An agricultural machine , comprising: a liquid reservoir that stores liquid to be applied to a field over which the agricultural machine is traveling; a supply line that defines a supply conduit; a plurality of valves disposed along the supply line , each valve having an inlet end and an outlet end and being controlled to move between an open position and a closed position by a valve control signal; a pump system that pumps the liquid from the liquid reservoir along the supply line to the inlet ends of the valves ; a plurality of nozzles , at least one nozzle corresponding to each valve so that when the corresponding valve is open, the liquid flows through the valve to the corresponding nozzle ; a plurality of row pressure sensors each sensing pressure at the outlet end of one of the plurality of valves and generating a corresponding row pressure signal indicative of the sensed pressure; a system flowmeter configured to identify an applied flowrate of the liquid material out of the liquid reservoir and through the supply conduit and generate an applied flowrate signal indicative of the applied flowrate; and a valve control signal generator that generates the valve control signal based on the row pressure signals and the applied flowrate signal; characterised by further comprising orifice size identifier logic configured to receive an active valve indicator indicative of a number of active valves on the agricultural machine and to identify a valve orifice size based on the sensed applied flowrate and the active valve indicator; valve row flowrate identifier logic configured to identify a valve row flowrate for each valve based on the valve orifice size and the row pressure signal from the corresponding row pressure sensor ; and a valve blockage detector configured to detect a blockage state of each of the plurality of nozzles based on the valve row flowrate for each of the corresponding valves . a liquid reservoir that stores liquid to be applied to a field over which the agricultural machine is traveling; a supply line that defines a supply conduit; a plurality of valves disposed along the supply line , each valve having an inlet end and an outlet end and being controlled to move between an open position and a closed position by a valve control signal; a pump system that pumps the liquid from the liquid reservoir along the supply line to the inlet ends of the valves ; a plurality of nozzles , at least one nozzle corresponding to each valve so that when the corresponding valve is open, the liquid flows through the valve to the corresponding nozzle ; a plurality of row pressure sensors each sensing pressure at the outlet end of one of the plurality of valves and generating a corresponding row pressure signal indicative of the sensed pressure; a system flowmeter configured to identify an applied flowrate of the liquid material out of the liquid reservoir and through the supply conduit and generate an applied flowrate signal indicative of the applied flowrate; and a valve control signal generator that generates the valve control signal based on the row pressure signals and the applied flowrate signal; characterised by further comprising orifice size identifier logic configured to receive an active valve indicator indicative of a number of active valves on the agricultural machine and to identify a valve orifice size based on the sensed applied flowrate and the active valve indicator; valve row flowrate identifier logic configured to identify a valve row flowrate for each valve based on the valve orifice size and the row pressure signal from the corresponding row pressure sensor ; and a valve blockage detector configured to detect a blockage state of each of the plurality of nozzles based on the valve row flowrate for each of the corresponding valves . <EOS>