reading notes for History Of Japanese Food, Naomichi Ishige.
The belief that rice is a plant in which a spirit dwells is found not only in Japan but also in Southeast Asian regions of wet rice cultivation, where the plant spirit is venerated through ritual involving the year's first-harvested ears of rice. In Southeast Asia and Japan it was commonly believed that disrespectful behaviour toward the spirit of the rice plant, believed especially to dwell in the first-harvested grains of rice, will cause people to waste away because the rice they eat will yield no nutrition and the seeds they plant will be barren. It appears that such ideas spread through all rice-growing regions in ancient times, but were displaced in China by the advent of other religions, while remaining in Southeast Asia and Japan.
Early agriculture in japan was not limited to rice. Other crops that have been found in excavations include millet (kibi), foxtail millet (awa), barley, wheat, and Deccan grass; soybeans and adzuki beans (Vigna angularis); melons of both fruit and vegetable types; and peaches. Some of those, such as peaches and melons, are thought to have been introduced from China, whereas others such as barley, adzuki beans and buckwheat are thought to have come by way of the Korean Peninsula. In sum, it seems that at the time of the arrival of rice growing, crops from those two regions came together in Japan to make up a discrete agricultural complex.