Abstract: Powdered organic fertilizers are characterized by excellent nutrient release, thereby enhancing soil fertility and structure for healthy plant growth in sustainable agriculture. However, material properties have posed serious challenges to the application in recent years, such as poor flowability, inconsistent particle characteristics, and sensitivity to mechanical vibrations. It is often required for the high spreading accuracy and sufficient uniformity during application. In this study, a dual-parameter electro-hydraulic proportional control system was developed for the large hydraulic-driven spreaders for powdered organic fertilizer. The control system synchronously adjusted the rotational speeds of the fertilizer delivery chain and spreading auger. Spreading precision and efficiency were also achieved for the stable and consistent fertilizer application rates under varying operations. An electro-hydraulic proportional control system with electromagnetic proportional valves was constructed to regulate the speed of hydraulic motors for fertilizer delivery and spreading. A transfer function model of the electro-hydraulic system was established to regulate the hydraulic motor speed. Frequency domain analysis revealed that insufficient phase margin under typical conditions resulted in slow response times and high sensitivity to external disturbances. A fuzzy control strategy was then introduced for high stability. The parameters were dynamically adjusted to enhance the system's adaptability under varying conditions, according to real-time error information and trends. Concurrently, a genetic algorithm (GA) was employed to optimize the fuzzy rule table. Parameters were globally optimized under different conditions. Manual tuning was reduced to improve the overall performance of the system. A numerical model was constructed using AMESim and Matlab/Simulink. A systematic evaluation was conducted on the performance under step input, time-varying tracking, and disturbance. Simulation results demonstrated that the optimal fuzzy PID controller significantly reduced settling time, overshoot, and steady-state error, compared with conventional proportional-integral-derivative (PID) control and fuzzy PID control. The system performed a settling time of 0.19 s higher than the fuzzy PID under step input conditions, with the overshoot of 0.3% (a 20.71% decrease, compared with classical PID) and steady-state error below 1%. The lags were markedly reduced for the high adaptability to sinusoidal tracking. A test bench was constructed to incorporate a hydraulic motor, proportional flow valve, and real-time feedback sensors. Experimental studies were conducted to validate the simulation reliability and engineering applicability under no-load and variable load. Test results showed that the high consistency was shared with the simulation. The optimal system achieved the target rotational speed within 1.2 s, indicating steady-state deviation within engineering tolerance limits. The superior robustness, repeatability, and disturbance rejection were observed under load fluctuations. Furthermore, fertilizer delivery and spreading were synchronically regulated to effectively reduce the speed mismatches among subsystems, providing for stable spreading. The precision and stability were significantly enhanced during variable-rate fertilizer application after optimization. A dual-parameter synchronous control strategy can provide a repeatable and adaptable engineering solution for the precise application of powdered organic fertilizers. Practical guidance and implications can also help optimize electro-hydraulic variable organic fertilizer application in medium-to-large agricultural machinery.