A method for dosing one or more treatment substances (S1, S2) in a system for washing and recycling plastics, comprises the steps of: - providing a container (2) supported in a freely oscillating manner and operationally connected to a programmable weighing and control unit (3), in which the container (2) is overtumable by gravity between a loading position (PI) and an unloading position (P2) of the treatment substances; - storing in the programmable weighing and control unit (3) a reference value (VR) of a suitable weight for overturning the container (2) to the unloading position (P2); - supplying a treatment substance (S1) to the container (2) detecting the weight thereof, and stopping the supply upon reaching a preset weight value (V1); and - supplying to the container (2) a washing substance (W) until the reference value (VR) is reached causing the container (2) to overturn through the force of gravity in the unloading position (P2) for unloading and sending the treatment substances (S1, S2) to plastics
