Abstract: Fertilizers play an important role in promoting crop growth and improving soil fertility. However, excessive use of fertilizers can cause serious damage to the environment. Precision fertilization can significantly reduce the excessive use of fertilizers in farmland, and the key to achieving precision fertilization lies in the accurate detection of fertilizers during the operation process. Conducting research on the granular fertilizer detection system to achieve stable, accurate, and reliable detection of high-flow granular fertilizers during field fertilization operations helps improve the quality of field fertilization and the utilization rate of fertilizers. However, during the field operation of the rapeseed direct seeder, influenced by the fertilization agronomy and the requirements for operation speed, the application amount of granular fertilizers is large and the fertilizer discharge frequency is high. At the same time, the sensor is easily affected by the environment and the type of fertilizer, which makes it quite difficult to achieve high-precision real-time detection during mechanical direct seeding operations. The detection device based on the capacitance method has a simple design, stable and reliable operation, and the characteristic of high sensitivity, and it has been widely used in the sowing, fertilization, and harvesting processes of precision agriculture. In terms of the detection of the mass flow rate of granular fertilizer application, the device based on the capacitance method has obvious advantages, but its stability and dynamic measurement accuracy in the field environment are still problems to be solved. Aiming at the problems of large granular fertilizer discharge amount, diversified types, and the fact that the sensitive components of the sensor are easily interfered with, resulting in low real-time detection accuracy during the rapeseed direct seeding operation process, this study developed a non-contact capacitive detection device for the discharge and application of granular fertilizers in rapeseed direct seeding machines with a self-calibration function. The key structural parameters of the capacitive plates and isolation sleeves were designed, a simplified Kalman filtering algorithm for capacitive signals was constructed, and the key parameters of the filtering algorithm were determined through experiments. A relationship model between the mass flow rate of granular fertilizers and the capacitive signals was established, and a three-point fitting calibration method for the detection parameters of the mass flow rate of granular fertilizers suitable for different temperature environments and types of granular fertilizers was designed. A detection system for the discharge and application of granular fertilizers in rapeseed direct seeding machines composed of an 8-channel detection device and a detection terminal was developed. The results of the bench tests show that under the environmental temperatures of 15℃, 25℃, and 35℃, the detection accuracy rate of the device is not lower than 97.35%. For Xiangyan compound fertilizer and Haosite compound fertilizer, the detection accuracy rates are not lower than 96.45% and 96.23% respectively; when the speed of the seeding and fertilization operation machine is 4-12 km/h and the average fertilizer discharge flow rate is 9.58-28.75 g/s, the detection accuracy rate of the system is not lower than 95.65%. The field tests show that when the granular fertilizer discharge amount is 9.82-28.99 g/s, the detection accuracy rate of the system is not lower than 93.14%, which is at least 3 percentage points higher than the accuracy rate of the existing granular fertilizer flow detection system. The data transmission between the detection system and the cloud database is accurate, complete, and stable, and the vibration of the machine and dust have little impact on the detection results of the detection system. This detection system can provide an effective means for the evaluation of fertilization quality.