## 3 Research Status of Fertilizer Discharge Devices  
The fertilizer discharge device is a key component of the pneumatic centralized fertilizer discharge system. During the working process, the fertilizer discharge device discharges the fertilizer into the pneumatic conveying system. Under the action of high-speed airflow, the fertilizer is transported along the pipeline at a fixed point, and, finally, it is evenly distributed to each row and discharged from the fertilizer outlet [20; 21; 22; 23]. In recent years, scholars at home and abroad have conducted extensive research on fertilizer discharge devices and achieved good results. Aiming to solve the problem of poor uniformity of fertilizer discharge caused by the fluctuation of fertilizer discharge with time in the process of fertilizer transportation, [PERSON] et al. [24] selected Russian Akang compound fertilizer (density 1320 kg/m\({}^{3}\), diameter 3.6 mm) and studied the influence of the length and angle of the fertilizer outlet on the uniformity of fertilizer discharge. Their results showed that the length and angle of the fertilizer outlet had a significant effect on the performance of fertilizer discharge. When the angle of the fertilizer outlet was 135\({}^{\circ}\) and the length was 60 mm, the effect of fertilizer discharge was the best. The structure of a spiral fertilizer discharg is shown in Figure 2.  
To achieve quantitative and variable fertilization, [PERSON] et al. [25] designed a screw-type fertilizer feeding device, as shown in Figure 3. This fertilizer feeding device can realize variable, quantitative and centralized fertilizer discharge, and it meets the requirements of fertilization stability in agricultural production, but it is not suitable for fertilizers with a high powder content. Figure 1: Structural schematic diagram of a pneumatic centralized fertilizing system. 1. Fertilizing pipe. 2. Air–fertilizer separation device. 3. Bellows. 4. Distributor outlet. 5. Distributor. 6. Fertilizer box. 7. Fertilizing motor. 8. Fan. 9. Airflow conveying pipe. 10. Air–fertil mixing device. 11. Outer–groove wheel fertilizer feeder. 12. Air–fertil conveying pipe. 13. Layered deep fertilizing shovel. Figure 2: Structural diagram of a spiral fertilizer device. 1. Drive motor. 2. Coupling. 3. Feeding inlet. 4. Screw. 5. Screw shell. 6. Discharge port. 7. Concentrator. To improve the efficiency of fertilizer utilization, [PERSON] et al. [26] developed a hole fertilizer device and, using the simulation analysis method, studied the influence of the length of the fertilizer hole, the working speed and the airflow speed on the amount of fertilizer discharged. Their results showed that the airflow had a significant effect on the performance of the hole. [PERSON] et al. [27] developed a double-gear fertilizer discharge. Taking the pressure angle of the fertilizer discharge wheel and the theoretical gap of the fertilizer discharge as the test factors, the quadratic general rotation combination test was used to establish a regression equation. The test results showed that changing the rotation speed of the fertilizer wheel could achieve the linear adjustment of the amount of fertilizer discharged. [PERSON] et al. [28] improved the screw fertilizer device, as shown in Figure 4. Through a comparative analysis of the three hole structures, namely the symmetrical hole, the inclined hole and the straight hole, it was determined that the inclined hole had the best fertilizer discharge performance, and, within a certain range, the fertilizer discharge rate had a linear relationship with the rotation speed. In order to improve the performance of fertilizer discharge and reduce the requirements of fertilizer discharge devices to accommodate for fertilizer morphological characteristics, [PERSON] et al. [29] developed a two-stage spiral fertilizer discharge device. Batian compound fertilizer and powdered rice special fertilizer developed by South China Agricultural University were selected as experimental fertilizers (moisture content 2.0-10.8%).