fertilize vs richen

fertilize

verb
  • To make more creative or intellectually productive. 

  • To make (the soil) more fertile by adding nutrients to it. 

  • To cause to produce offspring through insemination; to inseminate. 

richen

verb
  • To make or render rich or richer. 

  • To become rich or richer; become superior in quality, condition or effectiveness. 

  • To gain richness; become heightened or intensified in brilliancy. 

How often have the words fertilize and richen occurred in a corpus of books? (source: Google Ngram Viewer )