Here's why:
* Describes a noun: It describes a quality of something, like "fertile land" or "a fertile imagination."
* Modifies nouns: It can be used to modify nouns directly, like "the fertile soil" or "a fertile mind."
However, "fertile" can also be used as a noun in some contexts. For example:
* "Fertile" as a concept: You might say, "The book explores the concept of fertile ground for creativity."
* "Fertile" as a group: "The fertile of the land were responsible for the harvest."
While less common, these uses still demonstrate that "fertile" can act as a noun.