## 7 Future Research Directions  
In recent years, the technology of pneumatic centralized fertilizer discharge has developed rapidly. A large number of scholars at home and abroad have designed and improved the structure of the traditional pneumatic centralized fertilizer discharge system according to the characteristics of granular fertilizer, with strong hygroscopicity, cohesiveness, overhead and fluidity deviation. However, the research on the pneumatic centralized fertilizer discharge system in China started late, and the uniformity and stability of the fertilizer discharge system still need to be improved. In the future, theoretical innovation and technological breakthroughs can be explored from the following perspectives:  
(1) The structure of a fertilizer discharge device has poor adaptability to granular fertilizers. Due to the irregular shape and size of granular fertilizers, in a fertilizer discharge device, the crushing and uneven distribution of granular fertilizer can easily occur in the process of fertilizer discharge. At present, the structure of fertilizer dischargers is mostly designed according to the structural size of the fertilizer, and the adaptability is poor. Thus, it is necessary to continue to optimize the mechanical structure and key parameters of fertilizer discharge devices. (2) In air-fertilizer mixing devices, there are problems such as fertilizer backflow at the fertilizer outlet, uneven velocity distribution, accumulation and blockage of fertilizer particles. In the future, it is necessary to continue to optimize the internal mechanical structure of the air-fertil mixing chamber and determine the optimal combination of key parameters, such as the contraction angle of the nozzle port, the throat section structure, and the throat length of the air-fertil mixing device. (3) At present, domestic and foreign researchers are using CFD-DEM coupled simulation methods to study the flow characteristics of fertilizer particles in pneumatic centralized fertilizer discharge systems in order to simplify the fertilizer particles into a single diameter spherical model. Although this simplification assumption is convenient for calculation and analysis, it ignores the actual situation of irregular shape and size of fertilizer particles, which may lead to deviations between simulation results and actual situations. In future research, multiple sizes of fertilizer particle simulation models should be set up according to the actual proportions to more accurately reflect the distribution and flow status of fertilizer particles in the system. In addition to size differences, the shape of fertilizer particles is also an important factor affecting their flow characteristics. Future research can attempt to establish irregularly shaped fertilizer particle models to more comprehensively consider the influence of fertilizer particle shape on flow characteristics. (4) Because granular fertilizer has certain hygroscopicity and cohesiveness, adherence easily occurs between the particles, which leads to the blockage of the fertilizer in the pneumatic distribution device and affects distribution uniformity. Due to the evenness of the ground, the fertilizer applicator vibrates and deviates when walking, resulting in the pneumatic centralized fertilizer discharge system applying fertilizer unevenly to each row, and there are problems such as more fertilization, less fertilization and missed fertilization. When the device is tilted, there are problems such as uneven fertilizer distribution and poor consistency in each row in the pneumatic distribution device. In the future, developments regarding automation and intelligence can be targeted in order to keep the pneumatic distribution device level throughout the work engineering. In the future, intelligent pneumatic centralized fertilization systems can be developed, which can enable the fertilization system to apply fertilizer quantitatively based on the growth status of crops and the walking speed variables of the fertilization machine. Conceptualization, L.Z. and W.Y.