## 1 Introduction  
Chemical fertilizer as an important means of agricultural production, is made by chemical or physical methods to contain one or several nutrient elements required for the growth of crops. In order to ensure the yield and quality of crops, it plays an important role in ensuring the safety of food production and the high efficiency and high yield of agriculture [1]. However, in recent years, the excessive application of chemical fertilizers, the low utilization rate, and the unreasonable proportion of trace elements have caused more serious social problems such as environmental pollution, resource pressure, and ecological destruction. As a result, the organic matter content of cultivated land has been greatly reduced, the soil has become increasingly acidified and compacted, and the quality of agricultural products has declined, which is not conducive to the sustainable development of agriculture [2; 3; 4; 5]. Precision fertilization is based on the soil nutrient content and variable fertilization on demand. Proved by practice, it is an effective way to solve the unreasonable fertilizer input and ratio, low fertilizer utilization rate, and agricultural ecological environmental pollution. In addition, it can significantly improve economic,social, and ecological benefits, which is the development direction of fertilization technology in the future [6; 7; 8]. The main structural forms of the existing variable fertilizer discharging device include outer groove wheel type, centrifugal type, spiral type, vibrating type, star wheel type, etc. [9]. However, since most of the chemical fertilizers used in base fertilizer at home and abroad are granular, the outer groove wheel type fertilizer discharging device is mostly used. Mostly, the rotation speed of the outer groove wheel type fertilizer discharging shaft is changed to realize the adjustment of the fertilizer amount, that is, the single variable fertilizer discharging. Experts and scholars [10; 11] at home and abroad had done a lot of research on variable fertilization with grooved wheels, Japanese scholars had developed a variable fertilizer applicator suitable for paddy field operations. It used a Direct Current power (DC) stepper motor to drive the fertilizer discharging device. It implemented variable fertilization operations based on the onboard computer and the operation prescription map; the SOILEC-TION fertilizer applicator produced by Agchen in the United States could apply solid or liquid fertilizer as needed according to the fertilization prescription map [12]; Kyoto University [13] developed a variable rate fertilizer applicator. By controlling the rotation speed of the driving motor connected with the fertilizer discharging mechanism, the purpose of regulating the discharge amount of fertilizer was achieved. [PERSON] et al. [14] used a single-chip microcomputer to control the speed of the fertilizer discharging shaft of the fertilizer applicator and developed a continuously adjustable variable fertilization control system; [PERSON] et al. [15] designed a precision fertilization device and adopted an external trough type fertilizer device. There was an insert plate on the fertilizer pipe, which could adjust the amount of fertilizer according to the movement of the insert plate. At the same time, it could prevent the fertilizer pipe from clogging. The univariate fertilization method is simple in structure and easy to control. It has been widely used, but the accuracy and range of fertilization adjustment are small[PERSON] Especially when the amount of fertilization is small or the rotation speed of the fertilizer shaft is too low, it is easy to cause the pulsation of the fertilizer discharge of the outer groove wheel to be significant[PERSON] It seriously affects the uniformity of fertilization. Additionally, too high rotation speed can easily reduce the accuracy of fertilization and increase the rate of seed damage [16]. In order to solve the above problems, scholars have proposed a two-variable fertilization method.