## 6 Conclusions  
The mechanical structure design and working principle of a pneumatic centralized fertilizer discharge system, as the core part of pneumatic fertilization machinery, play a decisive role in the fertilization effect and equipment performance. The conveying performance of fertilizer particles in the pipeline of a pneumatic centralized fertilizer discharge system is affected by many parameters, mainly including the particle size, the density and shape of fertilizer particles, the airflow pressure, velocity and temperature, the pipeline diameter, length, bending, and branching, and other key parameters. A pneumatic centralized fertilizer discharge system mainly includes a fertilizer discharge device, an air-fertilizer mixing device and a pneumatic distribution device. This paper comprehensively analyzes the research progress made in pneumatic centralized fertilizer discharge systems at home and abroad, including the mechanical structure, key parameters and research methods of fertilizer discharge devices, air-fertilize mixing devices and pneumatic distribution devices. Through comparative analysis, the advantages and disadvantages of the existing devices are summarized, and the existing problems and improvement directions are pointed out. The mechanical structure and existing problems of the key components of pneumatic centralized fertilizer systems are discussed in depth from the following perspectives:  
(1) There are three main types of fertilizer discharge devices, namely the external groove wheel type, the spiral type, and the double tooth wheel type. Among these, external groove wheel-type and spiral-type fertilizer discharge devices are the most widely used. External groove wheel fertilizer discharge devices have the characteristics of a simple structure and convenient operation, with the main types including the straight groove, inclined groove, staggered groove and other types. The straight groove structure is simple, but there are problems with fertilization breakage and fluctuation in the fertilization amount. An inclined groove drives the fertilizer to be discharged through continuous spiral rotation, which can increase the uniformity of fertilization, but the rate of fertilizer particle damage is high. A staggered straight groove can effectively avoid the phenomenon of discontinuous and pulsating fertilizer discharge and improve the uniformity of fertilization. A spiral fertilizer discharge device has a simple structure,  
Figure 18: Schematic diagram of a distribution device structure: (**a**) overall structural diagram; (**b**) partial structure diagram. convenient operation, and is suitable for granular fertilizers with good fluidity. Spiral fertilizer discharge devices mainly include the spiral combination type, the oblique spiral type, the staggered symmetric spiral groove type, and other types. Spiral combination fertilizer discharge devices distribute a large amount of fertilizer with good uniformity, but their structure is complex and not suitable for fertilizers with a high powder content. The oblique spiral type is suitable for dry and loose fertilizers, with good fertilizer discharge performance, but is not suitable for fertilizers with strong moisture absorption. The staggered symmetric spiral groove reduces the filling rate of the gaps in the spiral blades, avoids the overfilling of fertilizer, and has the characteristics of uniform fertilizer discharge, less clogging, and a strong adaptability to fertilizer. (2) The air-fertilizer mixing device usually adopts the Venturi tube structure design to realize the mixing and transportation of air-solid two-phase flow. In the air-fertil mixing device, the air-solid two-phase flow involves a variety of motion mechanisms, including collision between solid particles, friction and resistance between particles and airflow and the suspension and diffusion of particles. The interaction of these mechanisms leads to the complex trajectories and distribution of granular fertilizers in the mixing device.