From The Collaborative International Dictionary of English v.0.48 [gcide]:
Usage: Fertile implies the inherent power of production; fruitful, the act. The prairies of the West are fertile by nature, and are turned by cultivation into fruitful fields. The same distinction prevails when these words are used figuratively. A man of fertile genius has by nature great readiness of invention; one whose mind is fruitful has resources of thought and a readiness of application which enable him to think and act effectively.
From WordNet (r) 3.0 (2006) [wn]:
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