## 3 Research Status of Fertilizer Discharge Devices
Figure 4: Screw fertilizer device: (**a**) fertilizer mechanism; (**b**) schematic diagram. 1. Fertilizer box. 2. Fertilizer shell. 3. Fertilizer plate. 4. Arc base brush. 5. Arc base. 6. Arc rear cover. 7. Fertilizer wheel. 8. Back cover arc brush. 9. D shaft. 10. Replace wheel. 11. Partition plate. 12. Through-shaft motor. Figure 3: Structural diagram of a screw-type fertilizer feeding device: (**a**) structure of fertilizer feeding device; (**b**) fertilizer feeding mechanism. 1. Fertilizer box. 2. Fertilizer churning mechanism. 3. Fertilizer filling room. 4. Shell. 5. Blocking plate. 6. Brush. 7. Unloaded plate. 8. Screw-type fertilizer feeding unit. 9. Feeding port. 10. Discharging shaft. 11. End cover. 12. Replace wheel. 13. Clapboard. The results showed that there was a linear relationship between the amount of fertilizer discharged by the spiral fertilizer discharger and the rotational speed at 10-200 r/min, and the overall linear determination coefficient was greater than 0.998. The spiral fertilizer device had a good effect on powder fertilizer and granular fertilizer after moisture absorption. [PERSON] et al. [30] used a spiral grooved wheel instead of a straight grooved wheel, and they optimized the key structural parameters of the spiral grooved wheel fertilizer discharger. Discrete element method was used to compare and analyze the different spiral angles of the spiral grooved wheel fertilizer discharger, and it was concluded that, when the spiral angle of the spiral grooved wheel was 60.83\({}^{\circ}\), the fertilizer discharge performance was the best. The structure of the spiral grooved wheel fertilizer discharger is shown in Figure 5.  
To address the issues of the poor uniformity and the unstable fertilizer discharge performance of side-deep fertilizer applicators in paddy fields, [PERSON] et al. [31] developed a conical disc push-plate double-row fertilizer discharger. By constructing a mechanical model and performing a simulation analysis, it was concluded that when the number of push plates was eight, the performance of the fertilizer discharger was the best. [PERSON] et al. [32] designed a spiral grooved wheel fertilizer device to replace the traditional straight grooved wheel fertilizer device. The measured three-axis sizes of the two fertilizers (compound fertilizer and urea fertilizer) were small, and the sphericity was greater than 90%. They used the discrete element method (DEM) to compare and analyze the fertilizer discharge performance of the two grooved wheel mechanisms. The research showed that the uniformity of the spiral grooved wheel was obviously better than that of the straight grooved wheel. [PERSON] et al. [33] designed a type of staggered symmetrical spiral groove seed feeding wheel, as shown in Figure 6. A full factor simulation test was carried out on the inclination angle and length of the spiral groove, and it was concluded that the staggered symmetrical spiral groove metering wheel greatly improved the uniformity of seeding. Aiming to solve the problem of fertilizer particles having strong hygroscopicity and often agglomerating, which leads to the problem of the outer groove wheel fertilizer  
Figure 5: Spiral groove wheel fertilizer feeder graph. Figure 3.0 Structure diagram of staggered symmetrical spiral grooved seed feeding. discharger easily becoming blocked, [PERSON] et al. [34] studied the influence of the different working parameters of a spiral grooved wheel on fertilizer discharge performance, and they determined the best working parameter combination for spiral grooved wheels. [PERSON] et al. [35] designed a pin-roller metering device, as shown in Figure 7, which can improve the discharge and uniformity of granular fertilizer. To solve the problem of uneven fertilization, [PERSON] et al. [36] designed a type of helical fertilizer discharger with an inclined mouth.