Abstract:      As organic-inorganic fertilization (that is, reducing chemical fertilizer and increasing organic fertilizer) has many advantages, such as realizing the fertilizer utilization of agricultural waste and reducing the application of chemical fertilizer at the same time, it has become an agricultural environmental protection measure actively advocated by many countries, and the Chinese government has also promoted it as a new agricultural planting technology. However, in order to prevent the formation of new pollution, it is necessary to transform farm manure into higher quality and safer commercial organic fertilizer. However, due to the high price, high labor intensity and poor economic benefits of commercial organic fertilizer, farmers′ application enthusiasm is restricted. As a result, the Chinese government proposes to provide farmers with reasonable subsidies for the application of organic fertilizer. At the same time in accordance with the principle of "who benefits, who compensates", conservationists can also obtain benefits through the transaction of ecological products. Subsidy standard is a key scientific issue in the design of commercial organic fertilizer subsidy policy, which is related to the enthusiasm of subsidy objects and the affordability of subsidy subjects. It is generally believed that the opportunity cost, as the lower limit of the subsidy standard, is the method with the highest subsidy benefit. In agricultural ecological compensation, the opportunity cost is often measured by the different utilization value of land and human resources.
   Firstly, under the conditions that the agricultural planting use of the land remains unchanged, the crop planting system remains unchanged, other planting technologies except fertilization remain unchanged, and fertilization can only be selected between organic-inorganic fertilization and single fertilization of chemical fertilizer, the opportunity cost of organic-inorganic fertilization is defined. Then, a micro-economic dynamic subsidy standard theoretical model for commercial organic fertilizer is established by using the cost-benefit analysis method. The theoretical model points out the core train of thought and key steps to improve the subsidy benefit of commercial organic fertilizer. Then, taking meeting the common concerns and objective reality of the government and farmers as the starting point, avoiding the unpredictable factors, unmeasurable components and complex factors that are difficult to calculate accurately in the theoretical model, an operational and executive subsidy standard calculation model is established. Finally, taking the rice wheat rotation experiment with organic-inorganic fertilization as an example, the application of the theory and calculation model was explained.
   At the same time, under the condition of stable output of agricultural products, the fertilization cost of reducing chemical fertilizer by 30% and 50% and increasing organic fertilizer is about 1.5 times and 2 times of that of single chemical fertilization; among the increased fertilization costs, the actual purchase price of fertilizer, transportation and miscellaneous expenses of organic fertilizer and field fertilization costs account for 55.7%, 7.3% and 37% respectively; the amount of organic fertilizer applied per unit area of cultivated land doubled, and the subsidy per ton of organic fertilizer increased by only 0.1 times; the agricultural products with 50% reduction of chemical fertilizer and increased application of organic fertilizer have a series of data such as the best input-output ratio. These data are of great significance for in-depth understanding of the key links of improving commercial organic fertilizer subsidies. The establishment of corresponding supporting systems, organizations and services can also help realizing the "government + society" joint compensation at the same time, so as to realize the stable yield and income growth of farmers with organic-inorganic fertilization.
    According to the above research conclusions, in order to improve the subsidy benefits of commercial organic fertilizer and realize social participation compensation, this study puts forward the following policy suggestions: (1) Efforts should be made to reduce the market price of commercial organic fertilizer through improving production process and expanding production scale. (2) The linkage mode of raw materials, production and consumption of organic fertilizer should be established to reduce the transportation and miscellaneous expenses of organic fertilizer. (3) Mechanical fertilization should be popularized as soon as possible. (4) Establish a dynamic adjustment mechanism for the subsidy standard of commercial organic fertilizer. (5) Create conditions for the realization of "government + society" joint compensation.

Key words: commercial organic fertilizer, subsidy standard, model, opportunity cost, social participation