The mass flow of fertilizer spreaders must be calibrated for the different types of fertilizers used. To obtain accurate fertilizer application manual calibration of actual mass flow must be repeated frequently. Automatic calibration is possible by measurement of the actual mass flow, based on dynamic weighing of the spreader. This paper describes the design and test of a dynamic weighing system. Under field conditions, this system was able to provide force due to spreader weight readings that have a standard deviation of 20 N, over a measurement range of 6-20 kN, at a maximum time delay of 1 s. Time-stamped data logging of the spreader weight and the theoretical application rate combined with position information allows determination of the realized spatial application of fertilizer. A comparison can be made between realized and prescribed application rates. This information is necessary for evaluation of fertilizer management in a precision agriculture farming system and to accurately target fertilizer application to crop and soil status. (C) 2001 Silsoe Research Institute.