**2. Materials and Methods**

Chinese milk vetch (*Astragalus sinicus* L.) is a legume plant. The growth states of Chinese milk vetch at different periods are shown in Figure 1. It is an expert at using rhizobia to fix nitrogen, as shown in Figure 1a. A large amount of nitrogen, required by rhizobia in the growth process, is converted into ammonia nitrogen through the nodules, and what it returns to the rhizobia is the carbohydrates they need for survival. In this way, every part of the whole unit enjoys mutual benefit and peaceful symbiosis. Fruit pods of Chinese milk vetch at harvest time are shown in Figure 1b. When the average black pod rate of Chinese milk vetch reaches more than 75%, it should be harvested by machine in time.

**Figure 1.** Growth states of Chinese milk vetch at different periods. (**a**) Nitrogen-fixation effect picture of Chinese milk vetch in seedling stage. (**b**) Chinese milk vetch during harvest.
