Answer:
The decline of the Roman Empire
Explanation:
The reason for the development of both feudalism and manorialism were both caused by the decline of the Roman Empire.
Manorialism and Feudalism are similar concepts based on land tenure system and land distribution among lord and vassal or subjects.
It was developed during the decline of Rome as an empire and at a time when countries were becoming more centralized.
I would say that it is the fertile soil, the question seems incomplete though
