"Fertile," as in fertile soil or fertile land, means that the soil and land is rich in nutrients for plants, and therefore is good for farming crops.
I believe the answer is: Integrative theory
Integrative theory view that society could never be exist unless each individuals fulfill their own roles. These happen because humans are unique in terms of our talent/capabilities. So in order to crate a well functioning society, different humans had to integrate our different capability to operate it.
Mencius' political philosophy basically results from an extension of his doctrine of moral feelings. As he told the rulers, “If you let people follow their feelings (original nature), they will be able to do good” (Mencius 6A6). When Xunzi later attacked Mencius, one of his arguments was based on the concept of natural feelings.
