New crops that had come to Asia from the Americas via the Spanish colonizers in the 16th century contributed to the Asia's population growth. Although the bulk of imports to China were silver, the Chinese also purchased New World crops from the Spanish Empire. This included sweet potatoes, maize, and peanuts, foods that could be cultivated in lands where traditional Chinese staple crops—wheat, millet, and rice—could not grow, hence facilitating a rise in the population of China. In the Song Dynasty (960–1279), rice had become the major staple crop of the poor; after sweet potatoes were introduced to China around 1560, it gradually became the traditional food of the lower classes.