remains equal thus supplying equal pressure to each of the pipes 90d the gate valves 94 can be operated to close off one or more of the pipe sections if the flow varies through the pipes due to the different lengths of the supply pipes in conjunction with the variations in flow rate and density of the material the gate valves can be operated by an actuator system not shown and the control of a central control unit for maintaining the flow rates substantially equal to prevent variations in flow rate along the length of the boom the pipe 29 supplying the liquid to the vehicle is supplied to a mixing valve 95 which supplies the pump 3 the mixing valve 95 also receives liquid fertilizer from the tank 32 supplied by a discharge pump 31 controlled by the control unit 96 thus the amount of liquid fertilizer added to the waste material from the pipe 29 can be varied by the control unit by actuating the discharge pump from zero addition up to a required addition to maintain the level of nutrients within the waste material at a required level the waste material from the pipe 29 passes firstly through a meter 97 of a type which is commercially available for measuring the nitrogen content particularly by measuring the nh4 content a meter of this type is disclosed in french patent application 90 00194 filed by etalissements maugin s a and a product of the type.