrigation piping (13) that runs between the branches (10) and (11) that form the bypass. The danger represented by pressurized compressible fluids (38-39) is also eliminated.
